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

["H. Peter Anvin" <hpa@zytor.com>]

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

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