[LLVMbugs] [Bug 3929] New: Rounding modes are ignored in gcc front end constant folding

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3929

           Summary: Rounding modes are ignored in gcc front end 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 <stdlib.h>
#include <fenv.h>

#include <math.h>

int main(int argc, char **argv) {
#pragma STDC FENV_ACCESS ON
  fesetround(FE_UPWARD);
  double g = sin(0.9);
  printf("%A\n", g);

  fesetround(FE_DOWNWARD);
  double h = sin(0.9);
  printf("%A\n", h);
  return 0;
}

produces the same bit patterns for g and h, which is obviously false
define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
entry:
        %0 = tail call i32 @fesetround(i32 2048) nounwind               ; <i32>
[#uses=0]
        %1 = tail call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([4 x
i8]* @.str, i32 0, i32 0), double 0x3FE91103985DA841) nounwind           ;
<i32> [#uses=0]
        %2 = tail call i32 @fesetround(i32 1024) nounwind               ; <i32>
[#uses=0]
        %3 = tail call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([4 x
i8]* @.str, i32 0, i32 0), double 0x3FE91103985DA841) nounwind           ;
<i32> [#uses=0]
        ret i32 0
}


here is the result on x86_32 :
nraynaud at macaron-2:~/Desktop/geek/fp$ gcc  test2.c && ./a.out
0X1.91103985DA842P-1
0X1.91103985DA858P-1


This is a common IEEE 754 mistake.

Either, fesetround should be interpreted before folding the constant, or IEEE
754 shouldn't be folded in the presence of the pragma.

It should be noted that only the pragma allows for toying with rounding mode
(and flag bits), careless folding is free if the pragma is not there. So people
who don't care about ieee stuff get their usual speed, those that know ieee
stuff get what they bargained for, and speed is often the price to pay. Those
who try to play with rounding modes without setting the pragma spend hours
finding why it doesn't work reliably.


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