[AggressiveInstCombine] Lower Table Based CTTZ

[djtodoro]

Summary

This patch introduces recognition of table-based ctz implementation during the AggressiveInstCombine.
This fixes the [0].

[0] https://bugs.llvm.org/show_bug.cgi?id=46434

TODO: Get the data on SPEC benchmark.

[djtodoro]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

371

OK, sure.

451

Agree with this.

483

Yep.

523

Actually, we don't need it.

[djtodoro]

• Refactor && update the tests
• Address the comments

[xbolva00]

llvm/test/Transforms/AggressiveInstCombine/AARCH64/lower-table-based-ctz-basics.ll

1 ↗

(On Diff #387204)

AggressiveInstCombine/AARCH64 -> AggressiveInstCombine/AArch64

[djtodoro]

• update the test dir name

[djtodoro]

any other comment here? :)

[fhahn]

TODO: Get the data on SPEC benchmark.

Did you manage to collect any perf data yet to motivate this change?

The tests already contain a few things that seem unrelated, it would be good to clean those things up.

llvm/test/Transforms/AggressiveInstCombine/AArch64/dereferencing-pointer.ll

28 ↗

(On Diff #387215)

That's out of date.

40 ↗

(On Diff #387215)

can the tests instead just take i64 %b or does it need to be a pointer? (

50 ↗

(On Diff #387215)

is all the mote data needed? Same for the other tests

llvm/test/Transforms/AggressiveInstCombine/AArch64/non-argument-value.ll

31 ↗

(On Diff #387215)

not needed

[djtodoro]

In D113291#3153219, @fhahn wrote:

TODO: Get the data on SPEC benchmark.

Did you manage to collect any perf data yet to motivate this change?

Not yet, but I will share ASAP.

Thanks for your comments!

llvm/test/Transforms/AggressiveInstCombine/AArch64/dereferencing-pointer.ll

40 ↗

(On Diff #387215)

This test is meant to test the pointer type. There are other tests checking non-ptr type.

50 ↗

(On Diff #387215)

Yep, I'll reduce the tests in the next update.

[djtodoro]

-Clean up the tests

[spatel]

AggressiveInstCombine is an extension of InstCombine. That is, it's a target-independent canonicalization pass. Therefore, we shouldn't use any target-specific cost model to enable the transform. Since we have a generic intrinsic for cttz, it's fine to create that for all targets as long as we can guarantee that a generic expansion of that intrinsic in the backend will not be worse than the original code.

But I'm not sure if we can make that guarantee? If not, this should be implemented as a late IR or codegen pass (as it was when first posted).

[craig.topper]

In D113291#3238355, @spatel wrote:

AggressiveInstCombine is an extension of InstCombine. That is, it's a target-independent canonicalization pass. Therefore, we shouldn't use any target-specific cost model to enable the transform. Since we have a generic intrinsic for cttz, it's fine to create that for all targets as long as we can guarantee that a generic expansion of that intrinsic in the backend will not be worse than the original code.

But I'm not sure if we can make that guarantee? If not, this should be implemented as a late IR or codegen pass (as it was when first posted).

Why is it ok to use DataLayout in InstCombine/AggressiveInstCombine, but not TTI? The cttz seems like it could enable other optimizations so I don't think we want it late. In particular, we should give the optimizer a chance to prove that the input isn't 0 to remove the select that gets generated after the cttz intrinsic. That could require computeKnownBits or CorrelatedValuePropagation. LoopIdiomRecognize queries TTI before generating cttz from loops.

[spatel]

In D113291#3238387, @craig.topper wrote:

In D113291#3238355, @spatel wrote:

AggressiveInstCombine is an extension of InstCombine. That is, it's a target-independent canonicalization pass. Therefore, we shouldn't use any target-specific cost model to enable the transform. Since we have a generic intrinsic for cttz, it's fine to create that for all targets as long as we can guarantee that a generic expansion of that intrinsic in the backend will not be worse than the original code.

But I'm not sure if we can make that guarantee? If not, this should be implemented as a late IR or codegen pass (as it was when first posted).

Why is it ok to use DataLayout in InstCombine/AggressiveInstCombine, but not TTI? The cttz seems like it could enable other optimizations so I don't think we want it late. In particular, we should give the optimizer a chance to prove that the input isn't 0 to remove the select that gets generated after the cttz intrinsic. That could require computeKnownBits or CorrelatedValuePropagation. LoopIdiomRecognize queries TTI before generating cttz from loops.

Yeah, it's fuzzy. I think we're only supposed to be using DataLayout to determine where creating an illegal type would obviously lead to worse codegen. But that's not much different than asking if some operation is legal on target X.
We've had several other requests for a cost-aware version of instcombine over the years, so maybe we should just use this an opportunity to reframe/rename AggressiveInstCombine.

[gsocshubham]

Does handling of "0", when accessing index 0 for x = 0 not acceptable?

What is the reason for this patch being on hold?

[dmgreen]

We've had several other requests for a cost-aware version of instcombine over the years, so maybe we should just use this an opportunity to reframe/rename AggressiveInstCombine.

It could help with doing thing like https://reviews.llvm.org/D114964 earlier too, where the transfroms are not always profitable and not reversible back to the original code, but would be beneficial to do earlier to get better cost modelling and vectorization.

[gsocshubham]

Hello,

I have a small query regarding this patch. The patch emits following llvm assembly for ctz table -

-----Patch assembly-------
// %bb.0:

rbit w8, w0
cmp w0, #0
clz w8, w8
csel w0, wzr, w8, eq
ret

---

but in gcc, we have the following assembly being emitted -

------------GCC---------------------
f(unsigned int):

rbit    w0, w0
clz     w0, w0
and     w0, w0, 31
ret

---

Reference - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838

[1] My question is - To solve the bug, do I have to generate assembly similar to GCC?

Please give me suggestions on moving forward in solving this bug.

[djtodoro]

In D113291#3274616, @gsocshubham wrote:

Hello,

I have a small query regarding this patch. The patch emits following llvm assembly for ctz table -

-----Patch assembly-------
// %bb.0:

rbit w8, w0
cmp w0, #0
clz w8, w8
csel w0, wzr, w8, eq
ret

---

but in gcc, we have the following assembly being emitted -

------------GCC---------------------
f(unsigned int):

rbit    w0, w0
clz     w0, w0
and     w0, w0, 31
ret

---

Reference - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838

[1] My question is - To solve the bug, do I have to generate assembly similar to GCC?

Please give me suggestions on moving forward in solving this bug.

Hi,

Is this really a bug?

[dmgreen]

I have a small query regarding this patch. The patch emits following llvm assembly for ctz table -

-----Patch assembly-------
// %bb.0:

rbit w8, w0
cmp w0, #0
clz w8, w8
csel w0, wzr, w8, eq
ret

---

but in gcc, we have the following assembly being emitted -

------------GCC---------------------
f(unsigned int):

rbit    w0, w0
clz     w0, w0
and     w0, w0, 31
ret

---

That sounds like a backend optimization that could happen given we know the semantics of the AArch64 instruction.

[djtodoro]

In D113291#3275222, @dmgreen wrote:

I have a small query regarding this patch. The patch emits following llvm assembly for ctz table -

-----Patch assembly-------
// %bb.0:

rbit w8, w0
cmp w0, #0
clz w8, w8
csel w0, wzr, w8, eq
ret

---

but in gcc, we have the following assembly being emitted -

------------GCC---------------------
f(unsigned int):

rbit    w0, w0
clz     w0, w0
and     w0, w0, 31
ret

---

That sounds like a backend optimization that could happen given we know the semantics of the AArch64 instruction.

Yep, +1 for this. I'd leave this job to backends.

[djtodoro]

@craig.topper @spatel Do you think this can go as is?

(P.S. I don't have access to any AARCH64 board currently, so I can see into the SPEC numbers.)

[xbolva00]

I am wondering about general direction..

Is it worth it? On way to become compiler just for benchmarks?

Spend compile time just to optimize one very very specific pattern from spec is bad thing imho.

Can you show us some other real world “hits”? I assume any sane project already uses builtin to compute this value efficiently.

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

383

getZExtValue may assert large ints

[xbolva00]

Can you please provide compile time data with @nikic's compile time tracker? This should be *very cheap* to be acceptable.

