[PATCH] Fixing improper conversion from sin() to sinf() in optimization mode.

[Cong Hou <congh at google dot com>]

When a sin() (cos(), log(), etc.) function is called on a value of
float type and the returned double value is converted to another value
of float type, GCC converts this function call into a float version
(sinf()) in the optimization mode. This avoids two type conversions
and the float version function call usually takes less time. However,
this can result in different result and therefore is unsafe.

For example, the following code produces different results using -O0
(correct), but the same results using -Ox other than -O0 (incorrect).


#include <stdio.h>
#include <math.h>

int main()
{
  float v = 1;
  printf("%.20f\n", (float)sin(v));
  printf("%.20f\n", sinf(v));
}


In this patch, we do this conversion only when the flag
-funsafe-math-optimizations is set. The patch is shown below.


thanks,

Cong




Index: gcc/testsuite/gcc.c-torture/execute/20030125-1.c
===================================================================
--- gcc/testsuite/gcc.c-torture/execute/20030125-1.c (revision 201891)
+++ gcc/testsuite/gcc.c-torture/execute/20030125-1.c (working copy)
@@ -44,11 +44,11 @@ __attribute__ ((noinline))
 double
 sin(double a)
 {
- abort ();
+ return a;
 }
 __attribute__ ((noinline))
 float
 sinf(float a)
 {
- return a;
+ abort ();
 }
Index: gcc/convert.c
===================================================================
--- gcc/convert.c (revision 201891)
+++ gcc/convert.c (working copy)
@@ -99,7 +99,7 @@ convert_to_real (tree type, tree expr)
   /* Disable until we figure out how to decide whether the functions are
      present in runtime.  */
   /* Convert (float)sqrt((double)x) where x is float into sqrtf(x) */
-  if (optimize
+  if (optimize && flag_unsafe_math_optimizations
       && (TYPE_MODE (type) == TYPE_MODE (double_type_node)
           || TYPE_MODE (type) == TYPE_MODE (float_type_node)))
     {

Attachment: diff.txt
Description: Text document