[llvm-bugs] [Bug 37340] New: /fp in CL-compat does not produce OrderedCompares

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37340

            Bug ID: 37340
           Summary: /fp in CL-compat does not produce OrderedCompares
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: alexandre.ganea at ubisoft.com
                CC: llvm-bugs at lists.llvm.org

The behavior between MSVC cl.exe and clang, in regards to the /fp flag, seems
to be different when comparing NaNs.

Consider:

// a.cpp
int main() {
    unsigned long a = 0xffc00000; // -Nan
    float f = *(float*)&a;
    if (f <= -1.f)
        return 1;
    return 0;
}

(MSVC) cl.exe a.cpp /arch:AVX2 /Z7 /fp:fast

    if (f <= -1.f)
00007FF6EA036586  vmovss      xmm0,dword ptr [f]  
00007FF6EA03658C  vcomiss     xmm0,dword ptr [__real at bf800000 (07FF6EA094F10h)] 
00007FF6EA036594  ja          main+2Dh (07FF6EA03659Dh)  

(MSVC) cl.exe a.cpp /arch:AVX2 /Z7 /fp:precise

    if (f <= -1.f)
00007FF70AF06586  vmovss      xmm0,dword ptr [__real at bf800000 (07FF70AF64F10h)] 
00007FF70AF0658E  vcomiss     xmm0,dword ptr [f]  
00007FF70AF06594  jb          main+2Dh (07FF70AF0659Dh) 

(Clang) clang-cl.exe a.cpp /arch:AVX2 /Z7 /fp:fast

    if (f <= -1.f)
00007FF7DAAF6598  vucomiss    xmm0,dword ptr [f]  
00007FF7DAAF659E  jb          main+41h (07FF7DAAF65B1h)  

(Clang) clang-cl.exe a.cpp /arch:AVX2 /Z7 /fp:precise

    if (f <= -1.f)
00007FF7DAAF6598  vucomiss    xmm0,dword ptr [f]  
00007FF7DAAF659E  jb          main+41h (07FF7DAAF65B1h)  
   (same as above)

Actually, no matter which /fp: or -f*math* you pass to clang, the result is the
same (when using the clang-cl driver). As you can see, Clang generates
'ucomiss' instructions in all cases, instead of 'comiss' as in cl.exe. Omiting
/arch:AVX2 gives the same result.

I am at r331502.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20180504/698ab3c7/attachment.html>