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.
----------------------------
%%%
Bruce
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