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SubjectRe: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC messing with mcount prologue
Jakub Jelinek wrote:
> On Tue, Nov 24, 2009 at 03:32:20PM +0000, Andrew Haley wrote:
>> Jakub Jelinek wrote:
>>> On Tue, Nov 24, 2009 at 03:55:49PM +0100, Thomas Gleixner wrote:
>>>>> you should compile your code with -maccumulate-outgoing-args, and there's
>>>>> no need to use -mtune=generic. Is that right?
>>>> Seems to work. What other side effects has that ?
>>> Faster code, significant increase in code size though.
>> Does it affect code size when we don't have to realign the stack pointer?
>
> Yes, a lot. The difference is that -maccumulate-outgoing-args allocates
> space for arguments of the callee with most arguments in the prologue, using
> subtraction from sp, then to pass arguments uses movl XXX, 4(%esp) etc.
> and the stack pointer doesn't usually change within the function (except for
> alloca/VLAs).
> With -mno-accumulate-outgoing-args args are pushed using push instructions
> and stack pointer is constantly changing.

Alright. So, it is possible in theory for gcc to generate code that
only uses -maccumulate-outgoing-args when it needs to realign SP.
And, therefore, we could have a nice option for the kernel: one with
(mostly) good code density and never generates the bizarre code
sequence in the prologue.

Andrew.


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