.

[craig.topper]

In D113291#3286784, @xbolva00 wrote:

I am wondering about general direction..

Is it worth it? On way to become compiler just for benchmarks?

Spend compile time just to optimize one very very specific pattern from spec is bad thing imho.

Can you show us some other real world “hits”? I assume any sane project already uses builtin to compute this value efficiently.

The same algorithm is documented here https://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup

It's also nearby in the code in primesieve from the recent tzcnt discusson. Around line 143 if you expand the context on Erat.hpp here https://github.com/kimwalisch/primesieve/pull/109/files Granted that code knows when to use the tzcnt builtin instead of that code. I'm only mentioning it to show it is a known way to implement tzcnt that is used in more than just spec.

[djtodoro]

In D113291#3286803, @xbolva00 wrote:

Can you please provide compile time data with @nikic's compile time tracker? This should be *very cheap* to be acceptable.

.

It is done: http://llvm-compile-time-tracker.com/compare.php?from=9920943ea201189f9b34918c7663d8a03d7e4676&to=666dd20021db313e4ead3e39ac4c8a12b9525521&stat=instructions

[djtodoro]

@xbolva00 ping :)

[xbolva00]

How hard is to add x86 support?

[xbolva00]

(Not blocking this)

[djtodoro]

Thanks!

In D113291#3493276, @xbolva00 wrote:

How hard is to add x86 support?

Is it even worth of implementing it for x86?

[craig.topper]

In D113291#3495933, @djtodoro wrote:

Thanks!

In D113291#3493276, @xbolva00 wrote:

How hard is to add x86 support?

Is it even worth of implementing it for x86?

Yes. Intel CPUs from about 2013 have a TZCNT instruction.

[djtodoro]

OK, great! it will be on my TODO list!

[djtodoro]

But I think that x86 support doesn’t block this.

[xbolva00]

Ideally this transformation should just emit proper intrinsic without need for target hook.

What is the real problem?

[dmgreen]

Hello - We were having a discussion about a very similar patch in D125755. I think the outcome for this patch is that either:

• We need to do this later (maybe in CodeGenPrepare).
• We need to do this unconditionally without the call to TTI.preferCTTZLowering() and have the reverse transform later for targets that do not have a cheaper alternative.
• We need to argue some more :)

There are more details about why in D125755. I would go for the first option if it doesn't lead to worse performance, as for the second I'm not sure when it would be profitable to transform back and emit the table. You may not want to do that for non-hot ctzs? It sounds like it may be difficult to get right, but maybe I'm overestimating it.

[efriedma]

as for the second I'm not sure when it would be profitable to transform back and emit the table

You really just have to weigh it against the current default expansion on targets where ctlz/cttz aren't legal, which is popcount(v & -v). It should be a straightforward comparison, generally. If you have popcount, use it. If multiply is legal, use a table lookup. Otherwise... maybe stick with the popcount expansion? Probably any approach is expensive at that point.

Compare the generated code for arm-eabi.

You may not want to do that for non-hot ctzs?

As opposed to what, calling into compiler-rt?

[djtodoro]

In D113291#3539840, @dmgreen wrote:

Hello - We were having a discussion about a very similar patch in D125755. I think the outcome for this patch is that either:

• We need to do this later (maybe in CodeGenPrepare).

The initial version of patch was implemented within CodeGenPrepare. And I think it should not introduce any performance regression.

• We need to do this unconditionally without the call to TTI.preferCTTZLowering() and have the reverse transform later for targets that do not have a cheaper alternative.

Hmm... I need to think about that.

• We need to argue some more :)

Seems that I need to find some time to catch up the conversation in D125755. :)

There are more details about why in D125755. I would go for the first option if it doesn't lead to worse performance, as for the second I'm not sure when it would be profitable to transform back and emit the table. You may not want to do that for non-hot ctzs?

I am not sure I get the question.

[djtodoro]

In D113291#3539965, @efriedma wrote:

as for the second I'm not sure when it would be profitable to transform back and emit the table

You really just have to weigh it against the current default expansion on targets where ctlz/cttz aren't legal, which is popcount(v & -v). It should be a straightforward comparison, generally. If you have popcount, use it. If multiply is legal, use a table lookup. Otherwise... maybe stick with the popcount expansion? Probably any approach is expensive at that point.

Compare the generated code for arm-eabi.

I guess we could measure something like that, but seems to me that it could introduce some performance regressions...

[dmgreen]

In D113291#3539965, @efriedma wrote:

as for the second I'm not sure when it would be profitable to transform back and emit the table

You really just have to weigh it against the current default expansion on targets where ctlz/cttz aren't legal, which is popcount(v & -v). It should be a straightforward comparison, generally. If you have popcount, use it. If multiply is legal, use a table lookup. Otherwise... maybe stick with the popcount expansion? Probably any approach is expensive at that point.

Compare the generated code for arm-eabi.

You may not want to do that for non-hot ctzs?

As opposed to what, calling into compiler-rt?

I was meaning - it can be difficult for the compiler to recognize _when_ a ctz is performance critical. If the size of the table is large (which I was possible over-estimating the size of in my mind), then you may not want to emit the table for every ctz in the program. Currently that places where this is used have said, from the fact that they wrote it this way, that these ctz's are important. It just depends on whether converting to a table is always better, and if the table it small enough to be reasonable in the common case. 32 x i8 doesn't sound too big compared to what I originally imagined, if that is all it needs.

[craig.topper]

In D113291#3540303, @dmgreen wrote:

In D113291#3539965, @efriedma wrote:

as for the second I'm not sure when it would be profitable to transform back and emit the table

You really just have to weigh it against the current default expansion on targets where ctlz/cttz aren't legal, which is popcount(v & -v). It should be a straightforward comparison, generally. If you have popcount, use it. If multiply is legal, use a table lookup. Otherwise... maybe stick with the popcount expansion? Probably any approach is expensive at that point.

Compare the generated code for arm-eabi.

You may not want to do that for non-hot ctzs?

As opposed to what, calling into compiler-rt?

I was meaning - it can be difficult for the compiler to recognize _when_ a ctz is performance critical. If the size of the table is large (which I was possible over-estimating the size of in my mind), then you may not want to emit the table for every ctz in the program. Currently that places where this is used have said, from the fact that they wrote it this way, that these ctz's are important. It just depends on whether converting to a table is always better, and if the table it small enough to be reasonable in the common case. 32 x i8 doesn't sound too big compared to what I originally imagined, if that is all it needs.

For the table lookup, is there an algorithm for creating the special constant that is used in the multiply? Or would we just hardcode known constants for common sizes.

[craig.topper]

In D113291#3539965, @efriedma wrote:

as for the second I'm not sure when it would be profitable to transform back and emit the table

You really just have to weigh it against the current default expansion on targets where ctlz/cttz aren't legal, which is popcount(v & -v). It should be a straightforward comparison, generally. If you have popcount, use it. If multiply is legal, use a table lookup. Otherwise... maybe stick with the popcount expansion? Probably any approach is expensive at that point.

Note, the popcount expansion already uses a multiply without checking if it is legal.

[efriedma]

For the table lookup, is there an algorithm for creating the special constant that is used in the multiply? Or would we just hardcode known constants for common sizes.

https://en.wikipedia.org/wiki/De_Bruijn_sequence has a description of the algorithm. Probably we'd just hard-code constants, though; practically speaking, the only sizes we actually care about are 16, 32, and 64. (For anything that doesn't fit in a single register, we probably just want to split the cttz.)

Note, the popcount expansion already uses a multiply without checking if it is legal.

It's a relatively cheap multiply to expand on a target with a shifter, though.

[djtodoro]

Thanks a lot for the comments! Can someone please sum the things up that need to be done for this?

By implementing this in the CodeGenPrepare doesn't include/imply that we should get rid of the call to TTI.preferCTTZLowering().
If we decide to go with the unconditional (not to care about Target) cttz lowering and to get rid of the call to TTI.preferCTTZLowering(), what should be done?

[efriedma]

Concretely, my preferred solution looks something like:

1. Perform the transform unconditionally in AggressiveInstCombine (so this patch without the preferCTTZLowering() bits).
2. Teach TargetLowering::expandCTTZ to emit a table lookup.

