Re: lib/39465: threads stack is not aligned properly for gcc on i386

[David Holland <dholland-bugs%netbsd.org@localhost>]

The following reply was made to PR lib/39465; it has been noted by GNATS.

From: David Holland <dholland-bugs%netbsd.org@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: 
Subject: Re: lib/39465: threads stack is not aligned properly for gcc on i386
Date: Sun, 28 Aug 2011 20:12:28 +0000

 On Sun, Aug 28, 2011 at 07:45:02AM +0000, Matthew Mondor wrote:
  >  I ran the test and this still occurs on recent netbsd-5.  I cannot test
  >  it on -current, though.  The fix appears simple, harmless and might
  >  enhance performance.
 
 The code in -current doesn't appear to be different.
 
  >  Perhaps that this would also fix warnings sent by ffmpeg about it
  >  possibly being slow because the stacks aren't aligned optimally...
 
 As I recall there's an issue about sse/mmx requiring this stack
 alignment and there was a lengthy discussion about it somewhere, but I
 forget the details.
 
 AIUI something like this patch should be committed. Although:
 
  : -       sp -= argc + 1;                 /* Make room for ret and args. */
  : +       sp -= argc;                     /* Make room for ret and args. */
  : +       sp  = (unsigned int *)((uintptr_t)sp & ~0xf);
  : +       sp --;                  /* Make room for ret and args. */
 
 Separating out an extra "sp--" seems bogus, and that should be
 
    (unsigned int *)((uintptr_t)sp & ~(uintptr_t)0xf);
 
 particularly if the code is going to get shared with or pasted into
 the amd64 version.
 
 -- 
 David A. Holland
 dholland%netbsd.org@localhost