Re: pkg/47906: lang/g95: SEGV occurs when stack address is not aligned 8 bytes at main().

[NONAKA Kimihiro <nonakap%gmail.com@localhost>]

The following reply was made to PR pkg/47906; it has been noted by GNATS.

From: NONAKA Kimihiro <nonakap%gmail.com@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: wennmach%netbsd.org@localhost, gnats-admin%netbsd.org@localhost, 
pkgsrc-bugs%netbsd.org@localhost
Subject: Re: pkg/47906: lang/g95: SEGV occurs when stack address is not
 aligned 8 bytes at main().
Date: Wed, 10 Jul 2013 20:49:17 +0900

 Hi,
 
 Do you think my first patch is wrong yet?
 Please discuss for speed at g95 project, not this PR.
 
 2013/7/10 Joerg Sonnenberger <joerg%britannica.bec.de@localhost>:
 > The following reply was made to PR pkg/47906; it has been noted by GNATS.
 >
 > From: Joerg Sonnenberger <joerg%britannica.bec.de@localhost>
 > To: gnats-bugs%NetBSD.org@localhost
 > Cc:
 > Subject: Re: pkg/47906: lang/g95: SEGV occurs when stack address is not
 >  aligned 8 bytes at main().
 > Date: Wed, 10 Jul 2013 12:12:02 +0200
 >
 >  On Wed, Jul 10, 2013 at 09:45:01AM +0000, SODA Noriyuki wrote:
 >  >  >  I don't beliebe that comment is true, I was more asking whether it 
 > just
 >  >  >  works...
 >  >
 >  >  The comment is true, at least at once.
 >
 >  s/is/was/
 >
 >  >  I remember some x86 program which heavily used double significantly
 >  >  ran slower occasionally.  And the reason of the slowness was that
 >  >  the program was accessing double on its stack, and the stack was not
 >  >  always 8 byte aligned.  It ran slowly, when the stack was not 8 byte
 >  >  aligned due to some environment variable settings.
 >
 >  Keep in mind that we don't keep the stack 64bit aligned on i386 at all.
 >  So chances are very high that it won't be preserved anyway. FP
 >  performance sensitive code wants to use SSE2 anyway.
 >
 >  >  That was more than 10 years ago.
 >  >  But it's better to keep the optimization, unless you are 100% sure
 >  >  that the optimization is really useless on all modern CPUs.
 >
 >  I'm a lot more concerned about a working state than a questionable
 >  optimisation. All modern CPUs do memory fetches in terms of cache lines
 >  and the stack is pretty much guaranteed to be in the cache anyway, so it
 >  should really not matter.
 >
 >  >  BTW, modern x86 CPUs still require some alignment restriction
 >  >  for efficiency about SSE2 and AVX instructions.
 >  >  See page 107 of http://www.agner.org/optimize/optimizing_cpp.pdf
 >  >  for example.
 >  >  16byte or 32byte alignment is better for those instructions, though.
 >
 >  Correct, but a separate issue. 128bit alignment is required for many of
 >  those. Newer GCC has support for doing adhoc realignment of the
 >  stack-frame, but that effectively would require using gfortran.
 >
 >  Joerg
 >