[djtodoro]

• drop target dependent hooks

[djtodoro]

In D113291#3542941, @efriedma wrote:

Concretely, my preferred solution looks something like:

1. Perform the transform unconditionally in AggressiveInstCombine (so this patch without the preferCTTZLowering() bits).

The latest update implements this.

2. Teach TargetLowering::expandCTTZ to emit a table lookup.

[gsocshubham]

In D113291#3566581, @djtodoro wrote:

In D113291#3542941, @efriedma wrote:

Concretely, my preferred solution looks something like:

1. Perform the transform unconditionally in AggressiveInstCombine (so this patch without the preferCTTZLowering() bits).

The latest update implements this.

2. Teach TargetLowering::expandCTTZ to emit a table lookup.

@djtodoro - Will you be sending patch for (2) "Teach TargetLowering::expandCTTZ to emit a table lookup."?

[djtodoro]

In D113291#3569114, @gsocshubham wrote:

In D113291#3566581, @djtodoro wrote:

In D113291#3542941, @efriedma wrote:

Concretely, my preferred solution looks something like:

1. Perform the transform unconditionally in AggressiveInstCombine (so this patch without the preferCTTZLowering() bits).

The latest update implements this.

2. Teach TargetLowering::expandCTTZ to emit a table lookup.

@djtodoro - Will you be sending patch for (2) "Teach TargetLowering::expandCTTZ to emit a table lookup."?

Unfortunately, I don’t have time to do it right now. If you are interested, please go ahead with the implementation.

[gsocshubham]

In D113291#3569225, @djtodoro wrote:

In D113291#3569114, @gsocshubham wrote:

In D113291#3566581, @djtodoro wrote:

In D113291#3542941, @efriedma wrote:

Concretely, my preferred solution looks something like:

1. Perform the transform unconditionally in AggressiveInstCombine (so this patch without the preferCTTZLowering() bits).

The latest update implements this.

2. Teach TargetLowering::expandCTTZ to emit a table lookup.

@djtodoro - Will you be sending patch for (2) "Teach TargetLowering::expandCTTZ to emit a table lookup."?

Unfortunately, I don’t have time to do it right now. If you are interested, please go ahead with the implementation.

@djtodoro - Sure. I am interested to do it. Can you elaborate (2) a bit in detail?

[djtodoro]

In D113291#3569456, @gsocshubham wrote:

In D113291#3569225, @djtodoro wrote:

In D113291#3569114, @gsocshubham wrote:

In D113291#3566581, @djtodoro wrote:

In D113291#3542941, @efriedma wrote:

Concretely, my preferred solution looks something like:

1. Perform the transform unconditionally in AggressiveInstCombine (so this patch without the preferCTTZLowering() bits).

The latest update implements this.

2. Teach TargetLowering::expandCTTZ to emit a table lookup.

@djtodoro - Will you be sending patch for (2) "Teach TargetLowering::expandCTTZ to emit a table lookup."?

Unfortunately, I don’t have time to do it right now. If you are interested, please go ahead with the implementation.

@djtodoro - Sure. I am interested to do it. Can you elaborate (2) a bit in detail?

Basically this patch implements lowering of the table-based cttz implementation into @llvm.cttz unconditionally. For some targets it won't be that beneficial, so during the TargetLowering::expandCTTZ we should emit table lookup again. @efriedma May have something to add.

[efriedma]

I don't think I have much further to say. Emitting a table lookup from TargetLowering::expandCTTZ should be straightforward, I think. See DAGCombiner::convertSelectOfFPConstantsToLoadOffset for an example of how to emit a constant table.

[craig.topper]

In D113291#3602808, @efriedma wrote:

I don't think I have much further to say. Emitting a table lookup from TargetLowering::expandCTTZ should be straightforward, I think. See DAGCombiner::convertSelectOfFPConstantsToLoadOffset for an example of how to emit a constant table.

Do we need to be careful with vector CTTZ which can also go through expand CTTZ?

[gsocshubham]

Is this patch ready to be merged? This is parent patch of https://reviews.llvm.org/D128911. Does child get merged before parent?

Anyways, https://reviews.llvm.org/D128911 can be merged independently of this patch.

[dmgreen]

In D113291#3670999, @gsocshubham wrote:

Is this patch ready to be merged? This is parent patch of https://reviews.llvm.org/D128911. Does child get merged before parent?

Anyways, https://reviews.llvm.org/D128911 can be merged independently of this patch.

D128911 needs to go in first, once that is done we can move forward with this one. It could do with a rebase and a clang-format.

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

444

We will need to support opaque pointers nowadays.

483

It's probably better to just say "64bit targets" as opposed to a specific target.

494

I believe it's the top Bitwidth - Log2(Bitwidth) bits.

[dmgreen]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

486

Does the extend between the lshr and the mul every happen? From what I can tell, the type of the VT should be based on the type of these operations.

690

The TTI's can be removed now (and if you rebase they may already be present, but still are not needed by the new code any more).

llvm/test/Transforms/AggressiveInstCombine/AArch64/lower-table-based-ctz-basics.ll

1 ↗

(On Diff #435145)

The tests can be moved out of the AArch64 directory, so long as they drop the AArch64 triple.

[dmgreen]

@djtodoro Do you have any time to update this? Otherwise do you mind we take it over and we can update it and get it reviewed. Thanks.

[djtodoro]

I promise I will find some time to update this - it is coming next week.

[djtodoro]

• support opaque pointers
• remove leftovers (since this was aarch64 only)
• move the tests in a non-target dir

[djtodoro]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

486

It does not happen in all the cases.

[djtodoro]

• remove a duplicated test

[spatel]

Why are some test files still specifying a triple in the RUN line?

It would be good to consolidate tests into less files if possible with better names/comments to explain exactly what differences are being tested in the sequence of tests. There should also be negative tests (wrong table constants, wrong magic multiplier, wrong shift amount, etc), so we know that the transform is not firing on mismatches.

[djtodoro]

In D113291#3728796, @spatel wrote:

Why are some test files still specifying a triple in the RUN line?

Leftovers. Thanks.

It would be good to consolidate tests into less files if possible with better names/comments to explain exactly what differences are being tested in the sequence of tests. There should also be negative tests (wrong table constants, wrong magic multiplier, wrong shift amount, etc), so we know that the transform is not firing on mismatches.

I will remove one redundant test. There are C producers as well as some top-level comments that explain what it should test (if the comment is needed). Furthermore, thanks for the suggestion for adding some negative tests - will add it.

[djtodoro]

• adding negative tests
• rename the tests
• clean the target triple leftovers from tests

[dmgreen]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

486

Do you have a test case where the extend is between the shift and the mul?

493

If the length of the table is larger than the InputBits, how are we sure that the matched elements will be the correct ones? Is this always guaranteed?
I think I would have expected a check that for each i=0..InputBits-1, Table[(Mul<<i)>>Shift] == i. With a check that the index is in range. Are they always equivalent with the larger tables too?

552

Can this always just use the Load type?

564

I think User here could be GlobalVariable

568

GEPUser->getOperand(0) -> Global->getInitializer(). It is worth adding a test where the global is extern.

590

It might be better to switch this logical around -

unsigned InputBits = X1->getType()->getScalarSizeInBits();
if (InputBits != 32 && InputBits != 64)
  return false;

593–594

Log2_32_Ceil -> Log2_Ceil if we know the InputBits is a power of 2.

The -1 case is for a larger table with more elements but that can handle zero values?

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-non-argument-value.ll

79

It is better to pass x as a parameter, although I'm not sure it matter much where x comes from for the rest of the pattern.

[spatel]

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-basics.ll

124

This is identical to the previous test, so it is not adding value here.

I realize that the source example it intended to model is slightly different. If you want to verify that we end up with cttz from the IR produced by clang, then I'd recommend adding a file to test/Transforms/PhaseOrdering and "RUN -O3".

When I create tests like that, I grab the unoptimized IR using "clang -S -o - -emit-llvm -Xclang -disable-llvm-optzns". Then reduce it by running it through "opt -mem2reg", so it's not completely full of junk IR.

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-non-argument-value.ll

79

Right - as far as this patch is concerned, this is identical to the previous test, so it shouldn't be here.

