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!
-hpa
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