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Re: RFC: [PATCH] X86: Add pseudo prefixes to control encoding
On Fri, Nov 11, 2016 at 1:28 PM, Florian Weimer <firstname.lastname@example.org> wrote:
> On 11/11/2016 10:24 PM, H.J. Lu wrote:
>>> Sure, but I think the instruction length specification would still be
>>> useful in some cases to lock things down (and document the length
>> Instruction length can a vague constraint:
>> 0: c4 e1 78 28 50 00 vmovaps 0x0(%rax),%xmm2
>> 6: 62 f1 7c 08 28 10 vmovaps (%rax),%xmm2
>> Both instructions with the same length do the same thing.
>> But one is AVX and the other is AVX512.
>> c: c4 e1 78 28 10 vmovaps (%rax),%xmm2
>> 11: c5 f8 28 50 00 vmovaps 0x0(%rax),%xmm2
>> These 2 are AVX. Majority of programmers don't care how
>> an instruction is encoded. If they want specific instruction
>> length, they can experiment with pseudo prefixes to get
>> exactly what they need.
> Oh, I didn't intend this as a replacement, just as an additional
Control instruction length is one usage of pseudo prefixes.
Specifying instruction length will make assembler unnecessarily
more complex, like how to encode "vmovaps (%rax),%xmm2"
with 7 bytes.