On 11/24/2009 07:46 AM, Andrew Haley wrote:
>>
>> 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.
>
If we're changing gcc anyway, then let's add the option of intercepting
the function at the point where the machine state is well-defined by
ABI, which is before the function stack frame is set up.
-maccumulate-outgoing-args sounds like it would be painful on x86 (not
using its cheap push/pop instructions), but I guess since it's only when
tracing it's less of an issue.
-hpa
--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.
