Messages in this thread | | | Date | Wed, 14 Nov 2007 09:16:35 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [patch 5/8] Immediate Values - x86 Optimization (update 2) |
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* 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 >
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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