> On Sat, Dec 05, 1998 at 09:48:44PM +0100, MOLNAR Ingo wrote:
> > on x86, we can do instructions like:
> >
> > movl 0xc0000028(%eax), %ebx
> >
> > the compiler expands it's patters wildly differently when it's a constant.
>
> Um, have you tried this Ingo? Given
[yes i've tried it before posting :)]
>
> #define foo ((int)&__foo)
> int bar(int *x)
> {
> return x[foo+45];
> }
>
> or suchlike you should see __foo combined with other operations.
>
> movl $__foo+45,%eax
> movl (%edx,%eax,4),%eax
>
> in this case due to the shift.
this is what it does:
0: 8b 54 24 04 movl 0x4(%esp,1),%edx
4: b8 2d 00 00 00 movl $0x2d,%eax
9: 8b 04 82 movl (%edx,%eax,4),%eax
c: c3 ret
so yes it has combined it into a constant, but not into an immediate
instruction constant. The thing we can get with
#define foo1 0x12340000
is:
0: 8b 44 24 04 movl 0x4(%esp,1),%eax
4: 8b 80 b4 00 d0 movl 0x48d000b4(%eax),%eax
9: 48
a: c3 ret
which is smaller and faster.
> My guess is that you can do fairly well with the immediates.
> Certainly better than having them be completely variable.
maybe EGCS already does such things? i have GCC 2.7.2.3.
-- mingo
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