[InlineCost] Increase the cost of Switch

[junbuml]

Summary

The motivation example is like below which has 13 cases but only 2 distinct targets

lor.lhs.false2:                                   ; preds = %if.then
  switch i32 %Status, label %if.then27 [
    i32 -7012, label %if.end35
    i32 -10008, label %if.end35
    i32 -10016, label %if.end35
    i32 15000, label %if.end35
    i32 14013, label %if.end35
    i32 10114, label %if.end35
    i32 10107, label %if.end35
    i32 10105, label %if.end35
    i32 10013, label %if.end35
    i32 10011, label %if.end35
    i32 7008, label %if.end35
    i32 7007, label %if.end35
    i32 5002, label %if.end35
  ]

which is compiled into a balanced binary tree like this on AArch64 (similar on X86)

.LBB853_9:                              // %lor.lhs.false2
        mov     w8, #10012
        cmp             w19, w8
        b.gt    .LBB853_14
// BB#10:                               // %lor.lhs.false2
        mov     w8, #5001
        cmp             w19, w8
        b.gt    .LBB853_18
// BB#11:                               // %lor.lhs.false2
        mov     w8, #-10016
        cmp             w19, w8
        b.eq    .LBB853_23
// BB#12:                               // %lor.lhs.false2
        mov     w8, #-10008
        cmp             w19, w8
        b.eq    .LBB853_23
// BB#13:                               // %lor.lhs.false2
        mov     w8, #-7012
        cmp             w19, w8
        b.eq    .LBB853_23
        b       .LBB853_3
.LBB853_14:                             // %lor.lhs.false2
        mov     w8, #14012
        cmp             w19, w8
        b.gt    .LBB853_21
// BB#15:                               // %lor.lhs.false2
        mov     w8, #-10105
        add             w8, w19, w8
        cmp             w8, #9          // =9
        b.hi    .LBB853_17
// BB#16:                               // %lor.lhs.false2
        orr     w9, wzr, #0x1
        lsl     w8, w9, w8
        mov     w9, #517
        and             w8, w8, w9
        cbnz    w8, .LBB853_23
.LBB853_17:                             // %lor.lhs.false2
        mov     w8, #10013
        cmp             w19, w8
        b.eq    .LBB853_23
        b       .LBB853_3
.LBB853_18:                             // %lor.lhs.false2
        mov     w8, #-7007
        add             w8, w19, w8
        cmp             w8, #2          // =2
        b.lo    .LBB853_23
// BB#19:                               // %lor.lhs.false2
        mov     w8, #5002
        cmp             w19, w8
        b.eq    .LBB853_23
// BB#20:                               // %lor.lhs.false2
        mov     w8, #10011
        cmp             w19, w8
        b.eq    .LBB853_23
        b       .LBB853_3
.LBB853_21:                             // %lor.lhs.false2
        mov     w8, #14013
        cmp             w19, w8
        b.eq    .LBB853_23
// BB#22:                               // %lor.lhs.false2
        mov     w8, #15000
        cmp             w19, w8
        b.ne    .LBB853_3

However, the inline cost model estimates the cost to be linear with the number of distinct targets and the cost of the above switch is just 2 InstrCosts. The function containing this switch is then inlined about 900 times.

This change modifies the model to be linear with the size of the balanced binary tree.

[junbuml]

Note that this change was originally written by @haicheng in D29870 and I updated his change by using a new TTI hook to get the number of case cluster based on D31080.

[junbuml]

I can still see the +8% in performance and -7.63% in size in spec2000/vortex. No significant performance and size change in other benchmarks in spec2000/spec2006.

[joerg]

Can you please also try this on top of the changes from D30333? That's changing one switch transformation to happen much later, making it more sensitive to inline cost estimates.

[junbuml]

Can you please also try this on top of the changes from D30333? That's changing one switch transformation to happen much later, making it more sensitive to inline cost estimates.

Thanks I will try this !

[junbuml]

While I'm working on the accurate version of case cluster calculation through the TTI hook in this change based on D31080, I also want to see if it make sense to apply different level of approximations depending on the number of case? So, for example, if the number of case is more than some threshold, we can use a very rough approximation (e.g, use the number case as number of cluster like D29870). Otherwise, we can use a somewhat accurate version of cluster calculation based on D31080.

[junbuml]

The below comment by Hans is copied from D31080 :

Before we get any further, I also would like to ask if you have done any measurements of compile-time with this set of patches. As I said before, I think this be quite an expensive hook to call for the inline cost analysis, and it would be nice to see some numbers. If it turns out that it is expensive, perhaps we could come up with some better inline cost heuristic, perhaps something based on the density of the switch.

I think there are three difference cost heuristics :

1. The cheapest yet inaccurate version is D29870 in which the number of case is simply used as number of cluster.
2. The most accurate yet expensive version is to use a hook from D31080.
3. I guess what Hans suggested in above comment (something based on the density of the switch) must be somewhere in between #1 and #2.

Both #1 and #2 consider forming a BTree, and don't require to update the cost heuristic for the changes in switch lowering. I'm not clear about #3 in terms of the accuracy, compile-time, and maintenance. Hans, can you give me little bit more detail?

For me, at least #1 is still better than the current heuristic which simply count the number of distinct successor blocks. If then, would it make sense to use #1 by default and add difference levels of cost heuristics with flags so that we can come up with the most reasonable heuristic and allow others to do experiments ?

[hans]

In D31085#713391, @junbuml wrote:

The below comment by Hans is copied from D31080 :

Before we get any further, I also would like to ask if you have done any measurements of compile-time with this set of patches. As I said before, I think this be quite an expensive hook to call for the inline cost analysis, and it would be nice to see some numbers. If it turns out that it is expensive, perhaps we could come up with some better inline cost heuristic, perhaps something based on the density of the switch.

I think there are three difference cost heuristics :

1. The cheapest yet inaccurate version is D29870 in which the number of case is simply used as number of cluster.
2. The most accurate yet expensive version is to use a hook from D31080.
3. I guess what Hans suggested in above comment (something based on the density of the switch) must be somewhere in between #1 and #2.

Both #1 and #2 consider forming a BTree, and don't require to update the cost heuristic for the changes in switch lowering. I'm not clear about #3 in terms of the accuracy, compile-time, and maintenance. Hans, can you give me little bit more detail?

For me, at least #1 is still better than the current heuristic which simply count the number of distinct successor blocks. If then, would it make sense to use #1 by default and add difference levels of cost heuristics with flags so that we can come up with the most reasonable heuristic and allow others to do experiments ?

Yes, counting the number of successor blocks doesn't seem right.

I think a decent heuristic might be:

1. Ask TTI about the native word width, and check if this switch is trivially lowered with a bit test (two successors, case range fits in a machine word); this is very cheap
2. Ask TTI about jump table density conditions; if the whole switch is dense enough, assume it's a jump table and compute a cost based on that
3. Otherwise assume it's a balanced tree and estimate the cost based on number of cases (it would be nice not to actually have to build the tree, but just compute an estimate cost using some formula)

This means the estimate will be off for switches that are lowered with a mix of jump tables, binary trees and bit-tests, but those aren't that common, and not being completely accurate is probably fine.

I think going with only #1 might not be so good because we'll overestimate the cost of many switches. But I think computing 1-3 would make for a good a cheap heuristic.

[junbuml]

If we are not worry about the mix of cases, I think this is reasonable cheap to be used.

[junbuml]

Chandler, do you agree with the heuristic Hans suggested above? Even though it do not cover switches that are lowered with a mix of jump table/bit test/BTree, I think this is reasonable compromise between accuracy and cost of the hook.

[junbuml]

Based on Hans' suggestion, checked if switch is suitable for either bit test or jump table. If not suitable for both, use BTree. Please take a look and let me know any comment.

[haicheng]

include/llvm/CodeGen/SwitchCaseCluster.h

123 ↗

(On Diff #94559)

Return

lib/Analysis/InlineCost.cpp

1005–1006

I think we can exit early if the number of cases is too large.

1082

If the estimation chooses to use jumptable, I think we also need to add the cost of the table which is proportional to the range.

lib/CodeGen/SelectionDAG/SwitchCaseCluster.cpp

86 ↗

(On Diff #94559)

We can start from begin()+1

test/Transforms/Inline/switch.ll

3 ↗

(On Diff #94559)

We may need to add tests for jump table and bit test.

[junbuml]

lib/Analysis/InlineCost.cpp

1005–1006

In SwitchCaseClusterFinder::getEstimatedNumberOfCluster(), we have early exit for a large number of cases. But I guess you mean something else. Can you specify little bit more about the "too large".

1082

I'm not sure if we really need to consider the size of table as a cost. I think just couple of instructions to look up the table and jump to actual blocks need to be considered as cost.

lib/CodeGen/SelectionDAG/SwitchCaseCluster.cpp

86 ↗

(On Diff #94559)

Thanks. I will do that.

test/Transforms/Inline/switch.ll

3 ↗

(On Diff #94559)

Yes. I will do that.

