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[SelectionDAG] Support multiple dangling debug info for one value
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Authored by bjope on Mar 18 2018, 12:56 PM.

Details

Summary

When building the selection DAG we sometimes need to postpone
the handling of a dbg.value until the value it should refer to
is created. This is done by using the DanglingDebugInfoMap.
In the past this map has been limited to hold one dangling
dbg.value per value. This patch removes that restriction.

Diff Detail

Event Timeline

bjope created this revision.Mar 18 2018, 12:56 PM
Ka-Ka added a subscriber: Ka-Ka.Mar 19 2018, 6:45 AM
JDevlieghere removed a subscriber: aprantl.
rnk added a comment.Mar 19 2018, 8:55 AM

Thanks, this seems like an important fix. :) Tons of dbg.values for redundant inlined copies of this use the same base value.

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
120

Embedding SmallVector in DenseMap is usually an efficiency trap. DenseMap stores keys and values in the table, not in separate allocations, so the value of every unfilled hashtable entry is wasted. Even then, filled slots typically have one dbg.value.

TinyPtrVector was created for this use case, but it looks like the elements are not pointers so it will not work.

I guess I'd suggest using std::vector or SmallVector<DangingDebugInfo, 0> to minimize wasted inline storage.

aprantl accepted this revision.Mar 19 2018, 9:04 AM

Thanks! LGTM with Reid's comment addressed.

This revision is now accepted and ready to land.Mar 19 2018, 9:04 AM
aprantl added a reviewer: vsk.Mar 19 2018, 9:04 AM
bjope added inline comments.Mar 19 2018, 9:20 AM
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
120

Thanks a lot for the feedback. I was a little bit worried about this part. I haven't spent any time on optimizing it really, and this first draft was aiming at getting it working in some simple way.

Now when I got the feedback that this patch is on track regarding the functionality I will go on doing some tests regarding impact on compile speed/memory consumption. After all, these problems were seen when analysing https://bugs.llvm.org/show_bug.cgi?id=36417, so it would be a pity if we end up with bad compilation performance again when solving this problem.

rnk added inline comments.Mar 19 2018, 10:37 AM
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
120

That's always possible, but I suspect any perf degradation will come from the extra DBG_VALUE MachineInstrs inserted after this pass, and not this data structure. Anyway, SmallVector 0 is three pointers big, and DanglingDbgInfo is three, so in the common case, there's not much change.

One way to build confidence that this isn't introducing a major regression is to measure the time to compile the optimized sqlite3.c amalgamation with debug info before and after your change. You can use perf -r 5 on Linux to get error bars to get a sense of how precise your comparison is, and whether it's in the noise or not.

bjope added inline comments.Mar 20 2018, 8:19 AM
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
120

std:vector seems to use less heap (compared to SmallVector) when running the small test/DebugInfo/X86/sdag-dangling-dbgvalue.ll test case.

The sqlite3.c test does not seem to result in any dangling debug info during the initial SDAG build, so this patch has no impact really.

When considering C++ and this pointers, as mentioned earlier by someone, we do not get any dangling debug info afaict. The this pointer will be an input arguement to the function, so the value is selected before any dbg.value instructions that reference it.

Normally the amount of dangling debug info should be quite small (there could be some nasty code out there so you never now for the general case). I do not think memory consumption should be a problem. The costly part (CPU wise) is probably the linear search in SelectionDAGBuilder::dropDanglingDebugInfo to find all dangling debug info that needs to be dropped. Now we can have more dangling debug info nodes to inspect, although the complexity is the same.

I'll will replace the the SmallVector by a std::vector and then I assume that I can land this.

bjope updated this revision to Diff 139131.Mar 20 2018, 8:24 AM

DanglingDebugInfoVector is not a std::vector.
Rebased to latest on trunk.

To see any difference you probably need to compile a heavily templated C++ program with lots of inlining. That said, I don't think that this patch will have a huge performance impact even there, since we don't propagate dbg.values beyond basic blocks outside of their lexical scopes.

This revision was automatically updated to reflect the committed changes.