[cfe-dev] [wasm32 target] Float cast after !! can be wrong when compiled with -x c++

Bernhard - via cfe-dev cfe-dev at lists.llvm.org
Wed Nov 6 09:33:48 PST 2019

Hi all

With all llvm/clang versions that support the wasm32 target (8, 9, current build of 10) the following function returns the correct 0.0f when compiled with '-x c' but it wrongfully returns 1.0f when compiled with '-x c++'.

    #ifdef __cplusplus
    extern "C"
    float test()
        static unsigned int mask = 2;
        mask |= 4;
        return (float)(!!(mask & 1));

Here is how I built it to a wasm module:
    clang -x c -nostdinc -O3 -target wasm32 -c -o test.c.o test.c
    wasm-ld -no-entry -export=test -o test.c.wasm test.c.o

    clang -x c++ -nostdinc -O3 -target wasm32 -c -o test.cpp.o test.c
    wasm-ld -no-entry -export=test -o test.cpp.wasm test.cpp.o

And here is how I run the test in node.js v12
    node -e "WebAssembly.instantiate(new Uint8Array(require('fs').readFileSync('test.c.wasm'))).then(m => { console.log('c: ' + m.instance.exports.test()); })"
    node -e "WebAssembly.instantiate(new Uint8Array(require('fs').readFileSync('test.cpp.wasm'))).then(m => { console.log('cpp: ' + m.instance.exports.test()); })"

The output I get is
    c: 0
    cpp: 1

And this is the disassembly when running it through the wasm2wat tool from wabt:
    wasm2wat -f test.c.wasm
      (func $test (type 0) (result f32)
        (local i32)
        (i32.store offset=1024
          (i32.const 0)
            (local.tee 0
              (i32.load offset=1024
                (i32.const 0)))
            (i32.const 5)))
            (local.get 0)
            (i32.const 1))))

    wasm2wat -f test.cpp.wasm
      (func $test (type 0) (result f32)
        (local i32)
        (i32.store offset=1024
          (i32.const 0)
            (local.tee 0
              (i32.load offset=1024
                (i32.const 0)))
            (i32.const 5)))
          (f32.const 0x1p+0 (;=1;))
          (f32.const 0x0p+0 (;=0;))
          (local.get 0)))

The disassembly of the '-x c' module reads basically what the original C code was.
But it looks like the '-x c++' module returns something like (value_of_mask_at_function_entry ? 1 : 0) which makes no sense to me.
If the code line `mask |= 4` is changed into `mask &= 1`, the function returns 1 the first time it is called, then 0 afterwards which matches that assumption (the '-x c' variant stays correct).

Playing with compilation flag variations like -O0 did not affect the result.

Here's the versions I tested with (all on Win64)
  LLD 10.0.0 / clang version 10.0.0 (trunk) Target: x86_64-pc-windows-msvc 
  LLD 8.0.0 / clang version 8.0.0 (tags/RELEASE_800/final) Target: x86_64-pc-windows-msvc
  LLD 9.0.0 / clang version 9.0.0 (tags/RELEASE_900/final) Target: x86_64-pc-windows-msvc

They all produced identical assembly as far as I can tell.

Thanks for checking this out, hopefully this is the right place to report this and greetings from Tokyo,
- Bernhard

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