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

Mathieu Desnoyers
2007-11-14T14:21:47Z

* H. Peter Anvin (hpa@zytor.com) 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 register—the 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