See my earlier comment about PhaseOrdering tests if we want more end-to-end coverage for opt -O3.

[djtodoro]

Thanks for the comments.

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

486

I was completely sure that I had a case for it, but I am not able to produce it actually -- so I deleted it for now.

493

Hmmm, can you please walk me through out an example?

552

I think yes, nothing comes up to my mind that can break it.

590

Sounds good.

593–594

Log2_32_Ceil -> Log2_Ceil if we know the InputBits is a power of 2.

Right...
Bu you meant Log2_64(), right? It is a power of 2, since it is either 32 or 64, so no need to add any assert here.

The -1 case is for a larger table with more elements but that can handle zero values?

int ctz2(unsigned x)
{
#define u 0
 static short table[64] =
  {
    32, 0, 1, 12, 2, 6, u, 13, 3, u, 7, u, u, u, u, 14,
    10, 4, u, u, 8, u, u, 25, u, u, u, u, u, 21, 27, 15,
    31, 11, 5, u, u, u, u, u, 9, u, u, 24, u, u, 20, 26,
    30, u, u, u, u, 23, u, 19, 29, u, 22, 18, 28, 17, 16, u
   };
 x = (x & -x) * 0x0450FBAF;
 return table[x >> 26];
 }

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-basics.ll

124

This is identical to the previous test, so it is not adding value here.

Right, thanks!

I realize that the source example it intended to model is slightly different. If you want to verify that we end up with cttz from the IR produced by clang, then I'd recommend adding a file to test/Transforms/PhaseOrdering and "RUN -O3".

When I create tests like that, I grab the unoptimized IR using "clang -S -o - -emit-llvm -Xclang -disable-llvm-optzns". Then reduce it by running it through "opt -mem2reg", so it's not completely full of junk IR.

I usually do create tests that way, but these may be stale a bit. I will add the `PhaseOrdering``` test, thanks for the suggestion.

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-non-argument-value.ll

79

Yes, agree.

[djtodoro]

• removed more duplicated tests
• added a llvm/test/Transforms/PhaseOrdering test
• refactor the code a bit

[dmgreen]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

493

Hmm. No, I'm not sure I can.

I was thinking about the ctz2 case, and whether there could be cases where the u's have different values that make them "match", but the values are different that make them wrong. So the items in the table accessed by the DeBruijn constant would produce incorrect values, but there are still InputBits number of matches.

;; int ctz2(unsigned x)
;; {
;; #define u 0
;;  static short table[64] =
;;   {
;;     32, 0, 1, 12, 2, 6, u, 13, 3, u, 7, u, u, u, u, 14,
;;     10, 4, u, u, 8, u, u, 25, u, u, u, u, u, 21, 27, 15,
;;     31, 11, 5, u, u, u, u, u, 9, u, u, 24, u, u, 20, 26,
;;     30, u, u, u, u, 23, u, 19, 29, u, 22, 18, 28, 17, 16, u
;;   };
;; x = (x & -x) * 0x0450FBAF;
;; return table[x >> 26];
;; }

But I don't think that is something that can come up. I was finding it hard to prove, but if the Mul is InputBits in length there are only at most InputBits separate elements that it can access. And multiple elements cannot map successfully back to the same i.

I ran a sat solver overnight, and it is still going but hasn't found any counter examples, which is a good sign. (It is able to find valid DeBruijn CTTZ tables given the chance).

It might be worth adding a comment explaining why this correctly matches the table in all cases.

547

One think I forgot to mention - llvm has a code style that is best explained as "just run clang-format on the patch". These returns are all in the wrong place, for example, and could do with a cleanup.

593–594

Ah, yeah - I meant Log2_32, but delete the wrong part of the function name.

595–596

This is true by definition now.

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-dereferencing-pointer.ll

24

I usually remove dso_local

llvm/test/Transforms/PhaseOrdering/lower-table-based-cttz.ll

23

Can remove the ; Function Attrs

[djtodoro]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

493

Yeah, good sign. I will try to make a reasonable comment. Thanks.

547

I've changed the style to Google, accidentally. Thanks.

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-dereferencing-pointer.ll

24

me as well, leftover :/ thanks!

[djtodoro]

• clang-format
• clean up tests

[dmgreen]

Thanks for the updates. I don't think I have anything else than is what is below. Any other comments from anyone else?

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

493

A comment explaining this function would still be useful.

552

This can be changed to just the Load type then.

570–573

if (!GVTable || !GVTable->hasInitializer())
  return false;

606–608

Remove this check, as it is always true as far as I can tell.

[spatel]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

476–477

There was a request to put a comment on this function, and I'll second that request. It's not clear why we are counting matches rather than just bailing out on the first mismatch. I think that's because you can construct/recognize a table with unaccessed/undefined elements?

[djtodoro]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

476–477

Yes, that is the reason - we are iterating over the elements of the table, so there could be mismatch that we can ignore. A comment is coming.

[djtodoro]

• addressing comments

[spatel]

LGTM

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

584–586

Could shorten this by using something like:

if (!match(GEP->idx_begin()->get(), m_ZeroInt())) return false;

[craig.topper]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

604

If we’re only handling 32 and 64, this comment should be 5..6

[craig.topper]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

486

I think this can be
Mask = APInt::getBitsSetFrom(InputBits , Shift)

[dmgreen]

Thanks for the updates. LGTM

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

604

I believe it is 7 because the table can be twice the size. Hence the -1 in the formula below. See the ctz2 test.

[djtodoro]

Thanks for your comments.

[djtodoro]

• addressing comments

[craig.topper]

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

485

extra space before comma. Looks like I mistyped it in my comment. Sorry.

[rsmith]

Hi, I'm seeing a heap-use-after-free in AggressiveInstCombine in a build shortly after this landed, within a function added here.

test/Transforms/AggressiveInstCombine/X86/sqrt.ll fails as follows under asan: https://gist.github.com/zygoloid/a270e65d32ab5b05504b3b0d5717f83b

Please can you fix or revert?

[rsmith]

In D113291#3768076, @rsmith wrote:

test/Transforms/AggressiveInstCombine/X86/sqrt.ll fails as follows under asan: https://gist.github.com/zygoloid/a270e65d32ab5b05504b3b0d5717f83b

I've gone ahead and reverted for now to unblock things. Let me know if you're not able to reproduce the ASan failure and I can dig more into it.

[djtodoro]

Thanks a lot. I will check ASAP.

[djtodoro]

@rsmith recommitted with f879939157. Thanks!

[spatel]

In D113291#3777119, @djtodoro wrote:

@rsmith recommitted with f879939157. Thanks!

What was the bug, how was it fixed, and is there a new test to verify the fix? That should have been mentioned in the new commit message.

[djtodoro]

In D113291#3777195, @spatel wrote:

In D113291#3777119, @djtodoro wrote:

@rsmith recommitted with f879939157. Thanks!

What was the bug, how was it fixed, and is there a new test to verify the fix? That should have been mentioned in the new commit message.

You are right. I reverted the recommit, and I will recommit it with proper message, sorry I missed it. :/

Actually, the issue was that your patch D129167 introduced eraseFromParent and the tryToRecognizeTableBasedCttz would try to use the instruction (dyn_cast) after free. I just moved the tryToRecognizeTableBasedCttz above foldSqrt. I guess it does not need any additional test case.

[spatel]

In D113291#3777356, @djtodoro wrote:

Actually, the issue was that your patch D129167 introduced eraseFromParent and the tryToRecognizeTableBasedCttz would try to use the instruction (dyn_cast) after free. I just moved the tryToRecognizeTableBasedCttz above foldSqrt. I guess it does not need any additional test case.

Ah, I see. Please put a comment on that call then - foldSqrt (or any other erasing transform) needs to be accounted for (last in the loop for now), or we might hit that bug. And yes, looks like we don't need another test since the existing regression test was flagged by the asan bot.

