[WebAssembly] Crash when compiling wasm32 with strict floating point exception behaviour

[zebreus]

When compiling C code for wasm32, a compiler crash occurs if the code has strict floating-point exceptions enabled.

The crash happens in the WebAssembly Instruction Selection pass. The issue can be reproduced directly with llc on the generated LLVM IR, confirming it's a backend bug and not specific to the Clang frontend. The crash does not occur when compiling for a native target like x86_64.

Environment

• LLVM Version: Ubuntu clang version 20.1.2 (0ubuntu1)
• Target: wasm32-unknown-wasi
• Host OS: Ubuntu (x86_64)

Steps to Reproduce

1. Create a C file named repro.c with the following content:

   #pragma clang fp exceptions(strict)
   float a;
   void b() {
     0 || a;
   }

2. Compile the file for the wasm32 target. The compiler will crash.

   clang -S -emit-llvm -c --target=wasm32-wasi repro.c

repro.ll

; ModuleID = 'repro.c'
source_filename = "repro.c"
target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-i128:128-n32:64-S128-ni:1:10:20"
target triple = "wasm32-unknown-wasi"

@a = hidden global float 0.000000e+00, align 4

; Function Attrs: noinline nounwind optnone strictfp
define hidden void @b() #0 {
  %1 = load float, ptr @a, align 4
  %2 = call i1 @llvm.experimental.constrained.fcmp.f32(float %1, float 0.000000e+00, metadata !"une", metadata !"fpexcept.strict") #2
  %3 = zext i1 %2 to i32
  ret void
}

; Function Attrs: nocallback nofree nosync nounwind strictfp willreturn memory(inaccessiblemem: readwrite)
declare i1 @llvm.experimental.constrained.fcmp.f32(float, float, metadata, metadata) #1

attributes #0 = { noinline nounwind optnone strictfp "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="generic" "target-features"="+bulk-memory,+bulk-memory-opt,+call-indirect-overlong,+multivalue,+mutable-globals,+nontrapping-fptoint,+reference-types,+sign-ext" }
attributes #1 = { nocallback nofree nosync nounwind strictfp willreturn memory(inaccessiblemem: readwrite) }
attributes #2 = { strictfp }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"Ubuntu clang version 20.1.2 (0ubuntu1)"}

3. Run llc on the generated repro.ll file. It will crash with a segmentation fault.

   $ llc repro.ll
   PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
   Stack dump:
   0.      Program arguments: llc repro.ll
   1.      Running pass 'Function Pass Manager' on module 'repro.ll'.
   2.      Running pass 'WebAssembly Instruction Selection' on function '@b'
    #0 0x000079fd5d41dd1f llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/usr/lib/llvm-20/bin/../lib/libLLVM.so.20.1+0x101dd1f)
    #1 0x000079fd5d41ba29 llvm::sys::RunSignalHandlers() (/usr/lib/llvm-20/bin/../lib/libLLVM.so.20.1+0x101ba29)
    #2 0x000079fd5d41e430 (/usr/lib/llvm-20/bin/../lib/libLLVM.so.20.1+0x101e430)
    #3 0x000079fd5bc45810 (/lib/x86_64-linux-gnu/libc.so.6+0x45810)
   ...

Expected Behavior

The code should compile or there should be a real error message

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Zebreus (zebreus)

When compiling C code for `wasm32`, a compiler crash occurs if the code has strict floating-point exceptions enabled.

The crash happens in the WebAssembly Instruction Selection pass. The issue can be reproduced directly with llc on the generated LLVM IR, confirming it's a backend bug and not specific to the Clang frontend. The crash does not occur when compiling for a native target like x86_64.

Environment

• LLVM Version: Ubuntu clang version 20.1.2 (0ubuntu1)
• Target: wasm32-unknown-wasi
• Host OS: Ubuntu (x86_64)

Steps to Reproduce

1. Create a C file named repro.c with the following content:

   #pragma clang fp exceptions(strict)
   float a;
   void b() {
     0 || a;
   }

2. Compile the file for the wasm32 target. The compiler will crash.

   clang -S -emit-llvm -c --target=wasm32-wasi repro.c

<details>
<summary>repro.ll</summary>

; ModuleID = 'repro.c'
source_filename = "repro.c"
target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-i128:128-n32:64-S128-ni:1:10:20"
target triple = "wasm32-unknown-wasi"

@<!-- -->a = hidden global float 0.000000e+00, align 4

; Function Attrs: noinline nounwind optnone strictfp
define hidden void @<!-- -->b() #<!-- -->0 {
  %1 = load float, ptr @<!-- -->a, align 4
  %2 = call i1 @<!-- -->llvm.experimental.constrained.fcmp.f32(float %1, float 0.000000e+00, metadata !"une", metadata !"fpexcept.strict") #<!-- -->2
  %3 = zext i1 %2 to i32
  ret void
}

; Function Attrs: nocallback nofree nosync nounwind strictfp willreturn memory(inaccessiblemem: readwrite)
declare i1 @<!-- -->llvm.experimental.constrained.fcmp.f32(float, float, metadata, metadata) #<!-- -->1

attributes #<!-- -->0 = { noinline nounwind optnone strictfp "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="generic" "target-features"="+bulk-memory,+bulk-memory-opt,+call-indirect-overlong,+multivalue,+mutable-globals,+nontrapping-fptoint,+reference-types,+sign-ext" }
attributes #<!-- -->1 = { nocallback nofree nosync nounwind strictfp willreturn memory(inaccessiblemem: readwrite) }
attributes #<!-- -->2 = { strictfp }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"Ubuntu clang version 20.1.2 (0ubuntu1)"}

</details>

3. Run llc on the generated repro.ll file. It will crash with a segmentation fault.

   $ llc repro.ll
   PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
   Stack dump:
   0.      Program arguments: llc repro.ll
   1.      Running pass 'Function Pass Manager' on module 'repro.ll'.
   2.      Running pass 'WebAssembly Instruction Selection' on function '@<!-- -->b'
    #<!-- -->0 0x000079fd5d41dd1f llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/usr/lib/llvm-20/bin/../lib/libLLVM.so.20.1+0x101dd1f)
    #<!-- -->1 0x000079fd5d41ba29 llvm::sys::RunSignalHandlers() (/usr/lib/llvm-20/bin/../lib/libLLVM.so.20.1+0x101ba29)
    #<!-- -->2 0x000079fd5d41e430 (/usr/lib/llvm-20/bin/../lib/libLLVM.so.20.1+0x101e430)
    #<!-- -->3 0x000079fd5bc45810 (/lib/x86_64-linux-gnu/libc.so.6+0x45810)
   ...

Expected Behavior

The code should compile or there should be a real error message