[junbuml]

lib/Analysis/InlineCost.cpp

1082

You are right Haicheng. Looks like we need to consider the cost of the table as well.

[haicheng]

lib/Analysis/InlineCost.cpp

1005–1006

One case needs at least one instruction. So if cost + numcases * instrcost > threshold, we can exit early.

[chandlerc]

In D31085#715294, @junbuml wrote:

Chandler, do you agree with the heuristic Hans suggested above? Even though it do not cover switches that are lowered with a mix of jump table/bit test/BTree, I think this is reasonable compromise between accuracy and cost of the hook.

Yes, I like this model.

lib/CodeGen/SelectionDAG/SwitchCaseCluster.cpp

68–69 ↗

(On Diff #94559)

inlining and loop unrolling. it's a generic cost model.

[chandlerc]

This really looks like it is going in the right direction. I'm going to work on reviewing some of teh code changes a bit more closely, but I wanted to mention one other thing.

This seems like a really good change to the inlining cost model, but it also seems likely to be a pretty big change. I think it is important to collect some benchmark data to make sure we're not going to uncover a significant regression by surprise. At the very least, I think running the LLVM test suite would be a good start and identifying:

1. How many benchmarks change
2. For the ones that change, what is the codesize impact
3. For the ones that change, what is the runtime impact

For #2 and #3 you probably want at least '-O2', but maybe also '-O3' and '-Os'.

It may be useful to ask others to benchmark other applications and/or various architectures as well. To facilitate that, I might suggest putting the code for this in under a flag that is off and then soliciting benchmark data on llvm-dev with the flag, and based on that data, enable the flag. But if this doesn't fire too often in the test suite, or the results are particularly good, might be easy to just try it and see.

Thanks again for working on this!

[junbuml]

Addressed Haicheng's comments and added a flag as Chandler asked. With this update, I kicked off performance tests for the llvm test suite, spec2000, spec2006 in aarch64, but I will be able to share organized data early next as I will be out of office rest of the week. Please let me know any comment.

[hans]

I was hoping we wouldn't need the refactoring to SwitchCaseCluster.cpp, and that InlineCost.cpp could just work with TLI to check whether jump tables are allowed, density requirements etc.

My concern is that separating the case cluster code from SelectionDAGBuilder might be more trouble than it's worth.

[junbuml]

I was hoping we wouldn't need the refactoring to SwitchCaseCluster.cpp, and that InlineCost.cpp could just work with TLI to check whether jump tables are allowed, density requirements etc.

Sure, I can make InlineCost work with just TLI, but I don't want to duplicate the same code for InlineCost from lowering. So I will refactor just a little bit on SelectionDAGBuilder to expose some util functions.

My concern is that separating the case cluster code from SelectionDAGBuilder might be more trouble than it's worth.

Can you give me little bit more details about your concern ?

[hans]

In D31085#728192, @junbuml wrote:

I was hoping we wouldn't need the refactoring to SwitchCaseCluster.cpp, and that InlineCost.cpp could just work with TLI to check whether jump tables are allowed, density requirements etc.

Sure, I can make InlineCost work with just TLI, but I don't want to duplicate the same code for InlineCost from lowering. So I will refactor just a little bit on SelectionDAGBuilder to expose some util functions.

That sounds good to me.

My concern is that separating the case cluster code from SelectionDAGBuilder might be more trouble than it's worth.

Can you give me little bit more details about your concern ?

The concern is just that the switch lowering code is fairly tightly integrated with SelectionDAGBuilder, and pulling it out isn't worth the effort if all we need from TLI is some basic info, like if the whole switch is dense enough for a lookup table.

[junbuml]

Addressed Hans' comments; now the switch cost heuristic just work with TLI.

[junbuml]

Please see list of benchmarks changed in LLVM test suite and spec2000/2006. No significant code size regression was found in any config. Overall minor positive impact on code size in LLVM test suite, but in O3 with LTO, there was -7.9% reduce in code size in spec2000/vortex.

In AArch64, I didn't see any clear performance impact in LLVM test suite, but in O3 with LTO, I observed +17.82% performance improvement in spec2000/vertex.

• O2 :

Benchmarks	Code size (- is better)
MultiSource/Applications/siod	-0.061%
MultiSource/Applications/hbd	0.000%
MultiSource/Applications/JM/lencod/lencod	-0.070%
MultiSource/Applications/JM/ldecod/ldecod	0.000%
MultiSource/Applications/lua/lua	0.001%
MultiSource/Applications/d/make_dparser	-0.081%
MultiSource/Applications/sqlite3/sqlite3	-0.217%
MultiSource/Benchmarks/Prolangs-C/bison/mybison	0.000%
MultiSource/Benchmarks/MiBench/consumer-typeset/consumer-typeset	0.000%
MultiSource/Benchmarks/MiBench/consumer-jpeg/consumer-jpeg	-0.082%
MultiSource/Benchmarks/mediabench/jpeg/jpeg-6a/cjpeg	0.000%
MultiSource/Benchmarks/MallocBench/gs/gs	0.000%
spec2000/perlbmk	-0.066%
spec2000/gcc	-0.183%
spec2000/parser	0.000%
spec2000/crafty	-0.080%
spec2000/mesa	-0.286%
spec2006/soplex	0.000%
spec2006/xalancbmk	0.000%
spec2006/hmmer	-0.072%
spec2006/gcc	-0.191%
spec2006/h264ref	0.000%
spec2006/povray	-0.057%
spec2006/perlbench	-0.060%

• Os :

Benchmarks	Code size(- is better)
MultiSource/Applications/siod/siod	-0.123%
MultiSource/Applications/JM/lencod/lencod	0.000%
MultiSource/Applications/lua/lua	0.000%
MultiSource/Applications/sqlite3/sqlite3	0.001%
MultiSource/Benchmarks/mafft/pairlocalalign	0.000%
MultiSource/Benchmarks/7zip/7zip-benchmark	0.001%
spec2000/perlbmk	0.000%
spec2000/gap	0.000%
spec2000/gcc	0.000%
spec2000/mesa	-0.071%
spec2000/vortex	0.000%
spec2006/gobmk	0.000%
spec2006/xalancbmk	0.000%
spec2006/hmmer	0.000%
spec2006/gcc	-0.048%
spec2006/omnetpp	0.000%
spec2006/h264ref	0.000%
spec2006/perlbench	-0.062%

• O3 :

Benchmarks	Code size(- is better)
MultiSource/Applications/kimwitu++/kc	0.000%
MultiSource/Applications/siod/siod	-0.061%
MultiSource/Applications/hbd/hbd	0.000%
MultiSource/Applications/JM/lencod/lencod	0.000%
MultiSource/Applications/lua/lua	0.001%
MultiSource/Applications/SIBsim4/SIBsim4	0.001%
MultiSource/Applications/d/make_dparser	0.000%
MultiSource/Applications/sqlite3/sqlite3	-0.212%
MultiSource/Benchmarks/mediabench/jpeg/jpeg-6a/cjpeg	0.000%
MultiSource/Benchmarks/MallocBench/gs/gs	0.000%
MultiSource/Benchmarks/7zip/7zip-benchmark	0.000%
spec2000/perlbmk	-0.066%
spec2000/gcc	-0.182%
spec2000/mesa	-0.356%
spec2000/vortex	0.000%
spec2006/soplex	0.000%
spec2006/xalancbmk	-0.019%
spec2006/bzip2	0.000%
spec2006/hmmer	0.000%
spec2006/gcc	-0.188%
spec2006/h264ref	0.000%
spec2006/povray	-0.056%
spec2006/perlbench	-0.121%

• Spec2000/2006 performance in O3 :

Benchmarks	Score(+ is better)
spec2000/perlbmk	+2.706%
spec2000/vortex	+2.028%
spec2000/mesa	-1.76%
spec2006/soplex	-1.334%
spec2006/povray	+1.509%
spec2006/perlbench	+0.757%

• Spec2000/2006 performance in O3 with LTO :

Benchmarks	Score(+ is better)
spec2000/gzip	+2.180%
spec2000/mesa	-4.093%
spec2000/vortex	+17.822%

[hans]

It's starting to look much simpler, which is great.

include/llvm/Analysis/TargetTransformInfo.h

773

The other function here return an estimated cost for lowering. I think that would be a better interface for this too.

include/llvm/CodeGen/BasicTTIImpl.h

175

I wish this could be much simpler. Maybe most of the code could defer to TLI::isSuitableForBitTest / isSuitableForJumpTable which could also be used from the DAG code.

225

If we have TLI::isSuitableForBitTests, maybe we should have isSuitableForJumpTable too, that way we don't have to duplicate as much logic, and that one could do what areJTsAllowed() as well.

lib/Analysis/InlineCost.cpp

1100

If the whole switch is suitable for a jump table, can't we just return after this?

1102

Why do we have to do this?