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

if (match(MulOp0, m_And(m_c_Add(m_LShr(m_Value(ShiftOp0), m_SpecificInt(4)),
       }
     }
   }
 
   return false;
 }
 
 /// Fold smin(smax(fptosi(x), C1), C2) to llvm.fptosi.sat(x), providing C1 and
 /// C2 saturate the value of the fp conversion. The transform is not reversable
 /// as the fptosi.sat is more defined than the input - all values produce a
 /// valid value for the fptosi.sat, where as some produce poison for original
 /// that were out of range of the integer conversion. The reversed pattern may
 /// use fmax and fmin instead. As we cannot directly reverse the transform, and
 /// it is not always profitable, we make it conditional on the cost being
 /// reported as lower by TTI.
 static bool tryToFPToSat(Instruction &I, TargetTransformInfo &TTI) {
   // Look for min(max(fptosi, converting to fptosi_sat.
   Value *In;
   const APInt *MinC, *MaxC;
   if (!match(&I, m_SMax(m_OneUse(m_SMin(m_OneUse(m_FPToSI(m_Value(In))),
                                         m_APInt(MinC))),
                         m_APInt(MaxC))) &&
       !match(&I, m_SMin(m_OneUse(m_SMax(m_OneUse(m_FPToSI(m_Value(In))),
                                         m_APInt(MaxC))),
                         m_APInt(MinC))))
     return false;
 
   // Check that the constants clamp a saturate.
   if (!(*MinC + 1).isPowerOf2() || -*MaxC != *MinC + 1)
foldSqrt(Instruction &I, TargetTransformInfo &TTI, TargetLibraryInfo &TLI) {
   // Match a call to sqrt mathlib function.
   auto *Call = dyn_cast<CallInst>(&I);
   if (!Call)
     return false;
 
   Module *M = Call->getModule();
   LibFunc Func;
   if (!TLI.getLibFunc(*Call, Func) || !isLibFuncEmittable(M, &TLI, Func))
     return false;
 
   if (Func != LibFunc_sqrt && Func != LibFunc_sqrtf && Func != LibFunc_sqrtl)
     return false;
 
   // If (1) this is a sqrt libcall, (2) we can assume that NAN is not created
   // (because NNAN or the operand arg must not be less than -0.0) and (2) we
   // would not end up lowering to a libcall anyway (which could change the value
   // of errno), then:
   // (1) errno won't be set.
   // (2) it is safe to convert this to an intrinsic call.
   Type *Ty = Call->getType();
   Value *Arg = Call->getArgOperand(0);
   if (TTI.haveFastSqrt(Ty) &&
       (Call->hasNoNaNs() || CannotBeOrderedLessThanZero(Arg, &TLI))) {
     IRBuilder<> Builder(&I);
     IRBuilderBase::FastMathFlagGuard Guard(Builder);
     Builder.setFastMathFlags(Call->getFastMathFlags());
 
     Function *Sqrt = Intrinsic::getDeclaration(M, Intrinsic::sqrt, Ty);
     Value *NewSqrt = Builder.CreateCall(Sqrt, Arg, "sqrt");
     I.replaceAllUsesWith(NewSqrt);
 
     // Explicitly erase the old call because a call with side effects is not
     // trivially dead.
     I.eraseFromParent();
     return true;
   }
 
   return false;
 }
 
+// Check if this array of constants represents a cttz table.
+// Iterate over the elements from \p Table by trying to find/match all
+// the numbers from 0 to \p InputBits that should represent cttz results.
+static bool isCTTZTable(const ConstantDataArray &Table, uint64_t Mul,
+                        uint64_t Shift, uint64_t InputBits) {
+  unsigned Length = Table.getNumElements();
+  if (Length < InputBits || Length > InputBits * 2)
+    return false;
+
+  APInt Mask = APInt::getBitsSetFrom(InputBits, Shift);
+  unsigned Matched = 0;
+
+  for (unsigned i = 0; i < Length; i++) {
+    uint64_t Element = Table.getElementAsInteger(i);
+    if (Element >= InputBits)
+      continue;
+
+    // Check if \p Element matches a concrete answer. It could fail for some
+    // elements that are never accessed, so we keep iterating over each element
+    // from the table. The number of matched elements should be equal to the
+    // number of potential right answers which is \p InputBits actually.
+    if ((((Mul << Element) & Mask.getZExtValue()) >> Shift) == i)
+      Matched++;
+  }
+
+  return Matched == InputBits;
+}
+
+// Try to recognize table-based ctz implementation.
+// E.g., an example in C (for more cases please see the llvm/tests):
+// int f(unsigned x) {
+//    static const char table[32] =
+//      {0, 1, 28, 2, 29, 14, 24, 3, 30,
+//       22, 20, 15, 25, 17, 4, 8, 31, 27,
+//       13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9};
+//    return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
+// }
+// this can be lowered to `cttz` instruction.
+// There is also a special case when the element is 0.
+//
+// Here are some examples or LLVM IR for a 64-bit target:
+//
+// CASE 1:
+// %sub = sub i32 0, %x
+// %and = and i32 %sub, %x
+// %mul = mul i32 %and, 125613361
+// %shr = lshr i32 %mul, 27
+// %idxprom = zext i32 %shr to i64
+// %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz1.table, i64 0,
+// i64 %idxprom %0 = load i8, i8* %arrayidx, align 1, !tbaa !8
+//
+// CASE 2:
+// %sub = sub i32 0, %x
+// %and = and i32 %sub, %x
+// %mul = mul i32 %and, 72416175
+// %shr = lshr i32 %mul, 26
+// %idxprom = zext i32 %shr to i64
+// %arrayidx = getelementptr inbounds [64 x i16], [64 x i16]* @ctz2.table, i64
+// 0, i64 %idxprom %0 = load i16, i16* %arrayidx, align 2, !tbaa !8
+//
+// CASE 3:
+// %sub = sub i32 0, %x
+// %and = and i32 %sub, %x
+// %mul = mul i32 %and, 81224991
+// %shr = lshr i32 %mul, 27
+// %idxprom = zext i32 %shr to i64
+// %arrayidx = getelementptr inbounds [32 x i32], [32 x i32]* @ctz3.table, i64
+// 0, i64 %idxprom %0 = load i32, i32* %arrayidx, align 4, !tbaa !8
+//
+// CASE 4:
+// %sub = sub i64 0, %x
+// %and = and i64 %sub, %x
+// %mul = mul i64 %and, 283881067100198605
+// %shr = lshr i64 %mul, 58
+// %arrayidx = getelementptr inbounds [64 x i8], [64 x i8]* @table, i64 0, i64
+// %shr %0 = load i8, i8* %arrayidx, align 1, !tbaa !8
+//
+// All this can be lowered to @llvm.cttz.i32/64 intrinsic.
+static bool tryToRecognizeTableBasedCttz(Instruction &I) {
+  LoadInst *LI = dyn_cast<LoadInst>(&I);
+  if (!LI)
+    return false;
+
+  Type *AccessType = LI->getType();
+  if (!AccessType->isIntegerTy())
+    return false;
+
+  GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(LI->getPointerOperand());
+  if (!GEP || !GEP->isInBounds() || GEP->getNumIndices() != 2)
+    return false;
+
+  if (!GEP->getSourceElementType()->isArrayTy())
+    return false;
+
+  uint64_t ArraySize = GEP->getSourceElementType()->getArrayNumElements();
+  if (ArraySize != 32 && ArraySize != 64)
+    return false;
+
+  GlobalVariable *GVTable = dyn_cast<GlobalVariable>(GEP->getPointerOperand());
+  if (!GVTable || !GVTable->hasInitializer())
+    return false;
+
+  ConstantDataArray *ConstData =
+      dyn_cast<ConstantDataArray>(GVTable->getInitializer());
+  if (!ConstData)
+    return false;
+
+  if (!match(GEP->idx_begin()->get(), m_ZeroInt()))
+    return false;
+
+  Value *Idx2 = std::next(GEP->idx_begin())->get();
+  Value *X1;
+  uint64_t MulConst, ShiftConst;
+  // FIXME: 64-bit targets have `i64` type for the GEP index, so this match will
+  // probably fail for other (e.g. 32-bit) targets.
+  if (!match(Idx2, m_ZExtOrSelf(
+                       m_LShr(m_Mul(m_c_And(m_Neg(m_Value(X1)), m_Deferred(X1)),
+                                    m_ConstantInt(MulConst)),
+                              m_ConstantInt(ShiftConst)))))
+    return false;
+
+  unsigned InputBits = X1->getType()->getScalarSizeInBits();
+  if (InputBits != 32 && InputBits != 64)
+    return false;
+
+  // Shift should extract top 5..7 bits.
+  if (InputBits - Log2_32(InputBits) != ShiftConst &&
+      InputBits - Log2_32(InputBits) - 1 != ShiftConst)
+    return false;
+
+  if (!isCTTZTable(*ConstData, MulConst, ShiftConst, InputBits))
+    return false;
+
+  auto ZeroTableElem = ConstData->getElementAsInteger(0);
+  bool DefinedForZero = ZeroTableElem == InputBits;
+
+  IRBuilder<> B(LI);
+  ConstantInt *BoolConst = B.getInt1(!DefinedForZero);
+  Type *XType = X1->getType();
+  auto Cttz = B.CreateIntrinsic(Intrinsic::cttz, {XType}, {X1, BoolConst});
+  Value *ZExtOrTrunc = nullptr;
+
+  if (DefinedForZero) {
+    ZExtOrTrunc = B.CreateZExtOrTrunc(Cttz, AccessType);
+  } else {
+    // If the value in elem 0 isn't the same as InputBits, we still want to
+    // produce the value from the table.
+    auto Cmp = B.CreateICmpEQ(X1, ConstantInt::get(XType, 0));
+    auto Select =
+        B.CreateSelect(Cmp, ConstantInt::get(XType, ZeroTableElem), Cttz);
+
+    // NOTE: If the table[0] is 0, but the cttz(0) is defined by the Target
+    // it should be handled as: `cttz(x) & (typeSize - 1)`.
+
+    ZExtOrTrunc = B.CreateZExtOrTrunc(Select, AccessType);
+  }
+
+  LI->replaceAllUsesWith(ZExtOrTrunc);
+
+  return true;
+}
+
 /// This is the entry point for folds that could be implemented in regular
 /// InstCombine, but they are separated because they are not expected to
 /// occur frequently and/or have more than a constant-length pattern match.
 static bool foldUnusualPatterns(Function &F, DominatorTree &DT,
                                 TargetTransformInfo &TTI,
                                 TargetLibraryInfo &TLI) {
   bool MadeChange = false;
   for (BasicBlock &BB : F) {
     // Ignore unreachable basic blocks.
     if (!DT.isReachableFromEntry(&BB))
       continue;
 
