On Tue, 1 Jul 2008, David Miller wrote:
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Date: Wed, 2 Jul 2008 00:39:35 -0400 (EDT)
The ABI is very vague about it. The V9 ABI just displays that 6-word space
in a figure bug doesn't say anything about it's usage. The V8 ABI just
says that "the function may write incoming arguments there". If it may
write anything other, it is unknown --- probably yes, but it is not said
in the document.
The document nicely specifies who owns which registers, but doesn't say
that about the stack space :-(
Actually, I know for a fact that you have to have those slots there.
A long time ago in the sparc64 kernel, in the trap entry code, I tried
only giving 128 bytes of stack frame as the trap entry called into C
code. And it did not work, I had to put the 6 slots there.
The bad thing is that gcc can't use those slots optimally. If you have for
example:
void f(int *x)
{
}
void g()
{
int a;
f(&a);
}
void h()
{
g();
}
Then the variable "a" can't be placed into one of the 6 implicit slots for
g->f call (beacuse "f" may overwrite that slot). But "a" could be placed
into one of those 6 slots that "h" allocates for "g" (because these slots
are owned by "g"). But it isn't --- additional place is allocated for "a"
:-/
Mikulas
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