[LLVMdev] Related constant folding of floating point values

[Arsen Hakobyan]

Hi all,

I have the following test case:
#define FLT_EPSILON   1.19209290E-7 

int err = -1;
int main()
{
 float a = 8.1;
 if (((a - 8.1) >= FLT_EPSILON) || ((a - 8.1) <= -FLT_EPSILON)) {    //I am
using FLT_EPSILON to check whether (a != 2.0).
    err = 1;
  } else {
    err = 0;
  }
  return 0;
}

with -O3 optimization level clang generates already incorrect LLVM IR:
; Function Attrs: nounwind uwtable
define i32 @main() #0 {
entry:
  store i32 1, i32* @err, align 4, !tbaa !0
  ret i32 0
}

BUT if I change the value of the variable 'a' to be '7.1'. then a correct IR 
is generated:
...
entry:
  store i32 0, i32* @err, align 4, !tbaa !0
  ret i32 0
...


I have already investigated the issue and found that during the EarlyCSE
transformation it seems replaces the FPExt instruction with an incorrect
hexadecimal value:
The LLVM IR generated with O0 opt. level is:
...
  store float 0x4020333340000000, float* %a, align 4
  %0 = load float* %a, align 4
  %conv = fpext float %0 to double
  %sub = fsub double %conv, 8.100000e+00
  %cmp = fcmp oge double %sub, 0x3E8000000102F4FD
  br i1 %cmp, label %if.then, label %lor.lhs.false

lor.lhs.false:                                    ; preds = %entry
  %1 = load float* %a, align 4
  %conv2 = fpext float %1 to double
  %sub3 = fsub double %conv2, 8.100000e+00
  %cmp4 = fcmp ole double %sub3, 0xBE8000000102F4FD
  br i1 %cmp4, label %if.then, label %if.else
...

during the transformation the %conv is replaced with "double
0x4020333340000000" and then the result of comparison is resolved
incorrectly. 

Is not this a bug?
I have also searched for a similar issue in llvm bug list, but could not
found anything.

Thank you,
Arsen



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