     // Walk the block backwards for efficiency. We're matching a chain of
     // use->defs, so we're more likely to succeed by starting from the bottom.
     // Also, we want to avoid matching partial patterns.
     // TODO: It would be more efficient if we removed dead instructions
     // iteratively in this loop rather than waiting until the end.
     for (Instruction &I : make_early_inc_range(llvm::reverse(BB))) {
       MadeChange |= foldAnyOrAllBitsSet(I);
       MadeChange |= foldGuardedFunnelShift(I, DT);
       MadeChange |= tryToRecognizePopCount(I);
       MadeChange |= tryToFPToSat(I, TTI);
       MadeChange |= foldSqrt(I, TTI, TLI);
+      MadeChange |= tryToRecognizeTableBasedCttz(I);
     }
   }
 
   // We're done with transforms, so remove dead instructions.
   if (MadeChange)
     for (BasicBlock &BB : F)
       SimplifyInstructionsInBlock(&BB);
 
static bool runImpl(Function &F, AssumptionCache &AC, TargetTransformInfo &TTI,
   return MadeChange;
 }
 
 void AggressiveInstCombinerLegacyPass::getAnalysisUsage(
     AnalysisUsage &AU) const {
   AU.setPreservesCFG();
   AU.addRequired<AssumptionCacheTracker>();
   AU.addRequired<DominatorTreeWrapperPass>();
   AU.addRequired<TargetLibraryInfoWrapperPass>();
   AU.addRequired<TargetTransformInfoWrapperPass>();
   AU.addPreserved<AAResultsWrapperPass>();
   AU.addPreserved<BasicAAWrapperPass>();
   AU.addPreserved<DominatorTreeWrapperPass>();
   AU.addPreserved<GlobalsAAWrapperPass>();
 }
 
