[Bug target/54446] New: Stack needlessly aligned with AVX

["cesarb at cesarb dot net" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54446

             Bug #: 54446
           Summary: Stack needlessly aligned with AVX
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: cesarb@cesarb.net


When compiling the following small function, gcc aligns the stack, even though
the function does not store any AVX values in the stack:

bool test(__m256i *p)
{
    __m256i x = _mm256_load_si256(p);
    return _mm256_testz_si256(x, x);
}

The generated assembly output is:

test:
    pushq    %rbp
    movq    %rsp, %rbp
    andq    $-32, %rsp
    addq    $16, %rsp
    vmovdqa    (%rdi), %ymm0
    vptest    %ymm0, %ymm0
    leave
    sete    %al
    vzeroupper
    ret

The assembly output should instead be something like:

test:
    vmovdqa    (%rdi), %ymm0
    vptest    %ymm0, %ymm0
    sete    %al
    vzeroupper
    ret

The command line is "gcc -mavx -std=c99 -O3 -Wall -Wextra -Werror -save-temps".

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.6.3/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin
--enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-ppl --with-cloog --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.6.3 20120306 (Red Hat 4.6.3-2) (GCC)