clang crashes at -Os on x86_64-linux-gnu: Assertion `... " VPlan cost model and legacy cost model disagreed"' failed

[zhendongsu]

It appears to be a regression from 19.1.0 and affects 20.1.0 and later.

Compiler Explorer: https://godbolt.org/z/K3Y3zn4hP

[515] % clangtk -v
clang version 21.0.0git (https://github.com/llvm/llvm-project.git da3ee9763266f9adedaac1b1b3162d14f951fc55)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /local/suz-local/software/local/clang-trunk/bin
Build config: +assertions
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64
[516] % 
[516] % clangtk -Os small.c
clang-21: /local/suz-local/software/clangbuild/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7574: llvm::VectorizationFactor llvm::LoopVectorizationPlanner::computeBestVF(): Assertion `(BestFactor.Width == LegacyVF.Width || PlanForEarlyExitLoop || planContainsAdditionalSimplifications(getPlanFor(BestFactor.Width), CostCtx, OrigLoop) || planContainsAdditionalSimplifications(getPlanFor(LegacyVF.Width), CostCtx, OrigLoop)) && " VPlan cost model and legacy cost model disagreed"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /local/suz-local/software/local/clang-trunk/bin/clang-21 -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -dumpdir a- -disable-free -clear-ast-before-backend -main-file-name small.c -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=none -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/local/suz-local/software/emitesting/bugs/20250313-clangtk-m64-Os-build-082148/delta -fcoverage-compilation-dir=/local/suz-local/software/emitesting/bugs/20250313-clangtk-m64-Os-build-082148/delta -resource-dir /local/suz-local/software/local/clang-trunk/lib/clang/21 -I /usr/local/include -I /local/suz-local/software/local/include -internal-isystem /local/suz-local/software/local/clang-trunk/lib/clang/21/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -Os -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcolor-diagnostics -vectorize-loops -vectorize-slp -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/small-d02bf4.o -x c small.c
1.	<eof> parser at end of file
2.	Optimizer
3.	Running pass "function<eager-inv>(float2int,lower-constant-intrinsics,loop(loop-rotate<header-duplication;no-prepare-for-lto>,loop-deletion),loop-distribute,inject-tli-mappings,loop-vectorize<no-interleave-forced-only;no-vectorize-forced-only;>,infer-alignment,loop-load-elim,instcombine<max-iterations=1;no-verify-fixpoint>,simplifycfg<bonus-inst-threshold=1;forward-switch-cond;switch-range-to-icmp;switch-to-lookup;no-keep-loops;hoist-common-insts;no-hoist-loads-stores-with-cond-faulting;sink-common-insts;speculate-blocks;simplify-cond-branch;no-speculate-unpredictables>,slp-vectorizer,vector-combine,instcombine<max-iterations=1;no-verify-fixpoint>,loop-unroll<O2>,transform-warning,sroa<preserve-cfg>,infer-alignment,instcombine<max-iterations=1;no-verify-fixpoint>,loop-mssa(licm<allowspeculation>),alignment-from-assumptions,loop-sink,instsimplify,div-rem-pairs,tailcallelim,simplifycfg<bonus-inst-threshold=1;no-forward-switch-cond;switch-range-to-icmp;no-switch-to-lookup;keep-loops;no-hoist-common-insts;hoist-loads-stores-with-cond-faulting;no-sink-common-insts;speculate-blocks;simplify-cond-branch;speculate-unpredictables>)" on module "small.c"
4.	Running pass "loop-vectorize<no-interleave-forced-only;no-vectorize-forced-only;>" on function "main"
 #0 0x0000563ced471fcf llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x4577fcf)
 #1 0x0000563ced46f7a4 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #2 0x00007f1c5b61c420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #3 0x00007f1c5b05300b raise /build/glibc-LcI20x/glibc-2.31/signal/../sysdeps/unix/sysv/linux/raise.c:51:1
 #4 0x00007f1c5b032859 abort /build/glibc-LcI20x/glibc-2.31/stdlib/abort.c:81:7
 #5 0x00007f1c5b032729 get_sysdep_segment_value /build/glibc-LcI20x/glibc-2.31/intl/loadmsgcat.c:509:8
 #6 0x00007f1c5b032729 _nl_load_domain /build/glibc-LcI20x/glibc-2.31/intl/loadmsgcat.c:970:34
 #7 0x00007f1c5b043fd6 (/lib/x86_64-linux-gnu/libc.so.6+0x33fd6)
 #8 0x0000563ceeecce93 llvm::LoopVectorizationPlanner::computeBestVF() (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5fd2e93)
 #9 0x0000563ceeee48a6 llvm::LoopVectorizePass::processLoop(llvm::Loop*) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5fea8a6)
#10 0x0000563ceeee72a1 llvm::LoopVectorizePass::runImpl(llvm::Function&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5fed2a1)
#11 0x0000563ceeee7943 llvm::LoopVectorizePass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5fed943)
#12 0x0000563ceea5a6d6 llvm::detail::PassModel<llvm::Function, llvm::LoopVectorizePass, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5b606d6)
#13 0x0000563cecde27c9 llvm::PassManager<llvm::Function, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x3ee87c9)
#14 0x0000563cea6cc7a6 llvm::detail::PassModel<llvm::Function, llvm::PassManager<llvm::Function, llvm::AnalysisManager<llvm::Function>>, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x17d27a6)
#15 0x0000563cecde1032 llvm::ModuleToFunctionPassAdaptor::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x3ee7032)
#16 0x0000563cea6ca3b6 llvm::detail::PassModel<llvm::Module, llvm::ModuleToFunctionPassAdaptor, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x17d03b6)
#17 0x0000563cecde0a51 llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x3ee6a51)
#18 0x0000563ced72fe4a (anonymous namespace)::EmitAssemblyHelper::RunOptimizationPipeline(clang::BackendAction, std::unique_ptr<llvm::raw_pwrite_stream, std::default_delete<llvm::raw_pwrite_stream>>&, std::unique_ptr<llvm::ToolOutputFile, std::default_delete<llvm::ToolOutputFile>>&, clang::BackendConsumer*) BackendUtil.cpp:0:0
#19 0x0000563ced7338a1 clang::emitBackendOutput(clang::CompilerInstance&, clang::CodeGenOptions&, llvm::StringRef, llvm::Module*, clang::BackendAction, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>, std::unique_ptr<llvm::raw_pwrite_stream, std::default_delete<llvm::raw_pwrite_stream>>, clang::BackendConsumer*) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x48398a1)
#20 0x0000563cede73d91 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x4f79d91)
#21 0x0000563cef8016dc clang::ParseAST(clang::Sema&, bool, bool) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x69076dc)
#22 0x0000563cede741b8 clang::CodeGenAction::ExecuteAction() (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x4f7a1b8)
#23 0x0000563cee14abc9 clang::FrontendAction::Execute() (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5250bc9)
#24 0x0000563cee0cbece clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x51d1ece)
#25 0x0000563cee23f996 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x5345996)
#26 0x0000563cea25f9ed cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x13659ed)
#27 0x0000563cea25715a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#28 0x0000563cea25b3c7 clang_main(int, char**, llvm::ToolContext const&) (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x13613c7)
#29 0x0000563cea1400db main (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x12460db)
#30 0x00007f1c5b034083 __libc_start_main /build/glibc-LcI20x/glibc-2.31/csu/../csu/libc-start.c:342:3
#31 0x0000563cea256bee _start (/local/suz-local/software/local/clang-trunk/bin/clang-21+0x135cbee)
clangtk: error: unable to execute command: Aborted
clangtk: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 21.0.0git (https://github.com/llvm/llvm-project.git da3ee9763266f9adedaac1b1b3162d14f951fc55)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /local/suz-local/software/local/clang-trunk/bin
Build config: +assertions
clangtk: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clangtk: note: diagnostic msg: /tmp/small-7bdf6a.c
clangtk: note: diagnostic msg: /tmp/small-7bdf6a.sh
clangtk: note: diagnostic msg: 