The code is basically constructing the tree and throwing it away. It should be possible to compute an estimate for the size of the tree with a closed-form mathematical expression.

[haicheng]

lib/Analysis/InlineCost.cpp

1102

If n is the case number, f(n) is the mapping from the case number to the node number of BTree. The recursion is
f(n) = 1 + f(n/2) + f (n - n/2), when n > 3.
So, f(n) is between n + 2^(log2(n) - 1) - 1 and n + 2^(log2(n)) - 1. The lower bound is about 1.5n - 1 and the upper bound is about 2n - 1

[haicheng]

lib/Analysis/InlineCost.cpp

1102

The exact equation is

f(n) = n, n <= 3
f(n) = n + 2^(log(n) - 1) - 1, n > 3 && 2^log(n) <= n <= 1.5*2^(log(n))
f(n) = 2n - 2^(log(n)) - 1, n > 3 && 1.5*2^(log(n)) < n < 2^(log(n)+1)

[junbuml]

include/llvm/Analysis/TargetTransformInfo.h

773

I doubt if this is good place to get the inline cost because other functions handle user costs which is different from inline cost. Mixing the user cost and inline cost here might be a bad choice. I believe the inline cost should be decided in InlineCost.

include/llvm/CodeGen/BasicTTIImpl.h

175

Tried to make it simpler, but we still need to find Min/MaxValue here without forming CaseClusters and it's also good to check IsJTAllowed early before doing the actual suitability check to avoid iterating the for loop to find Min/MaxValue in case not allowed. Please take a look and let me know if there is any part you want to move in either isSuitableForBitTest or isSuitableForJumpTable.

225

Added isSuitableForJumpTable in TLI which used in here and DAG, but keep areJTsAllowed() as a separate function because areJTsAllowed() need to be checked only once in findJumpTable() in DAG, and we can also hit the early exit in this function when JT is not allowed.

lib/Analysis/InlineCost.cpp

1102

Thanks Haicheng for this.

If n is a power of 2, the number of node should be n + 2^(log2(n) - 1) - 1. For non-power of 2 cases , the lower bound is n + 2^(floor(log2(n)) - 1) - 1 and the upper bound is n + 2^(ceiling(log2(n)) - 1) - 1. As a estimation, I think the use of upper bound is simple and conservative enough.

[hans]

include/llvm/Analysis/TargetTransformInfo.h

773

I was thinking it doesn't need to be used as the inlining cost directly, but exposing some metric the inline cost model could then make use of - that's kind of what we're doing anyway.

What I don't like about this one is that it feels a little out of place compared to other functions which return an estimate of the cost of lowering an instruction, whereas this seems to return details about the lowering instead.

I'm not sure how to make this better though.

include/llvm/Target/TargetLowering.h

766

NumCluster -> NumClusters please

lib/Analysis/InlineCost.cpp

1035

I'm not sure it's a BTree, but rather a regular binary search tree.

1048

Very nice! I think the code can be simplified though, and this will allow us to avoid the rounding from Log2_64_Ceil: (N = NumCaseCluster)

NumberOfNonLeaf = 2^(log2(N) - 1) - 1 = 2^log2(N) * 2^-1 - 1 = N/2 - 1
Adding NumCaseCluster to that yields N + N / 2 - 1 = N * 3 / 2 - 1.

I kind of wish this is all there was to this change and we didn't need all that other code: quick check if the switch is suitable for a bit test or jump table, otherwise compute a cost based on N * 3 / 2 - 1. I don't know if it's possible though.

1053

Can we really run into INT_MAX here? Especially given the Threshold check above?

1055

I would suggest handling the NumCaseCluster <= 3 first and returning early for that, since it's the less complicated case.

[junbuml]

Addressed Hans' comments.

include/llvm/Analysis/TargetTransformInfo.h

773

I see what you meant and I can see functions which return the lowering cost. However, this file also hold functions which expose details about the machine directly to be used in IR-level. For me it seems that exposing this information from TLI to InlineCost still keep the original intention of this class. I will be happy to hear any better suggestion about the way of exposing this lowering information.

lib/Analysis/InlineCost.cpp

1048

Thanks Hans for this. Updated it based your comments.

I think it's difficult to use a closed form which cover all lowering cases, especially in different targets. I believe we need to differentiate each lowering case as the costs varying depending on the lowering cases and targets.

1053

In very extreme case, it could happen. For a very large Threshold (e.g., INT_MAX), even though SI.getNumCases() * InlineConstants::InstrCost is smaller than the Threshold. ExpectedNumberOfCompare * 2 * InlineConstants::InstrCost could hit INT_MAX.

[hans]

Apologies for the reviews dragging out.

I think we've got the right idea with computing inline cost based on an estimate of the number of nodes in binary search tree, but the patch is still making a lot of changes all over the place making it hard to review.

The smaller you can make this change, the easier it will be to get this committed.

include/llvm/Target/TargetLowering.h

771

This check should probably be done in isSuitableForJumpTable. There should be no need to pass NumClusters to areJTsAllowed since whether jump tables are allowed only depends on the target and function. The switch instruction also shouldn't be passed in, just the function.

779

This is used in isSuitableForBitTests, but does it really need to be exposed in the TargetLowering interface?

792

What's the difference between Range and JumpTableSize? As far as I can tell, those are always the same.

lib/Analysis/InlineCost.cpp

1057

Aren't you adding Cost twice here? You're doing Cost += and also SwitchCost + Cost.

Oh, Cost is just a regular int; yeah then I see how it can overflow. But std::min is returnning an uint64_t here, so it seems you're still not handling the overflow?

[hans]

lib/Analysis/InlineCost.cpp

1057

Oh never mind, you're capping the uint64_t at INT_MAX so it shuold work. Probably still don't want to do Cost += though.

[junbuml]

I think we've got the right idea with computing inline cost based on an estimate of the number of nodes in binary search tree, but the patch is still making a lot of changes all over the place making it hard to review.
The smaller you can make this change, the easier it will be to get this committed.

For me, it seems that the code change in DAG and TLI only make sense when reviewed together with the changes in InlineCost. That's why I put them together. If you generally agree with the change in DAG side code, I can break it as a separate patch, and leave only inliner side change in here.

include/llvm/Target/TargetLowering.h

771

Yes, I agree that areJTsAllowed() should see only function and target, so I just pass a Function to areJTsAllowed(). But I don't think isSuitableForJumpTable is good place for this check because this check is for the whole clusters of a switch, but isSuitableForJumpTable should see a set of clusters, not necessary the whole clusters, especially when we try to build jump tables for split clusters in DAG. We also need to do an early exit when this check hit. So I do this check in findJumpTables() in DAG and getEstimatedNumberOfCaseClusters() in TTI.

779

This is also used in findBitTestClusters and buildBitTests(), and for me doing this check through TLI doesn't seem to be weird. Please let me know if you see any better place for this function.

792

Yes, these are the same.

lib/Analysis/InlineCost.cpp

1057

Thanks for this. Yes, it should be Cost =, instead of Cost +=.

[hans]

In D31085#737231, @junbuml wrote:

I think we've got the right idea with computing inline cost based on an estimate of the number of nodes in binary search tree, but the patch is still making a lot of changes all over the place making it hard to review.
The smaller you can make this change, the easier it will be to get this committed.

For me, it seems that the code change in DAG and TLI only make sense when reviewed together with the changes in InlineCost. That's why I put them together. If you generally agree with the change in DAG side code, I can break it as a separate patch, and leave only inliner side change in here.

Yes, reviewing them together makes total sense. I just wish the change we're making were smaller.

Anyway, I think this is really close now. Each time I read through it, it looks better :-)

include/llvm/CodeGen/BasicTTIImpl.h

194

Declaring CI and starting to increment it outside the for-loop is a little unusual. I realize this is to avoid repeating the first iteration (maybe I wrote this somewhere?), but I think it would be better if this were written as a straight-forward loop:

APInt MaxCaseVal = SI.case_begin()->getCaseValue()->getValue();
APInt MinCaseVal = SI.case_begin()->getCaseValue()->getValue();
for (auto CI : SI.cases()) {
  ...
}

include/llvm/Target/TargetLowering.h

771

isSuitableForJumpTable() seems like a good place for this to me. It should be able to handle the whole switch range or a subset in the same way.

lib/Analysis/InlineCost.cpp

1018

Could this overflow?

1057

Cool, makes sense now. Are the (uint64_t) casts strictly necessary though? SwitchCost is already uint64_t so I'd imagine INT_MAX to get promoted and everything to work out?

[junbuml]

include/llvm/CodeGen/BasicTTIImpl.h

194

Yes, I like this. I will do that.

include/llvm/Target/TargetLowering.h

771

If we use range for this check in isSuitableForJumpTable(), it will change the behavior in lowering. Current lowering code use the total number of cluster (N) for this check only once at the beginning of findJumpTables(), which is different from both Range and NumCases. And also when partitioning clusters for jump tables, doing this check in isSuitableForJumpTable() may change the behavior of the loop finding partitions. If either Range or NumCases should be used for this check, I think it should be a separate patch.

