Messages in this thread | | | Date | Wed, 9 Apr 2008 20:42:19 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 13/17] Immediate Values - x86 Optimization |
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* H. Peter Anvin (hpa@zytor.com) wrote: > Mathieu Desnoyers wrote: >> * H. Peter Anvin (hpa@zytor.com) wrote: >>> Mathieu Desnoyers wrote: >>>> Ok, so the most flexible solution that I see, that should fit for both >>>> x86 and x86_64 would be : >>>> 1 byte : "=q" : "a", "b", "c", or "d" register for the i386. For >>>> x86-64 it is equivalent to "r" class (for 8-bit >>>> instructions that do not use upper halves). >>>> 2, 4, 8 bytes : "=r" : A register operand is allowed provided that it is >>>> in a >>>> general register. >>> Any reason to keep carrying this completely misleading comment chunk >>> still? >>> >>> -hpa >> This comment explains why I use the =q constraint for the 1 bytes >> immediate value. It makes sure we use an instruction with 1-byte opcode, >> without REX.R prefix, on x86_64. > > No, it doesn't. That would be "=Q". > > -hpa
Ok. Sorry, it's been a few months since we looked at this. So the =q opcode lets the compiler choose instructions with or without REX prefix. We can allow this because
- We don't need the opcode length in the stop_machine_run() version - we support variable length opcode in the nmi-safe version
Am I remembering correctly now ?
Mathieu
-- 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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