[llvm] [InstCombine] Fold bitreverse.v8i8(bitcast(i64)) into bswap+bitreverse (PR #207828)

Mon Jul 6 13:49:48 PDT 2026

dtcxzyw wrote:

The following correctness issue was found by [llvm-hackme](https://github.com/dtcxzyw/llvm-hackme).

<!-- llvm-hackme-state: bug_found -->
<!-- llvm-hackme-baseline: 09f2062da3b34b7db167268b05c9a9a5bd3c7e17 -->
<!-- llvm-hackme-head-sha: 77153736c2355d23974208c9ca638dddd925bb12 -->
<!-- llvm-hackme-patch-sha256: 64cbbd252ae945d922769d5d05254a0d364a49fc2db9419c2cda4f6f60bf71cc -->
<!-- llvm-hackme-kind: crash -->

This comment is generated by an automated correctness checking service designed to help identify critical correctness bugs (opt crashes or Alive2 miscompilations) and improve PR review efficiency under limited reviewer bandwidth.

The reproducer was generated by LLM.

This bug is a regression introduced by this PR. It does not reproduce on the baseline commit.

## Reproducer

**Kind**: crash

**IR Reproducer**:
```llvm
; RUN: opt -passes=instcombine<no-verify-fixpoint> -S
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define i24 @test(i24 %x) {
  %1 = bitcast i24 %x to <3 x i8>
  %2 = call <3 x i8> @llvm.bitreverse.v3i8(<3 x i8> %1)
  %3 = bitcast <3 x i8> %2 to i24
  ret i24 %3
}
```

**Stacktrace**:
```
bswap must be an even number of bytes
  %1 = call i24 @llvm.bswap.i24(i24 %x)
LLVM ERROR: Broken module found, compilation aborted!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0.	Program arguments: /llvm-hackme/work/llvm-hackme/llvm-build-pr/bin/opt -S -o /dev/null /tmp/tmput5hx20m.ll -passes=instcombine<no-verify-fixpoint>
1.	Running pass "verify" on module "/tmp/tmput5hx20m.ll"
 #0 0x00007ffff7e176a2 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/lib/Support/Unix/Signals.inc:881:22
 #1 0x00007ffff7e1434c llvm::sys::RunSignalHandlers() /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/lib/Support/Signals.cpp:108:20
 #2 0x00007ffff7e14551 SignalHandler(int, siginfo_t*, void*) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/lib/Support/Unix/Signals.inc:448:14
 #3 0x00007ffff7a19520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007ffff7a6d9fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007ffff7a19476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007ffff79ff7f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007ffff7c758f8 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() /usr/include/c++/11/bits/basic_string.h:672:19
 #8 0x00007ffff7c758f8 llvm::report_fatal_error(llvm::Twine const&, bool) (.cold) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/lib/Support/ErrorHandling.cpp:117:36
 #9 0x00007ffff7d1b2e5 (/llvm-hackme/work/llvm-hackme/llvm-build-pr/bin/../lib/libLLVMSupport.so.23.0git+0x11b2e5)
#10 0x00007fffedf3bc60 llvm::VerifierPass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/lib/IR/Verifier.cpp:8014:79
#11 0x00007ffff7f84d36 llvm::detail::PassModel<llvm::Module, llvm::VerifierPass, llvm::AnalysisManager<llvm::Module> >::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/include/llvm/IR/PassManagerInternal.h:94:3
#12 0x00007fffedeffc97 llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module> >::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/include/llvm/IR/PassManagerImpl.h:80:18
#13 0x00007ffff7f91a5b llvm::SmallPtrSetImplBase::~SmallPtrSetImplBase() /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/include/llvm/ADT/SmallPtrSet.h:89:5
#14 0x00007ffff7f91a5b llvm::SmallPtrSetImpl<llvm::AnalysisKey*>::~SmallPtrSetImpl() /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/include/llvm/ADT/SmallPtrSet.h:365:35
#15 0x00007ffff7f91a5b llvm::SmallPtrSet<llvm::AnalysisKey*, 2u>::~SmallPtrSet() /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/include/llvm/ADT/SmallPtrSet.h:532:7
#16 0x00007ffff7f91a5b llvm::PreservedAnalyses::~PreservedAnalyses() /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/include/llvm/IR/Analysis.h:112:7
#17 0x00007ffff7f91a5b llvm::runPassPipeline(llvm::StringRef, llvm::Module&, llvm::TargetMachine*, llvm::TargetLibraryInfoImpl*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::StringRef, llvm::ArrayRef<llvm::PassPlugin>, llvm::ArrayRef<std::function<void (llvm::PassBuilder&)> >, llvm::opt_tool::OutputKind, llvm::opt_tool::VerifierKind, bool, bool, bool, bool, bool, bool, bool, bool) /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/tools/opt/NewPMDriver.cpp:572:10
#18 0x00007ffff7f9ee84 optMain /llvm-hackme/work/llvm-hackme/llvm-build-pr/../llvm-project-pr/llvm/tools/opt/optdriver.cpp:798:25
#19 0x00007ffff7a00d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#20 0x00007ffff7a00e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#21 0x0000555555555095 _start (/llvm-hackme/work/llvm-hackme/llvm-build-pr/bin/opt+0x1095)
```

**Baseline Revision**: `09f2062da3b34b7db167268b05c9a9a5bd3c7e17`
**PR Head SHA**: `77153736c2355d23974208c9ca638dddd925bb12`
**Patch SHA256**: `64cbbd252ae945d922769d5d05254a0d364a49fc2db9419c2cda4f6f60bf71cc`

https://github.com/llvm/llvm-project/pull/207828