[PATCH] D76319: [DAGCombiner] Fix non-determinism problem related to argument evaluation order in visitFDIV

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:13052-13055
+    SDValue Neg0 =
+        TLI.getNegatedExpression(N0, DAG, LegalOperations, ForCodeSize);
+    SDValue Neg1 =
+        TLI.getNegatedExpression(N1, DAG, LegalOperations, ForCodeSize);
----------------
spatel wrote:
> spatel wrote:
> > If you reverse the order of these statements, does the gcc-built-LLVM still crash? Ie, is this a reliable/reproducible failure and fix?
> Oops - just re-read the description text, and the answer is "yes".
> Can you post the exact assert/unreachable that you are seeing?
I didn't have the exact stack trace from the x86 reproducer at hand, but the below should give the picture.

And as you may have discovered I get the crash both when building llc using clang and gcc, as soon as I reverse the order here. So it should hopefully be easy to reproduce it (unless there is something even nastier involved).

```
Unknown code
UNREACHABLE executed at ../lib/CodeGen/SelectionDAG/TargetLowering.cpp:5812!
Stack dump:
 #0 0x00000000027c7de5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/.../compiler-clang/bin/llc+0x27c7de5)
 #1 0x00000000027c5a94 llvm::sys::RunSignalHandlers() (/.../compiler-clang/bin/llc+0x27c5a94)
 #2 0x00000000027c5bd3 SignalHandler(int) (/.../compiler-clang/bin/llc+0x27c5bd3)
 #3 0x00007fa8af489630 __restore_rt (/lib64/libpthread.so.0+0xf630)
 #4 0x00007fa8ae224377 raise (/lib64/libc.so.6+0x36377)
 #5 0x00007fa8ae225a68 abort (/lib64/libc.so.6+0x37a68)
 #6 0x00000000027463fa (/.../compiler-clang/bin/llc+0x27463fa)
 #7 0x000000000264fbcc llvm::TargetLowering::getNegatedExpression(llvm::SDValue, llvm::SelectionDAG&, bool, bool, unsigned int) const (/.../compiler-clang/bin/llc+0x264fbcc)
 #8 0x000000000264f620 llvm::TargetLowering::getNegatedExpression(llvm::SDValue, llvm::SelectionDAG&, bool, bool, unsigned int) const (/.../compiler-clang/bin/llc+0x264f620)
 #9 0x00000000025215c3 (anonymous namespace)::DAGCombiner::visitFDIV(llvm::SDNode*) (/.../compiler-clang/bin/llc+0x25215c3)
#10 0x0000000002523ed1 (anonymous namespace)::DAGCombiner::visit(llvm::SDNode*) (/.../compiler-clang/bin/llc+0x2523ed1)
#11 0x000000000252525c (anonymous namespace)::DAGCombiner::combine(llvm::SDNode*) (/.../compiler-clang/bin/llc+0x252525c)
#12 0x0000000002526d8d (anonymous namespace)::DAGCombiner::Run(llvm::CombineLevel) (/.../compiler-clang/bin/llc+0x2526d8d)
#13 0x0000000002529000 llvm::SelectionDAG::Combine(llvm::CombineLevel, llvm::AAResults*, llvm::CodeGenOpt::Level) (/.../compiler-clang/bin/llc+0x2529000)
#14 0x0000000002626330 llvm::SelectionDAGISel::CodeGenAndEmitDAG() (/.../compiler-clang/bin/llc+0x2626330)
#15 0x000000000262ad58 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/.../compiler-clang/bin/llc+0x262ad58)
#16 0x000000000262c709 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (.part.868) (/.../compiler-clang/bin/llc+0x262c709)
#17 0x000000000096e709 (anonymous namespace)::MyDAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/.../compiler-clang/bin/llc+0x96e709)
#18 0x0000000001d1b01f llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (/.../compiler-clang/bin/llc+0x1d1b01f)
#19 0x00000000020e564e llvm::FPPassManager::runOnFunction(llvm::Function&) (/.../compiler-clang/bin/llc+0x20e564e)
#20 0x00000000020e60f9 llvm::FPPassManager::runOnModule(llvm::Module&) (/.../compiler-clang/bin/llc+0x20e60f9)
#21 0x00000000020e64c1 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/.../compiler-clang/bin/llc+0x20e64c1)
#22 0x00000000006bb219 main (/.../compiler-clang/bin/llc+0x6bb219)
```






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