[hans]

include/llvm/Target/TargetLowering.h

771

Oh, I see what you mean.

It's annoying that we'll end up with a isSuitableForJumpTable() that doesn't take the minimum size into account though :-/

[junbuml]

lib/Analysis/InlineCost.cpp

1057

We need (uint64_t)INT_MAX, but we don't need (uint64_t) Cost as SwitchCost is uint64_t.

[hans]

include/llvm/Target/TargetLowering.h

787

It still seems backward that this is checking density and maximum table size, but that the minimum size is expected to be checked elsewhere :-/ There must be some better way to factor this.

[junbuml]

include/llvm/Target/TargetLowering.h

787

I agree that doing minSize check in isSuitableForJumpTable looks clear. However, I want to keep the changes in lowering side in this patch is NFC. That's why we cannot do this check in isSuitableForJumpTable().

If you want to add MinSize check in isSuitableForJumpTable(), I think we should also pass the cluster size (N) to isSuitableForJumpTable() in findJumpTable() because Range and NumCases could be different from the cluster size(N).

Only way I can think of doing this check in isSuitableForJumpTable() is :

1. pass the cluster size (N) to isSuitableForJumpTable() for the first call of isSuitableForJumpTable() in findJumpTable() because this is for the whole range.
2. Since the second calls of isSuitableForJumpTable() in findJumpTable for partitionized clusters should not check MinSize with sub clusters, we do not use isSuitableForJumpTable(), instead checking the max size and density separately just like current code.

However, I think this way is even worse than my current code. Please let me know your thought.

[hans]

lgtm

include/llvm/Target/TargetLowering.h

787

Let's put a FIXME in isSuitableForJumpTable that it would be nice if the min size check could somehow be combined with the other checks here.

[junbuml]

Added FIXME as Hans suggested.
I will commit it if there is any further comment from other reviewers by tomorrow. I will also post a follow-up patch to enable the flag (-inline-generic-switch-cost).

Thank you very much for the review.

include/llvm/Analysis/TargetTransformInfo.h

int getIntrinsicCost(Intrinsic::ID IID, Type *RetTy,
                        ArrayRef<Type *> ParamTys) const;
 
   /// \brief Estimate the cost of an intrinsic when lowered.
   ///
   /// Mirrors the \c getCallCost method but uses an intrinsic identifier.
   int getIntrinsicCost(Intrinsic::ID IID, Type *RetTy,
                        ArrayRef<const Value *> Arguments) const;
 
+  /// \return The estimated number of case clusters when lowering \p 'SI'.
+  /// \p JTSize Set a jump table size only when \p SI is suitable for a jump
+  /// table.
+  unsigned getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
+                                            unsigned &JTSize) const;
+
   /// \brief Estimate the cost of a given IR user when lowered.
   ///
   /// This can estimate the cost of either a ConstantExpr or Instruction when
   /// lowered. It has two primary advantages over the \c getOperationCost and
   /// \c getGEPCost above, and one significant disadvantage: it can only be
   /// used when the IR construct has already been formed.
   ///
   /// The advantages are that it can inspect the SSA use graph to reason more
public:
   virtual int getCallCost(const Function *F, int NumArgs) = 0;
   virtual int getCallCost(const Function *F,
                           ArrayRef<const Value *> Arguments) = 0;
   virtual unsigned getInliningThresholdMultiplier() = 0;
   virtual int getIntrinsicCost(Intrinsic::ID IID, Type *RetTy,
                                ArrayRef<Type *> ParamTys) = 0;
   virtual int getIntrinsicCost(Intrinsic::ID IID, Type *RetTy,
                                ArrayRef<const Value *> Arguments) = 0;
+  virtual unsigned getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
+                                                    unsigned &JTSize) = 0;
   virtual int getUserCost(const User *U) = 0;
   virtual bool hasBranchDivergence() = 0;
   virtual bool isSourceOfDivergence(const Value *V) = 0;
   virtual unsigned getFlatAddressSpace() = 0;
   virtual bool isLoweredToCall(const Function *F) = 0;
   virtual void getUnrollingPreferences(Loop *L, UnrollingPreferences &UP) = 0;
   virtual bool isLegalAddImmediate(int64_t Imm) = 0;
   virtual bool isLegalICmpImmediate(int64_t Imm) = 0;
unsigned getMinPrefetchStride() override {
     return Impl.getMinPrefetchStride();
   }
   unsigned getMaxPrefetchIterationsAhead() override {
     return Impl.getMaxPrefetchIterationsAhead();
   }
   unsigned getMaxInterleaveFactor(unsigned VF) override {
     return Impl.getMaxInterleaveFactor(VF);
   }
+  unsigned getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
+                                            unsigned &JTSize) override {
+    return Impl.getEstimatedNumberOfCaseClusters(SI, JTSize);
+  }
   unsigned
   getArithmeticInstrCost(unsigned Opcode, Type *Ty, OperandValueKind Opd1Info,
                          OperandValueKind Opd2Info,
                          OperandValueProperties Opd1PropInfo,
                          OperandValueProperties Opd2PropInfo,
                          ArrayRef<const Value *> Args) override {
     return Impl.getArithmeticInstrCost(Opcode, Ty, Opd1Info, Opd2Info,
                                        Opd1PropInfo, Opd2PropInfo, Args);

include/llvm/Analysis/TargetTransformInfoImpl.h

int getGEPCost(Type *PointeeType, const Value *Ptr,
     // into their uses via addressing modes.
     for (unsigned Idx = 0, Size = Operands.size(); Idx != Size; ++Idx)
       if (!isa<Constant>(Operands[Idx]))
         return TTI::TCC_Basic;
 
     return TTI::TCC_Free;
   }
 
+  unsigned getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
+                                            unsigned &JTSize) {
+    JTSize = 0;
+    return SI.getNumCases();
+  }
+
   unsigned getCallCost(FunctionType *FTy, int NumArgs) {
     assert(FTy && "FunctionType must be provided to this routine.");
 
     // The target-independent implementation just measures the size of the
     // function by approximating that each argument will take on average one
     // instruction to prepare.
 
     if (NumArgs < 0)

include/llvm/CodeGen/BasicTTIImpl.h

if (IID == Intrinsic::ctlz) {
       if (getTLI()->isCheapToSpeculateCtlz())
         return TargetTransformInfo::TCC_Basic;
       return TargetTransformInfo::TCC_Expensive;
     }
 
     return BaseT::getIntrinsicCost(IID, RetTy, ParamTys);
   }
 
+  unsigned getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
+                                            unsigned &JumpTableSize) {
+    /// Try to find the estimated number of clusters. Note that the number of
+    /// clusters identified in this function could be different from the actural
+    /// numbers found in lowering. This function ignore switches that are
+    /// lowered with a mix of jump table / bit test / BTree. This function was
+    /// initially intended to be used when estimating the cost of switch in
+    /// inline cost heuristic, but it's a generic cost model to be used in other
+    /// places (e.g., in loop unrolling).
+    unsigned N = SI.getNumCases();
+    const TargetLoweringBase *TLI = getTLI();
+    const DataLayout &DL = this->getDataLayout();
+
+    JumpTableSize = 0;
+    bool IsJTAllowed = TLI->areJTsAllowed(SI.getParent()->getParent());
+
+    // Early exit if both a jump table and bit test are not allowed.
+    if (N < 1 || (!IsJTAllowed && DL.getPointerSizeInBits() < N))
+      return N;
+
+    APInt MaxCaseVal = SI.case_begin()->getCaseValue()->getValue();
+    APInt MinCaseVal = MaxCaseVal;
+    for (auto CI : SI.cases()) {
+      const APInt &CaseVal = CI.getCaseValue()->getValue();
+      if (CaseVal.sgt(MaxCaseVal))
+        MaxCaseVal = CaseVal;
+      if (CaseVal.slt(MinCaseVal))
+        MinCaseVal = CaseVal;
+    }
+
+    // Check if suitable for a bit test
+    if (N <= DL.getPointerSizeInBits()) {
+      SmallPtrSet<const BasicBlock *, 4> Dests;
+      for (auto I : SI.cases())
+        Dests.insert(I.getCaseSuccessor());
+
+      if (TLI->isSuitableForBitTests(Dests.size(), N, MinCaseVal, MaxCaseVal,
+                                     DL))
+        return 1;
+    }
+
+    // Check if suitable for a jump table.
+    if (IsJTAllowed) {
+      if (N < 2 || N < TLI->getMinimumJumpTableEntries())
+        return N;
+      uint64_t Range =
+          (MaxCaseVal - MinCaseVal).getLimitedValue(UINT64_MAX - 1) + 1;
+      // Check whether a range of clusters is dense enough for a jump table
+      if (TLI->isSuitableForJumpTable(&SI, N, Range)) {
+        JumpTableSize = Range;
+        return 1;
+      }
+    }
+    return N;
+  }
+
   unsigned getJumpBufAlignment() { return getTLI()->getJumpBufAlignment(); }
 
   unsigned getJumpBufSize() { return getTLI()->getJumpBufSize(); }
 
   bool shouldBuildLookupTables() {
     const TargetLoweringBase *TLI = getTLI();
     return TLI->isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
            TLI->isOperationLegalOrCustom(ISD::BRIND, MVT::Other);

include/llvm/Target/TargetLowering.h

public:
 
