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Thread: gcc doesn't generate sse2 instructions ?

  1. #1
    Join Date
    May 2009

    gcc doesn't generate sse2 instructions ?

    I can't get gcc to generate sse2 instructions. To verify, I compiled my little test program with the -S option. I explicitly ask gcc to target the pentium-m architecture and generate sse2 instructions:

    <start test prog>
    #include <stdio.h>
    int main(int argc, char** argv)
    float f = atof(argv[1]);
    printf("%f\n", f * (float)2.0);
    return 0;
    <end test prog>

    I compile this with gcc -S -march=pentium-m -msse2, and I get:
    <start generated assembler>
    flds -8(%ebp)
    fadd %st(0), %st
    fstpl 4(%esp)
    <end generated assembler>

    This looks like standard i386 fpu math to me, not sse2 !! I've tried all sorts of switches, including -O or -O3, but the generated assembler is always identical.

    Anyone ????

    Ubuntu 9.04 32-bit running on a Pentium-M Dell Inspiron 1300
    catweazle@catweazle-laptop:~/catweazle$ gcc -v
    Using built-in specs.
    Target: i486-linux-gnu
    Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.3-5ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --enable-targets=all --with-tune=generic --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
    Thread model: posix
    gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4)

    catweazle@catweazle-laptop:~/catweazle$ cat /proc/cpuinfo
    processor : 0
    vendor_id : GenuineIntel
    cpu family : 6
    model : 13
    model name : Intel(R) Celeron(R) M processor 1.60GHz
    stepping : 8
    cpu MHz : 1596.043
    cache size : 1024 KB
    fdiv_bug : no
    hlt_bug : no
    f00f_bug : no
    coma_bug : no
    fpu : yes
    fpu_exception : yes
    cpuid level : 2
    wp : yes
    flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat clflush dts acpi mmx fxsr sse sse2 ss tm pbe nx up bts
    bogomips : 3192.08
    clflush size : 64
    power management:

  2. #2
    Join Date
    Feb 2007

    Lightbulb Re: gcc doesn't generate sse2 instructions ?

    You need to use "-mfpmath=sse" as well.

    For example on my AMD Athlon X2 I've been using:

    "-march=native -mfpmath=sse"

    The library will still be fp87 though unless you rebuild it and so will the function calling conventions unless you put:-

    "__attribute__ ((sseregparm))"

    .. after each function definition.

    Hope this helps.


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