cvs commit: src/sys/amd64/amd64 support.S

Bruce Evans bde at
Wed Aug 16 14:34:36 UTC 2006

On Tue, 15 Aug 2006, John Baldwin wrote:

> On Tuesday 15 August 2006 09:55, Kostik Belousov wrote:
>> So, it seems that xorq %rax, %rax and xorl %eax, %eax will make the
>> same results, but in the different ways. And xorq requires REX prefix,
>> that shall make the decoding longer.
> Ok, thanks!  David, can you revert your change?  I had almost gone through
> earlier and fixed all the places that did this a while back until I realized
> that it must have been intentional.

It is logged as intentional in the commit that went through and optimized
all places that used xorq:

RCS file: /home/ncvs/src/sys/amd64/amd64/support.S,v
Working file: support.S
head: 1.118
revision 1.116
date: 2005/09/27 18:32:46;  author: peter;  state: Exp;  lines: +11 -11
Fix a minor nit that has been bugging me for a while.  Fix the obvious
cases of using a 64 bit operation to zero a register.  32 bit opcodes are
smaller and supposedly faster, and clear the upper 32 bits for free.


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