   /// Return true if the specified operation is illegal but has a custom lowering
   /// on that type. This is used to help guide high-level lowering
   /// decisions.
   bool isOperationCustom(unsigned Op, EVT VT) const {
     return (!isTypeLegal(VT) && getOperationAction(Op, VT) == Custom);
   }
 
+  /// Return true if lowering to a jump table is allowed.
+  bool areJTsAllowed(const Function *Fn) const {
+    if (Fn->getFnAttribute("no-jump-tables").getValueAsString() == "true")
+      return false;
+
+    return isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
+           isOperationLegalOrCustom(ISD::BRIND, MVT::Other);
+  }
+
+  /// Check whether the range [Low,High] fits in a machine word.
+  bool rangeFitsInWord(const APInt &Low, const APInt &High,
+                       const DataLayout &DL) const {
+    // FIXME: Using the pointer type doesn't seem ideal.
+    uint64_t BW = DL.getPointerSizeInBits();
+    uint64_t Range = (High - Low).getLimitedValue(UINT64_MAX - 1) + 1;
+    return Range <= BW;
+  }
+
+  /// Return true if lowering to a jump table is suitable for a set of case
+  /// clusters which may contain \p NumCases cases, \p Range range of values.
+  /// Check if the table size and density is suitable.
+  bool isSuitableForJumpTable(const SwitchInst *SI, uint64_t NumCases,
+                              uint64_t Range) const {
+    const bool OptForSize = SI->getParent()->getParent()->optForSize();
+    const unsigned MinDensity = getMinimumJumpTableDensity(OptForSize);
+    const unsigned MaxJumpTableSize =
+        OptForSize || getMaximumJumpTableSize() == 0
+            ? UINT_MAX
+            : getMaximumJumpTableSize();
+    // Check whether a range of clusters is dense enough for a jump table.
+    if (Range <= MaxJumpTableSize &&
+        (NumCases * 100 >= Range * MinDensity)) {
+      return true;
+    }
+    return false;
+  }
+
+  /// Return true if lowering to a bit test is suitable for a set of case
+  /// clusters which contains \p NumDests unique destinations, \p Low and
+  /// \p High as its lowest and highest case values, and expects \p NumCmps
+  /// case value comparisons. Check if the number of destinations, comparison
+  /// metric, and range are all suitable.
+  bool isSuitableForBitTests(unsigned NumDests, unsigned NumCmps,
+                             const APInt &Low, const APInt &High,
+                             const DataLayout &DL) const {
+    // FIXME: I don't think NumCmps is the correct metric: a single case and a
+    // range of cases both require only one branch to lower. Just looking at the
+    // number of clusters and destinations should be enough to decide whether to
+    // build bit tests.
+
+    // To lower a range with bit tests, the range must fit the bitwidth of a
+    // machine word.
+    if (!rangeFitsInWord(Low, High, DL))
+      return false;
+
+    // Decide whether it's profitable to lower this range with bit tests. Each
+    // destination requires a bit test and branch, and there is an overall range
+    // check branch. For a small number of clusters, separate comparisons might
+    // be cheaper, and for many destinations, splitting the range might be
+    // better.
+    return (NumDests == 1 && NumCmps >= 3) || (NumDests == 2 && NumCmps >= 5) ||
+           (NumDests == 3 && NumCmps >= 6);
+  }
+
   /// Return true if the specified operation is illegal on this target or
   /// unlikely to be made legal with custom lowering. This is used to help guide
   /// high-level lowering decisions.
   bool isOperationExpand(unsigned Op, EVT VT) const {
     return (!isTypeLegal(VT) || getOperationAction(Op, VT) == Expand);
   }
 
   /// Return true if the specified operation is legal on this target.
public:
   /// Determine if we should use _longjmp or longjmp to implement llvm.longjmp.
   bool usesUnderscoreLongJmp() const {
     return UseUnderscoreLongJmp;
   }
 
   /// Return lower limit for number of blocks in a jump table.
   unsigned getMinimumJumpTableEntries() const;
 
+  /// Return lower limit of the density in a jump table.
+  unsigned getMinimumJumpTableDensity(bool OptForSize) const;
+
   /// Return upper limit for number of entries in a jump table.
   /// Zero if no limit.
   unsigned getMaximumJumpTableSize() const;
 
   virtual bool isJumpTableRelative() const {
     return TM.isPositionIndependent();
   }
 

lib/Analysis/InlineCost.cpp

static cl::opt<int> HintThreshold(
     "inlinehint-threshold", cl::Hidden, cl::init(325),
     cl::desc("Threshold for inlining functions with inline hint"));
 
 static cl::opt<int>
     ColdCallSiteThreshold("inline-cold-callsite-threshold", cl::Hidden,
                           cl::init(45),
                           cl::desc("Threshold for inlining cold callsites"));
 
+static cl::opt<bool>
+    EnableGenericSwitchCost("inline-generic-switch-cost", cl::Hidden,
+                            cl::init(false),
+                            cl::desc("Enable generic switch cost model"));
+
 // We introduce this threshold to help performance of instrumentation based
 // PGO before we actually hook up inliner with analysis passes such as BPI and
 // BFI.
 static cl::opt<int> ColdThreshold(
     "inlinecold-threshold", cl::Hidden, cl::init(225),
     cl::desc("Threshold for inlining functions with cold attribute"));
 
 static cl::opt<int>

 bool CallAnalyzer::visitSwitchInst(SwitchInst &SI) {
   // We model unconditional switches as free, see the comments on handling
   // branches.
   if (isa<ConstantInt>(SI.getCondition()))
     return true;
   if (Value *V = SimplifiedValues.lookup(SI.getCondition()))
     if (isa<ConstantInt>(V))
       return true;
 
-  // Otherwise, we need to accumulate a cost proportional to the number of
-  // distinct successor blocks. This fan-out in the CFG cannot be represented
-  // for free even if we can represent the core switch as a jumptable that
-  // takes a single instruction.
-  //
+  if (EnableGenericSwitchCost) {
+    // Assume the most general case where the swith is lowered into
+    // either a jump table, bit test, or a balanced binary tree consisting of
+    // case clusters without merging adjacent clusters with the same
+    // destination. We do not consider the switches that are lowered with a mix
+    // of jump table/bit test/binary search tree. The cost of the switch is
+    // proportional to the size of the tree or the size of jump table range.
+
+    // Exit early for a large switch, assuming one case needs at least one
+    // instruction.
+    // FIXME: This is not true for a bit test, but ignore such case for now to
+    // save compile-time.
+    int64_t CostLowerBound =
+        std::min((int64_t)INT_MAX,
+                 (int64_t)SI.getNumCases() * InlineConstants::InstrCost + Cost);
+
+    if (CostLowerBound > Threshold) {
+      Cost = CostLowerBound;
+      return false;
+    }
+
+    unsigned JumpTableSize = 0;
+    unsigned NumCaseCluster =
+        TTI.getEstimatedNumberOfCaseClusters(SI, JumpTableSize);
+
+    // If suitable for a jump table, consider the cost for the table size and
+    // branch to destination.
+    if (JumpTableSize) {
+      int64_t JTCost = (int64_t)JumpTableSize * InlineConstants::InstrCost +
+                       4 * InlineConstants::InstrCost;
+      Cost = std::min((int64_t)INT_MAX, JTCost + Cost);
+      return false;
+    }
+
+    // Considering forming a binary search, we should find the number of nodes
+    // which is same as the number of comparisons when lowered. For a given
+    // number of clusters, n, we can define a recursive function, f(n), to find
+    // the number of nodes in the tree. The recursion is :
+    // f(n) = 1 + f(n/2) + f (n - n/2), when n > 3,
+    // and f(n) = n, when n <= 3.
+    // This will lead a binary tree where the leaf should be either f(2) or f(3)
+    // when n > 3.  So, the number of comparisons from leaves should be n, while
+    // the number of non-leaf should be :
+    //   2^(log2(n) - 1) - 1
+    //   = 2^log2(n) * 2^-1 - 1
+    //   = n / 2 - 1.
+    // Considering comparisons from leaf and non-leaf nodes, we can estimate the
+    // number of comparisons in a simple closed form :
+    //   n + n / 2 - 1 = n * 3 / 2 - 1
+    if (NumCaseCluster <= 3) {
+      // Suppose a comparison includes one compare and one conditional branch.
+      Cost += NumCaseCluster * 2 * InlineConstants::InstrCost;
+      return false;
+    }
+    int64_t ExpectedNumberOfCompare = 3 * (uint64_t)NumCaseCluster / 2 - 1;
+    uint64_t SwitchCost =
+        ExpectedNumberOfCompare * 2 * InlineConstants::InstrCost;
+    Cost = std::min((uint64_t)INT_MAX, SwitchCost + Cost);
+    return false;
+  }
+
+  // Use a simple switch cost model where we accumulate a cost proportional to
+  // the number of distinct successor blocks. This fan-out in the CFG cannot
+  // be represented for free even if we can represent the core switch as a
+  // jumptable that takes a single instruction.
+  ///
   // NB: We convert large switches which are just used to initialize large phi
   // nodes to lookup tables instead in simplify-cfg, so this shouldn't prevent
   // inlining those. It will prevent inlining in cases where the optimization
   // does not (yet) fire.
   SmallPtrSet<BasicBlock *, 8> SuccessorBlocks;
   SuccessorBlocks.insert(SI.getDefaultDest());
   for (auto Case : SI.cases())
     SuccessorBlocks.insert(Case.getCaseSuccessor());
   // Add cost corresponding to the number of distinct destinations. The first
   // we model as free because of fallthrough.
   Cost += (SuccessorBlocks.size() - 1) * InlineConstants::InstrCost;
   return false;
 }
 
