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Re: [Qemu-devel] [PATCH 1/9] TCG "sync" op


From: Ulrich Hecht
Subject: Re: [Qemu-devel] [PATCH 1/9] TCG "sync" op
Date: Mon, 19 Oct 2009 19:17:33 +0200
User-agent: KMail/1.9.10

On Friday 16 October 2009, Aurelien Jarno wrote:
> This example is a bit biased, as registers are only saved, and never
> reused. Let's comment on it though.

Yeah, well, I searched from the top for the first case where it makes a 
difference. If it's of any help, I can upload a complete dump of both 
versions somewhere.

> and I don't understand what is the gain compared to 
> the use of tcg_gen_ld/st.

There are two sets of TCG values, tcgregs (which would arguably better 
called tcgregs64) and tcgregs32. When doing a 32-bit access, tcgregs(64) 
is synced, which is a nop if tcgregs(64) hasn't been touched. When doing 
a 64-bit access, tcgregs32 is synced, which is a nop if tcgregs32 hasn't 
been touched. In practice, 32-bit accesses followed by 64-bit accesses 
and vice versa are very rare, so in most cases, sync is a nop. 
tcg_gen_ld/st is never a nop. That's the benefit.

CU
Uli

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