The second issue I'm still not sure about is the magic nop marker
for frameless functions. In an earlier mail you wrote:
Both currently supported
architectures always emit split-stack code on every function.
At least for rs6000 this doesn't appear to be true; in
rs6000_expand_split_stack_prologue we have:
if (!info->push_p)
return;
so it does nothing for frameless routines.
Now on i386 we do indeed generate code for frameless routines;
in fact, the *same* full stack check is generated as for any
other routine. Now I'm wondering: is there are reason why
this check would be necessary (and there's simply a bug in
the rs6000 implementation)? Then we obviously should do the
same on s390.
Try that on powerpc64(le):
$ cat a.c
#include <stdio.h>
void f(void) {
}
typedef void (*fptr)(void);
fptr g(void);
int main() {
printf("%p\n", g());
}
$ cat b.c
void f(void);
typedef void (*fptr)(void);
fptr g(void) {
return f;
}
$ gcc -O3 -fsplit-stack -c b.c
$ gcc -O3 -c a.c
$ gcc a.o b.o -fuse-ld=gold
I don't have a recent enough gcc for powerpc, but from what I've seen in
the code, this should explode with a linker error.
Of course, mixing split-stack and non-split-stack code when function
pointers are involved is sketchy anyway, so what's one more bug...
That said, for s390, we can avoid the above problem by checking the
relocation in gold now that ESA paths are gone - for direct function
calls (the only ones we care about), we should be seeing a relocation in
brasl. So I'll remove the nopmark thing and add proper recognition in
gold.
Ugh. I take that back. For -fPIC, the load-address sequence is:
larl %r1,f@GOTENT
lg %r2,0(%r1)
br %r14
And (sibling) call sequence is:
larl %r1,f@GOTENT
lg %r1,0(%r1)
br %r1
It seems there's no proper way to recognize a call vs a load address -
so we can either go with emitting the marker, or have the same problem
as on ppc.
So - how much should we care?
On the other hand, if rs6000 works fine *without* any code
in frameless routines, why wouldn't that work for s390 too?
Emitting a nop (that is always executed) still looks weird to me.
Bye,
Ulrich