 bool CallAnalyzer::visitIndirectBrInst(IndirectBrInst &IBI) {
   // We never want to inline functions that contain an indirectbr.  This is
   // incorrect because all the blockaddress's (in static global initializers
   // for example) would be referring to the original function, and this
   // indirect jump would jump from the inlined copy of the function into the
   // original function which is extremely undefined behavior.
   // FIXME: This logic isn't really right; we can safely inline functions with
   // indirectbr's as long as no other function or global references the
   // blockaddress of a block within the current function.
   HasIndirectBr = true;
   return false;
 }
 
 bool CallAnalyzer::visitResumeInst(ResumeInst &RI) {
   // FIXME: It's not clear that a single instruction is an accurate model for
   // the inline cost of a resume instruction.
   return false;
 }
 
 bool CallAnalyzer::visitCleanupReturnInst(CleanupReturnInst &CRI) {
   // FIXME: It's not clear that a single instruction is an accurate model for
   // the inline cost of a cleanupret instruction.
   return false;
 }
 

lib/Analysis/TargetTransformInfo.cpp

 
 int TargetTransformInfo::getIntrinsicCost(
     Intrinsic::ID IID, Type *RetTy, ArrayRef<const Value *> Arguments) const {
   int Cost = TTIImpl->getIntrinsicCost(IID, RetTy, Arguments);
   assert(Cost >= 0 && "TTI should not produce negative costs!");
   return Cost;
 }
 
+unsigned
+TargetTransformInfo::getEstimatedNumberOfCaseClusters(const SwitchInst &SI,
+                                                      unsigned &JTSize) const {
+  return TTIImpl->getEstimatedNumberOfCaseClusters(SI, JTSize);
+}
+
 int TargetTransformInfo::getUserCost(const User *U) const {
   int Cost = TTIImpl->getUserCost(U);
   assert(Cost >= 0 && "TTI should not produce negative costs!");
   return Cost;
 }
 
 bool TargetTransformInfo::hasBranchDivergence() const {
   return TTIImpl->hasBranchDivergence();

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h

struct BitTestBlock {
     bool ContiguousRange;
     MachineBasicBlock *Parent;
     MachineBasicBlock *Default;
     BitTestInfo Cases;
     BranchProbability Prob;
     BranchProbability DefaultProb;
   };
 
-  /// Check whether a range of clusters is dense enough for a jump table.
-  bool isDense(const CaseClusterVector &Clusters,
-               const SmallVectorImpl<unsigned> &TotalCases,
-               unsigned First, unsigned Last, unsigned MinDensity) const;
+  /// Return the range of value in [First..Last].
+  uint64_t getJumpTableRange(const CaseClusterVector &Clusters, unsigned First,
+                             unsigned Last) const;
+
+  /// Return the number of cases in [First..Last].
+  uint64_t getJumpTableNumCases(const SmallVectorImpl<unsigned> &TotalCases,
+                                unsigned First, unsigned Last) const;
 
   /// Build a jump table cluster from Clusters[First..Last]. Returns false if it
   /// decides it's not a good idea.
   bool buildJumpTable(const CaseClusterVector &Clusters, unsigned First,
                       unsigned Last, const SwitchInst *SI,
                       MachineBasicBlock *DefaultMBB, CaseCluster &JTCluster);
 
   /// Find clusters of cases suitable for jump table lowering.
   void findJumpTables(CaseClusterVector &Clusters, const SwitchInst *SI,
                       MachineBasicBlock *DefaultMBB);
 
-  /// Check whether the range [Low,High] fits in a machine word.
-  bool rangeFitsInWord(const APInt &Low, const APInt &High);
-
-  /// Check whether these clusters are suitable for lowering with bit tests based
-  /// on the number of destinations, comparison metric, and range.
-  bool isSuitableForBitTests(unsigned NumDests, unsigned NumCmps,
-                             const APInt &Low, const APInt &High);
-
   /// Build a bit test cluster from Clusters[First..Last]. Returns false if it
   /// decides it's not a good idea.
   bool buildBitTests(CaseClusterVector &Clusters, unsigned First, unsigned Last,
                      const SwitchInst *SI, CaseCluster &BTCluster);
 
   /// Find clusters of cases suitable for bit test lowering.
   void findBitTestClusters(CaseClusterVector &Clusters, const SwitchInst *SI);
 

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

 static unsigned LimitFloatPrecision;
 
 static cl::opt<unsigned, true>
 LimitFPPrecision("limit-float-precision",
                  cl::desc("Generate low-precision inline sequences "
                           "for some float libcalls"),
                  cl::location(LimitFloatPrecision),
                  cl::init(0));
-
-/// Minimum jump table density for normal functions.
-static cl::opt<unsigned>
-JumpTableDensity("jump-table-density", cl::init(10), cl::Hidden,
-                 cl::desc("Minimum density for building a jump table in "
-                          "a normal function"));
-
-/// Minimum jump table density for -Os or -Oz functions.
-static cl::opt<unsigned>
-OptsizeJumpTableDensity("optsize-jump-table-density", cl::init(40), cl::Hidden,
-                        cl::desc("Minimum density for building a jump table in "
-                                 "an optsize function"));
-
-
 // Limit the width of DAG chains. This is important in general to prevent
 // DAG-based analysis from blowing up. For example, alias analysis and
 // load clustering may not complete in reasonable time. It is difficult to
 // recognize and avoid this situation within each individual analysis, and
 // future analyses are likely to have the same behavior. Limiting DAG width is
 // the safe approach and will be especially important with global DAGs.
 //
 // MaxParallelChains default is arbitrarily high to avoid affecting

 void SelectionDAGBuilder::updateDAGForMaybeTailCall(SDValue MaybeTC) {
   // If the node is null, we do have a tail call.
   if (MaybeTC.getNode() != nullptr)
     DAG.setRoot(MaybeTC);
   else
     HasTailCall = true;
 }
 
-bool SelectionDAGBuilder::isDense(const CaseClusterVector &Clusters,
-                                  const SmallVectorImpl<unsigned> &TotalCases,
-                                  unsigned First, unsigned Last,
-                                  unsigned Density) const {
+uint64_t
+SelectionDAGBuilder::getJumpTableRange(const CaseClusterVector &Clusters,
+                                       unsigned First, unsigned Last) const {
   assert(Last >= First);
-  assert(TotalCases[Last] >= TotalCases[First]);
-
   const APInt &LowCase = Clusters[First].Low->getValue();
   const APInt &HighCase = Clusters[Last].High->getValue();
   assert(LowCase.getBitWidth() == HighCase.getBitWidth());
 
   // FIXME: A range of consecutive cases has 100% density, but only requires one
   // comparison to lower. We should discriminate against such consecutive ranges
   // in jump tables.
 
