Messages in this thread | | | Date | Wed, 9 Apr 2008 15:08:16 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 13/17] Immediate Values - x86 Optimization |
| |
* 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.
That's required for the NMI-safe version of the immediate values, which uses a breakpoint, but not for this version based on stop_machine_run(). However, to minimize the amount of changes between the two versions, I left the =q constraint, which is more restrictive. Is it worth it to use =r instead ? It will typically let the compiler use a wider range of registers on x86_64.
Thanks,
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
| |