[Bug middle-end/21067] New: Excessive optimization of floating point expression

["bagnara at cs dot unipr dot it" <gcc-bugzilla at gcc dot gnu dot org>]

Compiling the following with GCC 3.4.3 (with "gcc -O2 -S file.c")

float mul2(float a, float b) {
  float v = -a * b;
  return -v;
}

produce the erroneous code

	pushl	%ebp
	movl	%esp, %ebp
	flds	8(%ebp)
	fmuls	12(%ebp)
	popl	%ebp
	ret

where the sign changes have been erroneously elided.
Notice that, depending on the rounding mode in effect,
this produces the wrong results.

Strangely enough, the code produced for

float div2(float a, float b) {
  float v = -a / b;
  return -v;
}

is instead correct.

-- 
           Summary: Excessive optimization of floating point expression
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bagnara at cs dot unipr dot it
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21067