On Wed, 25 Jun 2008, Helge Hafting wrote:
Mikulas Patocka wrote:
Hi
Here I'm sending 10 patches to inline various functions.
To give you some understanding of sparc64, every function there uses big
stack frame (at least 192 bytes). 128 bytes are required by architecture
(16 64-bit registers), 48 bytes are there due to mistake of Sparc64 ABI
designers (calling function has to allocate 48 bytes for called function)
and 16 bytes are some dubious padding.
I guess there is no way around those 128 bytes required by hardware - but
is it necessary to allocate the 48 bytes of "mistake" and the dubious
padding?
Linux kernel functions don't call any outside functions, and are not called
from
outside either. So violating the ABI should be ok - there is nothing else to
be compatible to. Similiar to how x86 experimented with --mregparm to get
a little more performance from changing the kernel calling convention.
Helge Hafting
So, ask gcc developers to do kernel-specific ABI with only 128-byte stack
frame.
BTW. could some gcc developer explain the reason for additional 16-bytes
on stack on sparc64? 64-bit ABI mandates 176 bytes, but gcc allocates 192
bytes.
Even worse, gcc doesn't use these additional bytes. If you try this:
extern void f(int *i);
void g()
{
int a;
f(&a);
}
, it allocates additional 16 bytes for the variable "a" (so there's total
208 bytes), even though it could place the variable into 48-byte
ABI-mandated area that it inherited from the caller or into it's own
16-byte padding that it made when calling "f".
Mikulas
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