-  uint64_t Diff = (HighCase - LowCase).getLimitedValue((UINT64_MAX - 1) / 100);
-  uint64_t Range = Diff + 1;
+  return (HighCase - LowCase).getLimitedValue((UINT64_MAX - 1) / 100) + 1;
+}
 
+uint64_t SelectionDAGBuilder::getJumpTableNumCases(
+    const SmallVectorImpl<unsigned> &TotalCases, unsigned First,
+    unsigned Last) const {
+  assert(Last >= First);
+  assert(TotalCases[Last] >= TotalCases[First]);
   uint64_t NumCases =
       TotalCases[Last] - (First == 0 ? 0 : TotalCases[First - 1]);
-
-  assert(NumCases < UINT64_MAX / 100);
-  assert(Range >= NumCases);
-
-  return NumCases * 100 >= Range * Density;
-}
-
-static inline bool areJTsAllowed(const TargetLowering &TLI,
-                                 const SwitchInst *SI) {
-  const Function *Fn = SI->getParent()->getParent();
-  if (Fn->getFnAttribute("no-jump-tables").getValueAsString() == "true")
-    return false;
-
-  return TLI.isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
-         TLI.isOperationLegalOrCustom(ISD::BRIND, MVT::Other);
+  return NumCases;
 }
 
 bool SelectionDAGBuilder::buildJumpTable(const CaseClusterVector &Clusters,
                                          unsigned First, unsigned Last,
                                          const SwitchInst *SI,
                                          MachineBasicBlock *DefaultMBB,
                                          CaseCluster &JTCluster) {
   assert(First <= Last);
if (I != First) {
         Table.push_back(DefaultMBB);
     }
     uint64_t ClusterSize = (High - Low).getLimitedValue() + 1;
     for (uint64_t J = 0; J < ClusterSize; ++J)
       Table.push_back(Clusters[I].MBB);
     JTProbs[Clusters[I].MBB] += Clusters[I].Prob;
   }
 
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   unsigned NumDests = JTProbs.size();
-  if (isSuitableForBitTests(NumDests, NumCmps,
-                            Clusters[First].Low->getValue(),
-                            Clusters[Last].High->getValue())) {
+  if (TLI.isSuitableForBitTests(
+          NumDests, NumCmps, Clusters[First].Low->getValue(),
+          Clusters[Last].High->getValue(), DAG.getDataLayout())) {
     // Clusters[First..Last] should be lowered as bit tests instead.
     return false;
   }
 
   // Create the MBB that will load from and jump through the table.
   // Note: We create it here, but it's not inserted into the function yet.
   MachineFunction *CurMF = FuncInfo.MF;
   MachineBasicBlock *JumpTableMBB =
       CurMF->CreateMachineBasicBlock(SI->getParent());
 
   // Add successors. Note: use table order for determinism.
   SmallPtrSet<MachineBasicBlock *, 8> Done;
   for (MachineBasicBlock *Succ : Table) {
     if (Done.count(Succ))
       continue;
     addSuccessorWithProb(JumpTableMBB, Succ, JTProbs[Succ]);
     Done.insert(Succ);
   }
   JumpTableMBB->normalizeSuccProbs();
 
-  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   unsigned JTI = CurMF->getOrCreateJumpTableInfo(TLI.getJumpTableEncoding())
                      ->createJumpTableIndex(Table);
 
   // Set up the jump table info.
   JumpTable JT(-1U, JTI, JumpTableMBB, nullptr);
   JumpTableHeader JTH(Clusters[First].Low->getValue(),
                       Clusters[Last].High->getValue(), SI->getCondition(),
                       nullptr, false);
#ifndef NDEBUG
   assert(!Clusters.empty());
   for (CaseCluster &C : Clusters)
     assert(C.Kind == CC_Range);
   for (unsigned i = 1, e = Clusters.size(); i < e; ++i)
     assert(Clusters[i - 1].High->getValue().slt(Clusters[i].Low->getValue()));
 #endif
 
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
-  if (!areJTsAllowed(TLI, SI))
+  if (!TLI.areJTsAllowed(SI->getParent()->getParent()))
     return;
 
-  const bool OptForSize = DefaultMBB->getParent()->getFunction()->optForSize();
-
   const int64_t N = Clusters.size();
   const unsigned MinJumpTableEntries = TLI.getMinimumJumpTableEntries();
   const unsigned SmallNumberOfEntries = MinJumpTableEntries / 2;
-  const unsigned MaxJumpTableSize =
-                   OptForSize || TLI.getMaximumJumpTableSize() == 0
-                   ? UINT_MAX : TLI.getMaximumJumpTableSize();
 
   if (N < 2 || N < MinJumpTableEntries)
     return;
  
   // TotalCases[i]: Total nbr of cases in Clusters[0..i].
   SmallVector<unsigned, 8> TotalCases(N);
   for (unsigned i = 0; i < N; ++i) {
     const APInt &Hi = Clusters[i].High->getValue();
     const APInt &Lo = Clusters[i].Low->getValue();
     TotalCases[i] = (Hi - Lo).getLimitedValue() + 1;
     if (i != 0)
       TotalCases[i] += TotalCases[i - 1];
   }
 
-  const unsigned MinDensity =
-    OptForSize ? OptsizeJumpTableDensity : JumpTableDensity;
-
   // Cheap case: the whole range may be suitable for jump table.
-  unsigned JumpTableSize = (Clusters[N - 1].High->getValue() -
-                            Clusters[0].Low->getValue())
-                           .getLimitedValue(UINT_MAX - 1) + 1;
-  if (JumpTableSize <= MaxJumpTableSize &&
-      isDense(Clusters, TotalCases, 0, N - 1, MinDensity)) {
+  uint64_t Range = getJumpTableRange(Clusters,0, N - 1);
+  uint64_t NumCases = getJumpTableNumCases(TotalCases, 0, N - 1);
+  assert(NumCases < UINT64_MAX / 100);
+  assert(Range >= NumCases);
+  if (TLI.isSuitableForJumpTable(SI, NumCases, Range)) {
     CaseCluster JTCluster;
     if (buildJumpTable(Clusters, 0, N - 1, SI, DefaultMBB, JTCluster)) {
       Clusters[0] = JTCluster;
       Clusters.resize(1);
       return;
     }
   }
 
for (int64_t i = N - 2; i >= 0; i--) {
     // Baseline: Put Clusters[i] into a partition on its own.
     MinPartitions[i] = MinPartitions[i + 1] + 1;
     LastElement[i] = i;
     PartitionsScore[i] = PartitionsScore[i + 1] + PartitionScores::SingleCase;
 
     // Search for a solution that results in fewer partitions.
     for (int64_t j = N - 1; j > i; j--) {
       // Try building a partition from Clusters[i..j].
-      JumpTableSize = (Clusters[j].High->getValue() -
-                       Clusters[i].Low->getValue())
-                      .getLimitedValue(UINT_MAX - 1) + 1;
-      if (JumpTableSize <= MaxJumpTableSize &&
-          isDense(Clusters, TotalCases, i, j, MinDensity)) {
+      uint64_t Range = getJumpTableRange(Clusters, i, j);
+      uint64_t NumCases = getJumpTableNumCases(TotalCases, i, j);
+      assert(NumCases < UINT64_MAX / 100);
+      assert(Range >= NumCases);
+      if (TLI.isSuitableForJumpTable(SI, NumCases, Range)) {
         unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
         unsigned Score = j == N - 1 ? 0 : PartitionsScore[j + 1];
         int64_t NumEntries = j - i + 1;
 
         if (NumEntries == 1)
           Score += PartitionScores::SingleCase;
         else if (NumEntries <= SmallNumberOfEntries)
           Score += PartitionScores::FewCases;
for (unsigned First = 0, Last; First < N; First = Last + 1) {
     } else {
       for (unsigned I = First; I <= Last; ++I)
         std::memmove(&Clusters[DstIndex++], &Clusters[I], sizeof(Clusters[I]));
     }
   }
   Clusters.resize(DstIndex);
 }
 
-bool SelectionDAGBuilder::rangeFitsInWord(const APInt &Low, const APInt &High) {
-  // FIXME: Using the pointer type doesn't seem ideal.
-  uint64_t BW = DAG.getDataLayout().getPointerSizeInBits();
-  uint64_t Range = (High - Low).getLimitedValue(UINT64_MAX - 1) + 1;
-  return Range <= BW;
-}
-
-bool SelectionDAGBuilder::isSuitableForBitTests(unsigned NumDests,
-                                                unsigned NumCmps,
-                                                const APInt &Low,
-                                                const APInt &High) {
-  // FIXME: I don't think NumCmps is the correct metric: a single case and a
-  // range of cases both require only one branch to lower. Just looking at the
-  // number of clusters and destinations should be enough to decide whether to
-  // build bit tests.
-
-  // To lower a range with bit tests, the range must fit the bitwidth of a
-  // machine word.
-  if (!rangeFitsInWord(Low, High))
-    return false;
-
-  // Decide whether it's profitable to lower this range with bit tests. Each
-  // destination requires a bit test and branch, and there is an overall range
-  // check branch. For a small number of clusters, separate comparisons might be
-  // cheaper, and for many destinations, splitting the range might be better.
-  return (NumDests == 1 && NumCmps >= 3) ||
-         (NumDests == 2 && NumCmps >= 5) ||
-         (NumDests == 3 && NumCmps >= 6);
-}
-
 bool SelectionDAGBuilder::buildBitTests(CaseClusterVector &Clusters,
                                         unsigned First, unsigned Last,
                                         const SwitchInst *SI,
                                         CaseCluster &BTCluster) {
   assert(First <= Last);
   if (First == Last)
     return false;
 
