[LLVMbugs] [Bug 3930] New: floating point exceptions flags are ignored in constant folding

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3930

           Summary: floating point exceptions flags are ignored in constant
                    folding
           Product: new-bugs
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: miscompilation
          Severity: normal
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: llvm at nraynaud.com
                CC: llvmbugs at cs.uiuc.edu


I toyed around with http://llvm.org/demo/index.cgi

and found that this code : 
#include <stdio.h>
#include <fenv.h>

int main(int argc, char **argv) {
#pragma STDC FENV_ACCESS ON
  feclearexcept(FE_DIVBYZERO);
  double d = 1.0/0.0;
  if (fetestexcept(FE_DIVBYZERO))
    printf("error\n");
  else
    printf("%e\n", d);
  return 0;
}

is compiled this way :

define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
entry:
        %0 = tail call i32 @feclearexcept(i32 4) nounwind               ; <i32>
[#uses=0]
        %1 = tail call i32 @fetestexcept(i32 4) nounwind                ; <i32>
[#uses=1]
        %2 = icmp eq i32 %1, 0          ; <i1> [#uses=1]
        br i1 %2, label %bb1, label %bb

bb:             ; preds = %entry
        %3 = tail call i32 @puts(i8* getelementptr ([6 x i8]* @.str, i32 0, i32
0)) nounwind            ; <i32> [#uses=0]
        ret i32 0

bb1:            ; preds = %entry
        %4 = tail call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([4 x
i8]* @.str1, i32 0, i32 0), double 0x7FF0000000000000) nounwind          ;
<i32> [#uses=0]
        ret i32 0
}

it as basically no chance to print "error" like it does under a normal C
compiler (tested on normal gcc x86_32).

the pragma is here to tell the compiler maker to be careful in floating point
handling because there will be some tampering with the FP subsystem.

exactly the same kind of bug as Bug 3929


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