[Bug c/21032] New: GCC 3.4.3 wrongly reorders floating-point operations

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

If you compile the function

void assign2(float* a, double b) {
  volatile float v = -b;
  *a = -v;
}

you will see that GCC 3.4.3, e.g., at -O2, produces

        fldl    12(%ebp)
        fstps   -20(%ebp)
        movl    8(%ebp), %eax
        flds    -20(%ebp)
        fchs
        fstps   -4(%ebp)
        flds    -4(%ebp)
        fchs
        fstps   (%eax)

where the first sign change is performed /after/ reducing the
precision and not /before/, as I believe it should (according
to ISO/IEC 9899, 5.1.2.3#13, 6.3.1.5#2 and 6.3.1.8#2).
The produced code seems also very badly optimized, considering
that something like

	fldl	12(%ebp)
	fchs
	fstps	-4(%ebp)
	flds	-4(%ebp)
	fchs
	fstps	(%eax)

would be significantly more efficient (besides being correct).
The same problem can be seen with gcc-4.0.0 20050406 (Fedora Core 3).

    Roberto Bagnara

-- 
           Summary: GCC 3.4.3 wrongly reorders floating-point operations
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        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=21032