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

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

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
>