   BitVector Dests(FuncInfo.MF->getNumBlockIDs());
   unsigned NumCmps = 0;
   for (int64_t I = First; I <= Last; ++I) {
     assert(Clusters[I].Kind == CC_Range);
     Dests.set(Clusters[I].MBB->getNumber());
     NumCmps += (Clusters[I].Low == Clusters[I].High) ? 1 : 2;
   }
   unsigned NumDests = Dests.count();
 
   APInt Low = Clusters[First].Low->getValue();
   APInt High = Clusters[Last].High->getValue();
   assert(Low.slt(High));
 
-  if (!isSuitableForBitTests(NumDests, NumCmps, Low, High))
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  const DataLayout &DL = DAG.getDataLayout();
+  if (!TLI.isSuitableForBitTests(NumDests, NumCmps, Low, High, DL))
     return false;
 
   APInt LowBound;
   APInt CmpRange;
 
-  const int BitWidth = DAG.getTargetLoweringInfo()
-                           .getPointerTy(DAG.getDataLayout())
-                           .getSizeInBits();
-  assert(rangeFitsInWord(Low, High) && "Case range must fit in bit mask!");
+  const int BitWidth = TLI.getPointerTy(DL).getSizeInBits();
+  assert(TLI.rangeFitsInWord(Low, High, DL) &&
+         "Case range must fit in bit mask!");
 
   // Check if the clusters cover a contiguous range such that no value in the
   // range will jump to the default statement.
   bool ContiguousRange = true;
   for (int64_t I = First + 1; I <= Last; ++I) {
     if (Clusters[I].Low->getValue() != Clusters[I - 1].High->getValue() + 1) {
       ContiguousRange = false;
       break;

 #endif
 
   // The algorithm below is not suitable for -O0.
   if (TM.getOptLevel() == CodeGenOpt::None)
     return;
 
   // If target does not have legal shift left, do not emit bit tests at all.
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
-  EVT PTy = TLI.getPointerTy(DAG.getDataLayout());
+  const DataLayout &DL = DAG.getDataLayout();
+
+  EVT PTy = TLI.getPointerTy(DL);
   if (!TLI.isOperationLegal(ISD::SHL, PTy))
     return;
 
   int BitWidth = PTy.getSizeInBits();
   const int64_t N = Clusters.size();
 
   // MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
   SmallVector<unsigned, 8> MinPartitions(N);
for (int64_t i = N - 2; i >= 0; --i) {
     LastElement[i] = i;
 
     // Search for a solution that results in fewer partitions.
     // Note: the search is limited by BitWidth, reducing time complexity.
     for (int64_t j = std::min(N - 1, i + BitWidth - 1); j > i; --j) {
       // Try building a partition from Clusters[i..j].
 
       // Check the range.
-      if (!rangeFitsInWord(Clusters[i].Low->getValue(),
-                           Clusters[j].High->getValue()))
+      if (!TLI.rangeFitsInWord(Clusters[i].Low->getValue(),
+                               Clusters[j].High->getValue(), DL))
         continue;
 
       // Check nbr of destinations and cluster types.
       // FIXME: This works, but doesn't seem very efficient.
       bool RangesOnly = true;
       BitVector Dests(FuncInfo.MF->getNumBlockIDs());
       for (int64_t k = i; k <= j; k++) {
         if (Clusters[k].Kind != CC_Range) {

lib/CodeGen/TargetLoweringBase.cpp

 static cl::opt<unsigned> MinimumJumpTableEntries
   ("min-jump-table-entries", cl::init(4), cl::Hidden,
    cl::desc("Set minimum number of entries to use a jump table."));
 
 static cl::opt<unsigned> MaximumJumpTableSize
   ("max-jump-table-size", cl::init(0), cl::Hidden,
    cl::desc("Set maximum size of jump tables; zero for no limit."));
 
+/// Minimum jump table density for normal functions.
+static cl::opt<unsigned>
+    JumpTableDensity("jump-table-density", cl::init(10), cl::Hidden,
+                     cl::desc("Minimum density for building a jump table in "
+                              "a normal function"));
+
+/// Minimum jump table density for -Os or -Oz functions.
+static cl::opt<unsigned> OptsizeJumpTableDensity(
+    "optsize-jump-table-density", cl::init(40), cl::Hidden,
+    cl::desc("Minimum density for building a jump table in "
+             "an optsize function"));
+
 // Although this default value is arbitrary, it is not random. It is assumed
 // that a condition that evaluates the same way by a higher percentage than this
 // is best represented as control flow. Therefore, the default value N should be
 // set such that the win from N% correct executions is greater than the loss
 // from (100 - N)% mispredicted executions for the majority of intended targets.
 static cl::opt<int> MinPercentageForPredictableBranch(
     "min-predictable-branch", cl::init(99),
     cl::desc("Minimum percentage (0-100) that a condition must be either true "

 unsigned TargetLoweringBase::getMinimumJumpTableEntries() const {
   return MinimumJumpTableEntries;
 }
 
 void TargetLoweringBase::setMinimumJumpTableEntries(unsigned Val) {
   MinimumJumpTableEntries = Val;
 }
 
+unsigned TargetLoweringBase::getMinimumJumpTableDensity(bool OptForSize) const {
+  return OptForSize ? OptsizeJumpTableDensity : JumpTableDensity;
+}
+
 unsigned TargetLoweringBase::getMaximumJumpTableSize() const {
   return MaximumJumpTableSize;
 }
 
 void TargetLoweringBase::setMaximumJumpTableSize(unsigned Val) {
   MaximumJumpTableSize = Val;
 }
 

test/Transforms/Inline/AArch64/switch.ll

+; RUN: opt < %s -inline -inline-threshold=20 -S -mtriple=aarch64-none-linux -inline-generic-switch-cost=true | FileCheck %s
+; RUN: opt < %s -passes='cgscc(inline)' -inline-threshold=20 -S -mtriple=aarch64-none-linux -inline-generic-switch-cost=true | FileCheck %s
+
+define i32 @callee_range(i32 %a, i32* %P) {
+  switch i32 %a, label %sw.default [
+    i32 0, label %sw.bb0
+    i32 1000, label %sw.bb1
+    i32 2000, label %sw.bb1
+    i32 3000, label %sw.bb1
+    i32 4000, label %sw.bb1
+    i32 5000, label %sw.bb1
+    i32 6000, label %sw.bb1
+    i32 7000, label %sw.bb1
+    i32 8000, label %sw.bb1
+    i32 9000, label %sw.bb1
+  ]
+
+sw.default:
+  store volatile i32 %a, i32* %P
+  br label %return
+sw.bb0:
+  store volatile i32 %a, i32* %P
+  br label %return
+sw.bb1:
+  store volatile i32 %a, i32* %P
+  br label %return
+return:
+  ret i32 42
+}
+
+define i32 @caller_range(i32 %a, i32* %P) {
+; CHECK-LABEL: @caller_range(
+; CHECK: call i32 @callee_range
+  %r = call i32 @callee_range(i32 %a, i32* %P)
+  ret i32 %r
+}
+
+define i32 @callee_bittest(i32 %a, i32* %P) {
+  switch i32 %a, label %sw.default [
+    i32 0, label %sw.bb0
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+    i32 3, label %sw.bb0
+    i32 4, label %sw.bb1
+    i32 5, label %sw.bb2
+    i32 6, label %sw.bb0
+    i32 7, label %sw.bb1
+    i32 8, label %sw.bb2
+  ]
+
+sw.default:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+sw.bb0:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+sw.bb1:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+sw.bb2:
+  br label %return
+
+return:
+  ret i32 42
+}
+
+
+define i32 @caller_bittest(i32 %a, i32* %P) {
+; CHECK-LABEL: @caller_bittest(
+; CHECK-NOT: call i32 @callee_bittest
+  %r= call i32 @callee_bittest(i32 %a, i32* %P)
+  ret i32 %r
+}
+
+define i32 @callee_jumptable(i32 %a, i32* %P) {
+  switch i32 %a, label %sw.default [
+    i32 1001, label %sw.bb101
+    i32 1002, label %sw.bb102
+    i32 1003, label %sw.bb103
+    i32 1004, label %sw.bb104
+    i32 1005, label %sw.bb101
+    i32 1006, label %sw.bb102
+    i32 1007, label %sw.bb103
+    i32 1008, label %sw.bb104
+    i32 1009, label %sw.bb101
+    i32 1010, label %sw.bb102
+    i32 1011, label %sw.bb103
+    i32 1012, label %sw.bb104
+ ]
+
+sw.default:
+  br label %return
+
+sw.bb101:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+sw.bb102:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+sw.bb103:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+sw.bb104:
+  store volatile i32 %a, i32* %P
+  br label %return
+
+return:
+  ret i32 42
+}
+
+define i32 @caller_jumptable(i32 %a, i32 %b, i32* %P) {
+; CHECK-LABEL: @caller_jumptable(
+; CHECK: call i32 @callee_jumptable
+  %r = call i32 @callee_jumptable(i32 %b, i32* %P)
+  ret i32 %r
+}
+