http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46198
Summary: movd xmm, r (xmm -> GPR) may hit the stack
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: tbp...@gmail.com
Host: x86_64-pc-linux-gnu
Target: x86_64-pc-linux-gnu
Under somewhat elusive (target) conditions...
$ cat movd.c
#include <emmintrin.h>
int foo(__m128i x) { return _mm_cvtsi128_si32(x); }
$ gcc -O3 movd.c -S -o -
foo:
.LFB521:
.cfi_startproc
movd %xmm0, -12(%rsp)
movl -12(%rsp), %eax
ret
$ gcc -Os movd.c -S -o -
foo:
.LFB514:
.cfi_startproc
movd %xmm0, -12(%rsp)
movl -12(%rsp), %eax
ret
$ gcc -O3 -march=native movd.c -S -o -
foo:
.LFB521:
.cfi_startproc
movd %xmm0, %eax
ret
... movd may or may not pay a trip to the stack, apparently depending on some
target cpu+SSE level condition (here native is a corei7).
I can't see any good reason for that (and certainly not for size).
Known to happen from gcc 4.4.4 to
$ /usr/local/gcc-4.6-20101026/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/usr/local/gcc-4.6-20101026/bin/gcc
COLLECT_LTO_WRAPPER=/usr/local/gcc-4.6-20101026/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.6.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --prefix=/usr/local/gcc-4.6.0
--enable-languages=c,c++ --enable-threads=posix --disable-nls
--with-system-zlib --disable-bootstrap --enable-mpfr --enable-gold --enable-lto
--with-ppl --with-cloog --with-arch=native --enable-checking=release
Thread model: posix
gcc version 4.6.0 20101026 (experimental) (GCC)