 bool AggressiveInstCombinerLegacyPass::runOnFunction(Function &F) {

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-basics.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -aggressive-instcombine -S < %s | FileCheck %s
+
+;; These cases test lowering of various implementations of table-based ctz
+;; algorithms to the llvm.cttz instruction.
+
+;; C reproducers:
+;; int ctz1 (unsigned x)
+;; {
+;; static const char table[32] =
+;;    {
+;;     0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
+;;      31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+;;    };
+;;   return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
+;; }
+
+;; int ctz2(unsigned x)
+;; {
+;; #define u 0
+;;  static short table[64] =
+;;   {
+;;     32, 0, 1, 12, 2, 6, u, 13, 3, u, 7, u, u, u, u, 14,
+;;     10, 4, u, u, 8, u, u, 25, u, u, u, u, u, 21, 27, 15,
+;;     31, 11, 5, u, u, u, u, u, 9, u, u, 24, u, u, 20, 26,
+;;     30, u, u, u, u, 23, u, 19, 29, u, 22, 18, 28, 17, 16, u
+;;   };
+;; x = (x & -x) * 0x0450FBAF;
+;; return table[x >> 26];
+;; }
+
+;; int ctz3(unsigned x)
+;;{
+;; static int table[32] =
+;;   {
+;;     0, 1, 2, 24, 3, 19, 6, 25, 22, 4, 20, 10, 16, 7, 12, 26,
+;;      31, 23, 18, 5, 21, 9, 15, 11, 30, 17, 8, 14, 29, 13, 28, 27
+;;    };
+;;  if (x == 0) return 32;
+;;  x = (x & -x) * 0x04D7651F;
+;;  return table[x >> 27];
+;; }
+
+;; static const unsigned long long magic = 0x03f08c5392f756cdULL;
+;;
+;; static const int table[64] = {
+;;     0,  1, 12,  2, 13, 22, 17,  3, 14, 33, 23, 36, 18, 58, 28, 4,
+;;    62, 15, 34, 26, 24, 48, 50, 37, 19, 55, 59, 52, 29, 44, 39,  5,
+;;    63, 11, 21, 16, 32, 35, 57, 27, 61, 25, 47, 49, 54, 51, 43, 38,
+;;    10, 20, 31, 56, 60, 46, 53, 42, 9, 30, 45, 41,  8, 40,  7,  6,
+;; };
+;;
+;; int ctz4 (unsigned long long b)
+;; {
+;;    unsigned long long lsb = b & -b;
+;;    return table[(lsb * magic) >> 58];
+;; }
+;;
+;; int ctz5(unsigned x)
+;; {
+;;  static char table[32] =
+;;    {
+;;      0, 1, 2, 24, 3, 19, 6, 25, 22, 4, 20, 10, 16, 7, 12, 26,
+;;      31, 23, 18, 5, 21, 9, 15, 11, 30, 17, 8, 14, 29, 13, 28, 27
+;;    };
+;;  x = (x & -x)*0x04D7651F;
+;;  return table[x >> 27];
+;; }
+
+;; int indexes[] = {
+;;     63, 0, 58, 1, 59, 47, 53, 2,60, 39, 48, 27, 54, 33, 42, 3,
+;;     61, 51, 37, 40, 49, 18, 28, 20, 55, 30, 34, 11, 43, 14, 22, 4,
+;;     62, 57, 46, 52, 38, 26, 32, 41, 50, 36, 17, 19, 29, 10, 13, 21,
+;;     56, 45, 25, 31, 35, 16, 9, 12, 44, 24, 15, 8, 23, 7, 6, 5
+;; };
+;;
+;; int ctz6(unsigned long n)
+;; {
+;;       return indexes[((n & (~n + 1)) * 0x07EDD5E59A4E28C2ull) >> 58];
+;; }
+;;
+;; int ctz8(unsigned v)
+;; {
+;; static const int table[] =
+;; {
+;;  31 ,0 ,1 ,23 ,2 ,18 ,5 ,24 ,21 ,3 ,19 ,9 ,15 ,6 ,11 ,25 ,30 ,22 ,17 ,4 ,20 ;,8 ,14 ,10 ,29 ,16 ,7 ,13 ,28 ,12 ,27 ,26
+;; };
+;; unsigned x =(-v & v);
+;; return table[(unsigned)(x * 0x9AECA3EU) >> 27];
+;; }
+
[e-mail] = internal unnamed_addr constant [32 x i8] c"\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 1
+
+define i32 @ctz1(i32 %x) {
+; CHECK-LABEL: @ctz1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[X:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i32 0, i32 [[TMP0]]
+; CHECK-NEXT:    [[TMP3:%.*]] = trunc i32 [[TMP2]] to i8
+; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz7.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
[e-mail] = internal unnamed_addr constant [64 x i16] [i16 32, i16 0, i16 1, i16 12, i16 2, i16 6, i16 0, i16 13, i16 3, i16 0, i16 7, i16 0, i16 0, i16 0, i16 0, i16 14, i16 10, i16 4, i16 0, i16 0, i16 8, i16 0, i16 0, i16 25, i16 0, i16 0, i16 0, i16 0, i16 0, i16 21, i16 27, i16 15, i16 31, i16 11, i16 5, i16 0, i16 0, i16 0, i16 0, i16 0, i16 9, i16 0, i16 0, i16 24, i16 0, i16 0, i16 20, i16 26, i16 30, i16 0, i16 0, i16 0, i16 0, i16 23, i16 0, i16 19, i16 29, i16 0, i16 22, i16 18, i16 28, i16 17, i16 16, i16 0], align 2
+
+define i32 @ctz2(i32 %x) {
+; CHECK-LABEL: @ctz2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[X:%.*]], i1 false)
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i32 [[TMP0]] to i16
+; CHECK-NEXT:    [[CONV:%.*]] = sext i16 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 72416175
+  %shr = lshr i32 %mul, 26
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [64 x i16], [64 x i16]* @ctz2.table, i64 0, i64 %idxprom
+  %0 = load i16, i16* %arrayidx, align 2
+  %conv = sext i16 %0 to i32
+  ret i32 %conv
+}
+
[e-mail] = internal unnamed_addr constant [32 x i32] [i32 0, i32 1, i32 2, i32 24, i32 3, i32 19, i32 6, i32 25, i32 22, i32 4, i32 20, i32 10, i32 16, i32 7, i32 12, i32 26, i32 31, i32 23, i32 18, i32 5, i32 21, i32 9, i32 15, i32 11, i32 30, i32 17, i32 8, i32 14, i32 29, i32 13, i32 28, i32 27], align 4
+
+define i32 @ctz3(i32 %x) {
+; CHECK-LABEL: @ctz3(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[X:%.*]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[RETURN:%.*]], label [[IF_END:%.*]]
+; CHECK:       if.end:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[X]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X]], 0
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = phi i32 [ [[TMP0]], [[IF_END]] ], [ 32, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i32 [[RETVAL_0]]
+;
+entry:
+  %cmp = icmp eq i32 %x, 0
+  br i1 %cmp, label %return, label %if.end
+
+if.end:                                           ; preds = %entry
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 81224991
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i32], [32 x i32]* @ctz3.table, i64 0, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  br label %return
+
+return:                                           ; preds = %entry, %if.end
+  %retval.0 = phi i32 [ %0, %if.end ], [ 32, %entry ]
+  ret i32 %retval.0
+}
+
+@table = internal unnamed_addr constant [64 x i32] [i32 0, i32 1, i32 12, i32 2, i32 13, i32 22, i32 17, i32 3, i32 14, i32 33, i32 23, i32 36, i32 18, i32 58, i32 28, i32 4, i32 62, i32 15, i32 34, i32 26, i32 24, i32 48, i32 50, i32 37, i32 19, i32 55, i32 59, i32 52, i32 29, i32 44, i32 39, i32 5, i32 63, i32 11, i32 21, i32 16, i32 32, i32 35, i32 57, i32 27, i32 61, i32 25, i32 47, i32 49, i32 54, i32 51, i32 43, i32 38, i32 10, i32 20, i32 31, i32 56, i32 60, i32 46, i32 53, i32 42, i32 9, i32 30, i32 45, i32 41, i32 8, i32 40, i32 7, i32 6], align 4
+
+define i32 @ctz4(i64 %b) {
+; CHECK-LABEL: @ctz4(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.cttz.i64(i64 [[B:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i64 [[B]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i64 0, i64 [[TMP0]]
+; CHECK-NEXT:    [[TMP3:%.*]] = trunc i64 [[TMP2]] to i32
+; CHECK-NEXT:    ret i32 [[TMP3]]
+;
+entry:
+  %sub = sub i64 0, %b
+  %and = and i64 %sub, %b
+  %mul = mul i64 %and, 283881067100198605
+  %shr = lshr i64 %mul, 58
+  %arrayidx = getelementptr inbounds [64 x i32], [64 x i32]* @table, i64 0, i64 %shr
+  %0 = load i32, i32* %arrayidx, align 4
+  ret i32 %0
+}
+
[e-mail] = internal unnamed_addr constant [32 x i8] c"\00\01\02\18\03\13\06\19\16\04\14\0A\10\07\0C\1A\1F\17\12\05\15\09\0F\0B\1E\11\08\0E\1D\0D\1C\1B", align 1
+
+define i32 @ctz5(i32 %x) {
+; CHECK-LABEL: @ctz5(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[X:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i32 0, i32 [[TMP0]]
+; CHECK-NEXT:    [[TMP3:%.*]] = trunc i32 [[TMP2]] to i8
+; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 81224991
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz5.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
[e-mail] = global [64 x i32] [i32 63, i32 0, i32 58, i32 1, i32 59, i32 47, i32 53, i32 2, i32 60, i32 39, i32 48, i32 27, i32 54, i32 33, i32 42, i32 3, i32 61, i32 51, i32 37, i32 40, i32 49, i32 18, i32 28, i32 20, i32 55, i32 30, i32 34, i32 11, i32 43, i32 14, i32 22, i32 4, i32 62, i32 57, i32 46, i32 52, i32 38, i32 26, i32 32, i32 41, i32 50, i32 36, i32 17, i32 19, i32 29, i32 10, i32 13, i32 21, i32 56, i32 45, i32 25, i32 31, i32 35, i32 16, i32 9, i32 12, i32 44, i32 24, i32 15, i32 8, i32 23, i32 7, i32 6, i32 5], align 4
+
+define i32 @ctz6(i64 %n) {
+; CHECK-LABEL: @ctz6(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i64 @llvm.cttz.i64(i64 [[N:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i64 [[N]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i64 63, i64 [[TMP0]]
+; CHECK-NEXT:    [[TMP3:%.*]] = trunc i64 [[TMP2]] to i32
+; CHECK-NEXT:    ret i32 [[TMP3]]
+;
+entry:
+  %add = sub i64 0, %n
+  %and = and i64 %add, %n
+  %mul = mul i64 %and, 571347909858961602
+  %shr = lshr i64 %mul, 58
+  %arrayidx = getelementptr inbounds [64 x i32], [64 x i32]* @ctz6.table, i64 0, i64 %shr
+  %0 = load i32, i32* %arrayidx, align 4
+  ret i32 %0
+}
+
[e-mail] = internal unnamed_addr constant [32 x i32] [i32 31, i32 0, i32 1, i32 23, i32 2, i32 18, i32 5, i32 24, i32 21, i32 3, i32 19, i32 9, i32 15, i32 6, i32 11, i32 25, i32 30, i32 22, i32 17, i32 4, i32 20, i32 8, i32 14, i32 10, i32 29, i32 16, i32 7, i32 13, i32 28, i32 12, i32 27, i32 26], align 4
+
+define i32 @ctz8(i32 %v) {
+; CHECK-LABEL: @ctz8(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[V:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[V]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i32 31, i32 [[TMP0]]
+; CHECK-NEXT:    ret i32 [[TMP2]]
+;
+entry:
+  %sub = sub i32 0, %v
+  %and = and i32 %sub, %v
+  %mul = mul i32 %and, 162449982
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i32], [32 x i32]* @ctz8.table, i64 0, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  ret i32 %0
+}

