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RE: A proposal to align GCC stack
- From: "Ye, Joey" <joey dot ye at intel dot com>
- To: "Ross Ridge" <rridge at csclub dot uwaterloo dot ca>, <gcc at gcc dot gnu dot org>
- Date: Wed, 19 Dec 2007 09:46:30 +0800
- Subject: RE: A proposal to align GCC stack
- References: <20071218233126.3EA1973D2A@caffeine.csclub.uwaterloo.ca>
Ross Ridge wrote:
> I'm currently using -fpreferred-stack-boundary without any trouble.
> Your proposal would in fact generate code to align stack when it's not
> necessary. This would change the behaviour of
-fpreferred-stack-boundary,
> hurting performance and that's unacceptable to me.
This proposal values correctness at first place. So when compile can't
make
sure a function is only called from functions with the same or bigger
preferred-stack-boundary, it will conservatively align the stack. One
optimization
is to set INCOMING = PREFERRED for local functions. Do you think it more
acceptable?
>> Ok, if people are using this flag to change the alignment to
something
>> smaller than used by the standard ABI, then INCOMING should be
>> MAX(STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY).
>
> On x86-64, ABI_STACK_BOUNDARY is 16byte, but the Linux kernel may
> want to use 8 byte for PREFERRED_STACK_BOUNDARY. INCOMING will
> be MIN(STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY) == 8 byte.
> Using MAX(STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY) also equals 8 in
that
> case and preserves the behaviour -fpreferred-stack-boundary in every
case.
I think HJ means MIN(ABI_STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY).
MAX(ABI, PREFERRED) == 16 in this case.
Thanks - Joey