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From: | Richard Henderson |
Subject: | Re: [Qemu-devel] [PATCH 2/2] tcg-x86_64: Avoid unnecessary REX.B prefixes. |
Date: | Tue, 05 Jan 2010 20:16:35 -0800 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.5) Gecko/20091209 Fedora/3.0-4.fc12 Thunderbird/3.0 |
On 01/05/2010 04:31 PM, Richard Henderson wrote:
A while ago Laurent pointed out that the setcc opcode emitted by the setcond patch had unnecessary REX prefixes. The existing P_REXB internal opcode flag unconditionally emits the REX prefix. Technically it's not needed if the register in question is %al, %bl, %cl, %dl.
I should perhaps mention that this patch doesn't help out all that much. Only EBX is free for use by the code generator, so more often than not we're using %bpl, which does require the REX prefix. r~
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