Re: [patch 5/8] Immediate Values - x86 Optimization (update 2)

[Mathieu Desnoyers]

* H. Peter Anvin (hpa@xxxxxxxxx) wrote:
> Mathieu Desnoyers wrote:
>> Immediate Values - x86 Optimization
>> x86 optimization of the immediate values which uses a movl with code 
>> patching
>> to set/unset the value used to populate the register used as variable 
>> source.
>> Changelog:
>> - Use text_poke_early with cr0 WP save/restore to patch the bypass. We are 
>> doing
>>   non atomic writes to a code region only touched by us (nobody can 
>> execute it
>>   since we are protected by the immediate_mutex).
>> - Put immediate_set and _immediate_set in the architecture independent 
>> header.
>> - Use $0 instead of %2 with (0) operand.
>> - Add x86_64 support, ready for i386+x86_64 -> x86 merge.
>> - Use asm-x86/asm.h.
>> Ok, so the most flexible solution that I see, that should fit for both
>> i386 and x86_64 would be :
>> 1 byte  : "=Q" : Any register accessible as rh: a, b, c, and d.
>> 2, 4 bytes : "=R" : Legacy registerâthe eight integer registers 
>> available
>>                  on all i386 processors (a, b, c, d, si, di, bp, sp). 8
>> bytes : (only for x86_64)
>>           "=r" : A register operand is allowed provided that it is in a
>>                  general register.
>> That should make sure x86_64 won't try to use REX prefixed opcodes for
>> 1, 2 and 4 bytes values.
>
> I just had a couple of utterly sick ideas.
>
> Consider this variant (this example is for a 32-bit immediate on x86-64, 
> but the obvious macroizations apply):
>
> 	.section __discard,"a",@progbits
> 1:	movl $0x12345678,%r9d
> 2:
> 	.previous
>
> 	.section __immediate,"a",@progbits
> 	.quad foo_immediate, (3f)-4, 4
> 	.previous
> 	
> 	.org . + ((-.-(2b-1b)) & 3), 0x90
> 	movl $0x12345678,%r9d
> 3:
>
>
> The idea is that the instruction is emitted into a section, which is marked 
> DISCARD in the linker script.  That lets us actually measure the length, 
> and since we know the immediate is always at the end of the instruction... 
> done!
>

Wow! I like this idea. I'll give it a try in a test sandbox to see if
gas or ld complains. I'll keep you posted.

Mathieu

>


> 	-hpa
>

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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