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    DateTue, 03 Jul 2007 13:18:39 -0700
    From"H. Peter Anvin" <>
    SubjectRe: [patch 06/10] Immediate Value - i386 Optimization
    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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