********************
[517] % 
[517] % cat small.c
int printf(const char *, ...);
int a, c, d, e;
static int b = 2;
static void f() {
  printf("0\n");
  for (c = 0; c < 3; c++)
    if (d && c >= b && e)
      a = 0;
}
int main() {
  b = 0;
  f();
  return 0;
}

[cardigan1008]

Here is another case that triggers the same crash at -O2:

int a, b;
char c;
void f(int g) {
  int d;
  for (int e = 0; e < g; e++)
    d = e;
  a = d;
}
void h() {
  if (c)
    b = 0;
  else
    b = 1;
  f(b + 42);
}
void main() {}

Compiler Explorer: https://godbolt.org/z/zY6GT5eox

Bisected to 26324bc, which was committed by @lukel97

[lukel97]

Hi @zhendongsu @cardigan1008, thanks for reporting/reducing/bisecting this. Can you confirm that this is fixed now after eef5ea0?

[cardigan1008]

Hi @lukel97 my case has been fixed. But the first case seems to still trigger the same crash at -Os.

My clang version: clang version 21.0.0git (https://github.com/llvm/llvm-project.git b3c5031)

[lukel97]

@cardigan1008 Looks like that's a separate vplan-legacy cost model mismatch, it seems to be reproducible before 26324bc. I'll leave this issue open