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-dereferencing-pointer.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -aggressive-instcombine -S < %s | FileCheck %s
+
+;; static const unsigned long long magic = 0x03f08c5392f756cdULL;
+;;
+;; static const int table[64] = {
+;;     0,  1, 12,  2, 13, 22, 17,  3,
+;;   14, 33, 23, 36, 18, 58, 28,  4,
+;;    62, 15, 34, 26, 24, 48, 50, 37,
+;;    19, 55, 59, 52, 29, 44, 39,  5,
+;;    63, 11, 21, 16, 32, 35, 57, 27,
+;;    61, 25, 47, 49, 54, 51, 43, 38,
+;;    10, 20, 31, 56, 60, 46, 53, 42,
+;;     9, 30, 45, 41,  8, 40,  7,  6,
+;; };
+;;
+;; int ctz6 (unsigned long long * const b) {
+;;    return table[(((*b) & -(*b)) * magic) >> 58];
+;; }
+
+@table = internal unnamed_addr constant [64 x i32] [i32 0, i32 1, i32 12, i32 2, i32 13, i32 22, i32 17, i32 3, i32 14, i32 33, i32 23, i32 36, i32 18, i32 58, i32 28, i32 4, i32 62, i32 15, i32 34, i32 26, i32 24, i32 48, i32 50, i32 37, i32 19, i32 55, i32 59, i32 52, i32 29, i32 44, i32 39, i32 5, i32 63, i32 11, i32 21, i32 16, i32 32, i32 35, i32 57, i32 27, i32 61, i32 25, i32 47, i32 49, i32 54, i32 51, i32 43, i32 38, i32 10, i32 20, i32 31, i32 56, i32 60, i32 46, i32 53, i32 42, i32 9, i32 30, i32 45, i32 41, i32 8, i32 40, i32 7, i32 6], align 4
+
+define i32 @ctz6(i64* nocapture readonly %b) {
+; CHECK-LABEL: @ctz6(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = load i64, i64* [[B:%.*]], align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = call i64 @llvm.cttz.i64(i64 [[TMP0]], i1 true)
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i64 [[TMP0]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP2]], i64 0, i64 [[TMP1]]
+; CHECK-NEXT:    [[TMP4:%.*]] = trunc i64 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[TMP4]]
+;
+entry:
+  %0 = load i64, i64* %b, align 8
+  %sub = sub i64 0, %0
+  %and = and i64 %0, %sub
+  %mul = mul i64 %and, 283881067100198605
+  %shr = lshr i64 %mul, 58
+  %arrayidx = getelementptr inbounds [64 x i32], [64 x i32]* @table, i64 0, i64 %shr
+  %1 = load i32, i32* %arrayidx, align 4
+  ret i32 %1
+}

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-non-argument-value.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -aggressive-instcombine -S < %s | FileCheck %s
+
+;; C reproducers:
+;; #include "stdio.h"
+;; unsigned x;
+;;
+;; int test ()
+;; {
+;;  static const char table[32] =
+;;    {
+;;      0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
+;;      31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+;;    };
+;;  return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
+;; }
+;;
+
+@x = global i32 0, align 4
+@.str = private constant [3 x i8] c"%u\00", align 1
[e-mail] = internal constant [32 x i8] c"\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 1
+
+define i32 @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* @x, align 4
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.cttz.i32(i32 [[TMP0]], i1 true)
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[TMP0]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP2]], i32 0, i32 [[TMP1]]
+; CHECK-NEXT:    [[TMP4:%.*]] = trunc i32 [[TMP3]] to i8
+; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[TMP4]] to i32
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %0 = load i32, i32* @x, align 4
+  %sub = sub i32 0, %0
+  %and = and i32 %0, %sub
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @test.table, i64 0, i64 %idxprom
+  %1 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %1 to i32
+  ret i32 %conv
+}

llvm/test/Transforms/AggressiveInstCombine/lower-table-based-cttz-zero-element.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -aggressive-instcombine -S < %s | FileCheck %s
+
[e-mail] = internal constant [32 x i8] c"\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 1
+
+define i32 @ctz1(i32 %x) {
+; CHECK-LABEL: @ctz1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[X:%.*]], i1 true)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP1]], i32 0, i32 [[TMP0]]
+; CHECK-NEXT:    [[TMP3:%.*]] = trunc i32 [[TMP2]] to i8
+; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz1.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}

llvm/test/Transforms/AggressiveInstCombine/negative-lower-table-based-cttz.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -aggressive-instcombine -S < %s | FileCheck %s --implicit-check-not=llvm.cttz
+
+;; These cases should ensure we are not lowering of some wrong implementations
+;; of table-based ctz algorithms to the llvm.cttz instruction.
+
[e-mail] = internal unnamed_addr constant [32 x i8] c"\05\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 1
+
+;; This is a negative test with a wrong table constant.
+
+define i32 @ctz1(i32 %x) {
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz7.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
+;; These are some negative tests with a wrong instruction sequences.
+
[e-mail] = internal unnamed_addr constant [32 x i8] c"\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 1
+
+define i32 @ctz2(i32 %x) {
+entry:
+  %sub = sub i32 1, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz1.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
+define i32 @ctz3(i32 %x) {
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613362
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz1.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
+define i32 @ctz4(i32 %x) {
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 26
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @ctz1.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
+;; This is a negative test with a wrong table size and constants.
+
[e-mail] = internal unnamed_addr constant [128 x i8] c"\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 1
+
+define i32 @ctz5(i32 %x) {
+entry:
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [128 x i8], [128 x i8]* @ctz3.table, i64 0, i64 %idxprom
+  %0 = load i8, i8* %arrayidx, align 1
+  %conv = zext i8 %0 to i32
+  ret i32 %conv
+}
+
+;; A test with an extern global variable representing the table.
+;; extern int table[32];
+;;
+;;  int ctz6(unsigned x) {
+;;    if (x == 0) return 32;
+;;    x = (x & -x) * 0x04D7651F;
+;;    return table[x >> 27];
+;;  }
+
+@table = external global [32 x i32], align 16
+define i32 @ctz6(i32 noundef %x) {
+entry:
+  %cmp = icmp eq i32 %x, 0
+  br i1 %cmp, label %return, label %if.end
+
+if.end:                                           ; preds = %entry
+  %sub = sub i32 0, %x
+  %and = and i32 %sub, %x
+  %mul = mul i32 %and, 81224991
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i32], [32 x i32]* @table, i64 0, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  br label %return
+
+return:                                           ; preds = %entry, %if.end
+  %retval.0 = phi i32 [ %0, %if.end ], [ 32, %entry ]
+  ret i32 %retval.0
+}

llvm/test/Transforms/PhaseOrdering/lower-table-based-cttz.ll

+;; This tests lowering of the implementations of table-based ctz
+;; algorithm to the llvm.cttz instruction in the -O3 case.
+
+;; C producer:
+;; int ctz1 (unsigned x)
+;; {
+;; static const char table[32] =
+;;    {
+;;     0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
+;;      31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+;;    };
+;;   return table[((unsigned)((x & -x) * 0x077CB531U)) >> 27];
+;; }
+;; Compiled as: clang -O3 test.c -S -emit-llvm -Xclang -disable-llvm-optzns
+
+; RUN: opt -O3 -S < %s | FileCheck %s
+
+; CHECK: call i32 @llvm.cttz.i32
+
[e-mail] = internal constant [32 x i8] c"\00\01\1C\02\1D\0E\18\03\1E\16\14\0F\19\11\04\08\1F\1B\0D\17\15\13\10\07\1A\0C\12\06\0B\05\0A\09", align 16
+
+define i32 @ctz1(i32 noundef %x) {
+entry:
+  %x.addr = alloca i32, align 4
+  store i32 %x, ptr %x.addr, align 4
+  %0 = load i32, ptr %x.addr, align 4
+  %1 = load i32, ptr %x.addr, align 4
+  %sub = sub i32 0, %1
+  %and = and i32 %0, %sub
+  %mul = mul i32 %and, 125613361
+  %shr = lshr i32 %mul, 27
+  %idxprom = zext i32 %shr to i64
+  %arrayidx = getelementptr inbounds [32 x i8], ptr @ctz1.table, i64 0, i64 %idxprom
+  %2 = load i8, ptr %arrayidx, align 1
+  %conv = sext i8 %2 to i32
+  ret i32 %conv
+}