Re: [patch 06/10] Immediate Value - i386 Optimization

H. Peter Anvin
2007-07-03T20:18:57Z

Mathieu Desnoyers wrote:
>
> Hi Peter,
>
> I understand your concern. If you find a way to let the code be compiled
> by gcc, put at the end of the functions (never being a branch target)
> and then, dynamically, get the address of the branch instruction and
> patch it, all that in cooperation with gcc, I would be glad to hear from
> it. What I found is that gcc lets us do anything that touches
> variables/registers in an inline assembly, but does not permit to place
> branch instructions ourselves; it does not expect the execution flow to
> be changed in inline asms.
>

I believe this is correct. It probably would require requesting a gcc
builtin, which might be worthwhile to do if we

> <branch site>
> 77: b8 00 00 00 00 mov $0x0,%eax
> 7c: 85 c0 test %eax,%eax
> 7e: 0f 85 16 03 00 00 jne 39a <schedule+0x39a>
> here, we just loaded 0 in eax (movl used to make sure we populate the
> whole register so we do not stall the pipeline)a
> When we activate the site,
> line 77 becomes: b8 01 00 00 00 mov $0x1,%eax
> </branch site>

One could, though, use an indirect jump to achieve, if not as good, at
least most of the effect:

movl $<patchable>,<reg>
jmp *<reg>

Some x86 cores will be able to detect the movl...jmp forwarding, and
collapse it into a known branch target; however, on the ones that can't,
it might be worse, since one would have to rely on the indirect branch
predictor.

This would, however, provide infrastructure that could be combined with
a future gcc builtin.

-hpa