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

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

[bagnara at cs dot unipr dot it]

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

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

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-17 07:34 -------
Note GCC does not know about the rounding mode, in fact the round mode is only changeable in C99 
by the #pragma which GCC does not do right now and I thought that is a different PR already.

-- 


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

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

[bagnara at cs dot unipr dot it]

------- Additional Comments From bagnara at cs dot unipr dot it  2005-04-17 08:52 -------
Subject: Re:  Excessive optimization of floating point
 expression

pinskia at gcc dot gnu dot org wrote:
> Note GCC does not know about the rounding mode,

This seems a good reason not to attempt optimizations
that only work with a given rounding mode.

> in fact the round mode is only changeable in C99 
> by the #pragma which GCC does not do right now and  I thought that is a different PR already.

I do not see the connection with the #pragma you are talking about.

IMHO, a program that uses the services of <fenv.h>, which is
covered by section 7.6 of the C99 standard, is a perfectly
legal C99 program, and thus deserves to be compiled correctly
as prescribed by that standard.
Am I missing something?
All the best,

     Roberto



-- 


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

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

[vincent at vinc17 dot org]

------- Additional Comments From vincent at vinc17 dot org  2005-06-15 16:42 -------
Even without <fenv.h>, the function could be in a library called in a directed
rounding mode.

And one can change the rounding mode via a standard function in the glibc, no
need for a pragma.

-- 


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

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

[vincent at vinc17 dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vincent at vinc17 dot org


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