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From: Tim Prince <tprince@computer.org> To: nobody@gcc.gnu.org Cc: gcc-prs@gcc.gnu.org, Subject: Re: target/6624: SSE misalignment with -O0 Date: Fri, 10 May 2002 10:56:00 -0000 [thread overview] Message-ID: <20020510175602.4969.qmail@sources.redhat.com> (raw) The following reply was made to PR target/6624; it has been noted by GNATS. From: Tim Prince <tprince@computer.org> To: jdhall@uiuc.edu, gcc-gnats@gcc.gnu.org Cc: Subject: Re: target/6624: SSE misalignment with -O0 Date: Fri, 10 May 2002 10:49:20 -0700 On Friday 10 May 2002 09:12, Jesse Hall wrote: > >Number: 6624 > >Category: target > >Synopsis: SSE mis-alignment with -O0 > >Confidential: no > >Severity: serious > >Priority: medium > >Responsible: unassigned > >State: open > >Class: wrong-code > >Submitter-Id: net > >Arrival-Date: Fri May 10 09:16:01 PDT 2002 > >Closed-Date: > >Last-Modified: > >Originator: Jesse Hall > >Release: 3.1 20020426 (prerelease) > >Organization: > >Environment: > > System: Linux espresso 2.4.18-xfs #1 SMP Tue Mar 12 20:03:55 CST 2002 i686 > unknown Architecture: i686 (more precisely, dual AthlonMP) > > > host: i386-pc-linux-gnu > build: i386-pc-linux-gnu > target: i386-pc-linux-gnu > configured with: ../src/configure -v > --enable-languages=c,c++,java,f77,proto,objc --prefix=/usr/lib/gcc-snapshot > --infodir=/share/info --mandir=/share/man --enable-shared --with-gnu-as > --with-gnu-ld --with-system-zlib --enable-long-long --enable-nls > --without-included-gettext --disable-checking --enable-threads=posix > --enable-java-gc=boehm --with-cpp-install-dir=bin --enable-objc-gc > i386-linux > > >Description: > > The SSE intrinsic _mm_loadu_ps can do an aligned SSE load (LOADAPS) > from an unaligned address when compiling with -O0. The problem seems to go > away with -O1 and higher. This causes a segfault when the program is run. > The assembly generated looks like: > > call 804834a <_mm_loadu_ps> > add $0x10,%esp > movaps %xmm0,0xffffffe8(%ebp) > > On my machine, the movaps address is consistently not on a 16-byte > boundary like it should be. > > >How-To-Repeat: > > This simple program exhibits the problem on my machine: > > #include <xmmintrin.h> > int main(int argc, char** argv) { > __m128 x; > float a[4] = {1.0f, 1.0f, 1.0f, 1.0f}; > x = _mm_loadu_ps(a); > return 0; > } > > Compiled with "gcc -g -O0 -msse foo.c -o foo". > > >Fix: > > Compiling with -O1 or higher makes the problem go away, at least in the > simple example above. > > >Release-Note: > >Audit-Trail: > >Unformatted: According to my understanding, alignments aren't supported by gcc in main(), as the OS doesn't necessarily present an aligned stack. The necessary alignments are supported in functions which are called by a function built with -mpreferred-stack-boundary=4. The option -Os is intended to present a lesser alignment; -O2 usually (but not always?) passes a fully aligned stack to its callees. I'm not sure of all this, and better documentation seems needed. -- Tim Prince
next reply other threads:[~2002-05-10 17:56 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2002-05-10 10:56 Tim Prince [this message] -- strict thread matches above, loose matches on Subject: below -- 2002-05-10 9:16 Jesse Hall
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