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Re: pkg/47906: lang/g95: SEGV occurs when stack address is not aligned 8 bytes at main().
The following reply was made to PR pkg/47906; it has been noted by GNATS.
From: Joerg Sonnenberger <joerg%britannica.bec.de@localhost>
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.
> 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.
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