[Bug ipa/107196] [13 Regression] llvm-14.0.6 is miscompiles by gcc-13 in -O3: hangs llvm testsuite (inliner seems to break it)

[slyfox at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107196

--- Comment #6 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #4)
> llvm/lib/Target/RISCV/RISCVISelLowering.cpp is huge source and it might have
> undefined code in it.
> Maybe narrow down which function is going wrong.

What would be the best way to do it? I initially tried '#pragma GCC
push_options/#pragma GCC optimize(0)/#pragma GCC pop_options', but the final
result still requires a few headers and RISCVISelLowering.cpp functions to be
outside optimize(0).

I'm afraid I spent on in a few days and filed the bug in hopes there is a nicer
technique to nail down the specific transformation that changes the behaviour.

If there is no better way to do it I'll start sprinkling
__attribute__((noinline, noipa)) around to get broken and working variants
closer.

> Maybe fire up gdb and see where the infinite loop is located too.

Looking at gdb the problem is not very clear: generic (not RISCV-specific code)
at '#8  Run () at llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:1538'
implements some instruction selection logic to pick better instructions
(DAGCombiner). Normally it stabilizes, but in this case it does not finish.
Heavy use of STL-like data structures do not make it easier to trace through in
gdb to see where first deviation bet ween good/bad happens. llvm has debugging
mode to dump the transformations applied but enabling it at build time make the
bug to go away.

Full backtrace:

llvm::DenseMapBase<llvm::DenseMap<llvm::SDNode*, unsigned int,
llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >, llvm::SDNode*,
unsigned int, llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int>
>::LookupBucketFor<llvm::SDNode*> () at llvm/include/llvm/ADT/DenseMap.h:632
632           if (LLVM_LIKELY(KeyInfoT::isEqual(Val, ThisBucket->getFirst())))
{
(gdb) bt
#0  llvm::DenseMapBase<llvm::DenseMap<llvm::SDNode*, unsigned int,
llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >, llvm::SDNode*,
unsigned int, llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int>
>::LookupBucketFor<llvm::SDNode*> ()
    at llvm/include/llvm/ADT/DenseMap.h:632
#1  llvm::DenseMapBase<llvm::DenseMap<llvm::SDNode*, unsigned int,
llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >, llvm::SDNode*,
unsigned int, llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int>
>::LookupBucketFor<llvm::SDNode*> ()
    at llvm/include/llvm/ADT/DenseMap.h:663
#2  llvm::DenseMapBase<llvm::DenseMap<llvm::SDNode*, unsigned int,
llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >, llvm::SDNode*,
unsigned int, llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >::try_emplace<unsigned
int> ()
    at llvm/include/llvm/ADT/DenseMap.h:225
#3  llvm::DenseMapBase<llvm::DenseMap<llvm::SDNode*, unsigned int,
llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >, llvm::SDNode*,
unsigned int, llvm::DenseMapInfo<llvm::SDNode*, void>,
llvm::detail::DenseMapPair<llvm::SDNode*, unsigned int> >::insert ()
    at llvm/include/llvm/ADT/DenseMap.h:216
#4  AddToWorklist () at llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:273
#5  0x00000000014f12e8 in AddToWorklist () at
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:268
#6  AddUsersToWorklist () at llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:195
#7  AddToWorklistWithUsers () at
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:200
#8  Run () at llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:1538
#9  llvm::SelectionDAG::Combine () at
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:24210
#10 0x00000000015ea7bf in llvm::SelectionDAGISel::CodeGenAndEmitDAG ()
    at llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp:958
#11 0x00000000015ed2c5 in llvm::SelectionDAGISel::SelectAllBasicBlocks ()
    at llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp:1634
#12 0x00000000015efd8b in llvm::SelectionDAGISel::runOnMachineFunction ()
    at llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp:515
#13 0x0000000000ba4f34 in llvm::MachineFunctionPass::runOnFunction () at
llvm/lib/CodeGen/MachineFunctionPass.cpp:72
#14 llvm::MachineFunctionPass::runOnFunction () at
llvm/lib/CodeGen/MachineFunctionPass.cpp:37
#15 0x0000000000ff87b0 in llvm::FPPassManager::runOnFunction () at
llvm/lib/IR/LegacyPassManager.cpp:1434
#16 0x0000000000ff8b3c in llvm::FPPassManager::runOnModule () at
llvm/lib/IR/LegacyPassManager.cpp:1480
#17 0x0000000000ffa39e in runOnModule () at
llvm/lib/IR/LegacyPassManager.cpp:1549
#18 llvm::legacy::PassManagerImpl::run () at
llvm/lib/IR/LegacyPassManager.cpp:539
#19 0x000000000065a38a in compileModule () at llvm/tools/llc/llc.cpp:719
#20 main () at llvm/tools/llc/llc.cpp:417