c++/3299: __attribute__((aligned(16))) seems to be ignored on x86

[Jan Hubicka]

The following reply was made to PR c++/3299; it has been noted by GNATS.

From: Jan Hubicka <jh@suse.cz>
To: Gabriel Dos Reis <gdr@integrable-solutions.net>
Cc: hubicka@gcc.gnu.org, gcc-bugs@gcc.gnu.org, ryants@home.com,
	gcc-gnats@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: c++/3299: __attribute__((aligned(16))) seems to be ignored on x86
Date: Fri, 11 Oct 2002 16:17:58 +0200

 > hubicka@gcc.gnu.org writes:
 > 
 > | Synopsis: __attribute__((aligned(16))) seems to be ignored on x86
 > | 
 > | State-Changed-From-To: open->closed
 > | State-Changed-By: hubicka
 > | State-Changed-When: Thu Oct 10 11:13:35 2002
 > | State-Changed-Why:
 > |     This is runtime problem that missaligns stack frame of function main.  You need to update to runtime compiled with gcc 3.0+ or not use aligned data in the function main (gcc aligns outgoing frame so the called functions will be aligned properly)
 > 
 > Jan,
 >   
 >   Please could you add a note to that effect in the documentation?
 > Thanks,
 Good idea.  I was thinking about adding it to FAQ too.  Here is my
 attempt for the description.  WOuld you mind to correct it?
 
 Fri Oct 11 16:15:51 CEST 2002  Jan Hubicka  <jh@suse.cz>
 	* extend.texi (SSE vector operations and stack alignment): New subsection.
 Index: extend.texi
 ===================================================================
 RCS file: /cvs/gcc/egcs/gcc/doc/extend.texi,v
 retrieving revision 1.106
 diff -c -3 -p -r1.106 extend.texi
 *** extend.texi	11 Oct 2002 04:15:14 -0000	1.106
 --- extend.texi	11 Oct 2002 14:15:33 -0000
 *************** v2sf __builtin_ia32_pswapdsf (v2sf)
 *** 5241,5246 ****
 --- 5241,5277 ----
   v2si __builtin_ia32_pswapdsi (v2si)
   @end example
   
 + @subsection SSE vector operations and stack alignment
 + 
 + Unlike other i386 instructions, SSE operations often strictly require the
 + memory operands to be 16 byte aligned.  When this restriction is not met, program
 + crash.
 + 
 + In order to avoid crash, all data structures accessed by SSE operations needs
 + to be properly aligned.  It is common mistake, for instance, to access array of
 + 4 floats directly by single SSE operation.  In order to get this working,
 + attribute @code{alignment} needs to be used.
 + 
 + Another common case is situation, where compiler spills SSE register into stack
 + frame that is not itself properly aligned.  Unfortunately the Application
 + Binary Interface mandates only 4 byte alignment of stack frames and thus it is
 + never safe to spill register without precautions.  GCC makes this possible by
 + keeping stack pointer aligned to 16 byte boundary at each function call (see
 + @code{-mpreffered-stack-boundary} for details), however this scheme can break
 + when code compiled with lower stack pointer alignment restrictions is used to
 + call code with higher ones.  This may be the case of call-backs called from
 + library compiled using different compiler or runtime not aligning properly
 + stack pointer when invoking @code{main()}.  Unfortunately GCC 2.95 did contain
 + bug that miss-compiled startup code of glibc and some other libraries to
 + misalign stack.  To avoid problems with on such systems, GCC automatically
 + instrument function @code{main()} to align outgoing stack pointer dynamically.
 + This makes stack frame of all functions called from @code{main()} to be
 + properly aligned, however it does not align the frame of @code{main*(} itself.
 + 
 + In short when your runtime is not 16 byte alignment safe, it is impossible to
 + use SSE inside the function @code{main()}, any other function inlined to it, or
 + any function called via some kind of call-back from library code.
 + 
   @node PowerPC AltiVec Built-in Functions
   @subsection PowerPC AltiVec Built-in Functions