[Bug c/17394] New: Implicit SSE/SSE2 FP code reordering in Placement Independent Code error (SSE/optimisation/PIC)

[ivan at vmfacility dot fr]

Here is a simple code :
<testcase>
#include <stdio.h>
float foo;
int main()
{
    unsigned int y;
    *((unsigned int *)&foo)=0x40555555;
    y=(int)foo;
    printf("%g %d\n",foo,y);
}
</testcase>
Note : Direct FP manipulation is required by my application as it needs to 
perform conversion from a different FP encoding.

Straight compiling (gcc foo.c -o foo) gives the expected result :
"3.3333 3"

However, compiling with the following flags give various results :
"gcc -O3 -march=pentium4 -fPIC foo.c foo"
gives :
"0 0"
Using gcc 3.3.4 with the same flags give :
"3.3333 0"

Examination of the generated assembly indicate various cases :

Starting with 3.4.1 (comments are my own)
... prolog ...
    movss   (%ecx), %xmm0         Put *%ecx in MMX Reg 0
    movl    $1079334229, (%ecx)   Put constant in *%ecx - UGH ! Backwards
    subl    $16, %esp
    movss   %xmm0, -8(%ebp)       Save MMX Reg 0 in %ebp[-8]
    cvttss2si   %xmm0, %edx       Convert Single Prec to Signed Int to %edx
    movl    %eax, (%esp)
    flds    -8(%ebp)
    movl    %edx, 12(%esp)
    fstpl   4(%esp)
    call    printf@PLT
... epilog ...

and now with gcc 3.3.4 :
... prolog ...
    cvttss2si   (%ecx), %eax        Convert Single Prec float @%ecx to %eax
    movl    $1079334229, (%ecx)     Put constant value@%ecx - Backwards !
    movl    %eax, 12(%esp)
    flds    (%ecx)
    movl    %edx, (%esp)
    fstpl   4(%esp)
    call    printf@PLT
... epilog ...

Notes
- No problem shows if -fPIC is not specified
- Tried -mfpmath=387 but didn't help
- -msse or -msse2 are also sufficient to trigger the problem
- with -march=pentium2 and no -msseX, the problem goes away
- Problem only shows at -O2 or better

I tried with
- gcc 3.3.4 from debian
- gcc 3.4.1 from debian
- gcc 3.4.1 from Mandrake
(haven't tried stock gcc - but I suspect I might be getting identical results 
as I am getting similar results from different vendors).

Thanks,

--Ivan

-- 
           Summary: Implicit SSE/SSE2 FP code reordering in Placement
                    Independent Code error (SSE/optimisation/PIC)
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ivan at vmfacility dot fr
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-linux-gnu
  GCC host triplet: i686-linux-gnu
GCC target triplet: i686-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17394