On Wed, 18 Nov 2009 19:41:56 +0100
Heiko Carstens <heiko.carstens@xxxxxxxxxx> wrote:
> On Wed, Nov 18, 2009 at 06:34:00PM +0100, Martin Schwidefsky wrote:
> > used for parameter passing. 64 bit values are passed as registers pairs
> > with the first register an even numbered register. The effect of that
> > rule is that parameter registers may be skipped or that the whole 64 bit
> > value is passed on the stack. Examples:
>
> Erm, did I miss something? The 32 bit ABI doesn't say anything about
> even/odd register pairs for 64 bit values. Also gcc doesn't generate
> such code:
Odd, I can remember that there has been a rule like that. It is indeed
not in the ABI, seems like we dropped it/never added it. There are quite
a few 32 bit instruction that operate on 64 bit values in even/odd
registers which makes it beneficial to pass 64 bit values in a way that
they already are located in even/odd and not in odd/even registers.
> > fn(int a, long long b, int c)
> > a is passed in %r2, b is passed in %r4/%r5, c is passed in %r6, %r3 is
> > skipped.
>
> extern void fn(int a, long long b, int c);
>
> void bla(void)
> {
> fn(0, 1, 2);
> }
>
> 00000000 <bla>:
> 0: 90 ef f0 38 stm %r14,%r15,56(%r15)
> 4: a7 fa ff a0 ahi %r15,-96
> 8: a7 28 00 00 lhi %r2,0
> c: a7 38 00 00 lhi %r3,0
> 10: a7 48 00 01 lhi %r4,1
> 14: a7 58 00 02 lhi %r5,2
> 18: c0 e5 00 00 00 00 brasl %r14,18 <bla+0x18>
> 1e: 98 ef f0 98 lm %r14,%r15,152(%r15)
> 22: 07 fe br %r14
>
> Not that it really matters, since other archs enforce the rule anyway.
Perhaps that is what confused me. Anyway, the thing to remember here is
that passing a long long as the last parameter which would end up in
%r6/%r7 requires a system call wrapper for the standard 32 bit system
call for a 32 bit kernel. So please don't do that.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
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