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Re: [Qemu-devel] [PATCH] target-arm: Move VLD/VST multiple into helper f
From: |
Aurelien Jarno |
Subject: |
Re: [Qemu-devel] [PATCH] target-arm: Move VLD/VST multiple into helper functions |
Date: |
Fri, 3 Jun 2011 22:47:22 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Mon, May 02, 2011 at 05:01:24PM +0100, Peter Maydell wrote:
> On 20 April 2011 15:52, Peter Maydell <address@hidden> wrote:
> > Move VLD/VST multiple into helper functions, as some cases can
> > generate more TCG ops than the maximum per-instruction limit
> > and certainly more than the recommended 20.
>
> I've had a review comment in private email that this patch
> slows down ffmpeg by a little over 4% (in a linux-user execution
> run where about 5% of instructions were an affected vld/vst). So
> perhaps the right approach is to increase the max-ops-per-insn limit
> and update the guidance in tcg/README instead?
>
Does this patch fixes a real issue (ie most probably a crash), or it is
just to make the arm target compliant with the README?
Two remarks there:
- The guidance in tcg/README is probably true for complex helpers in the
sense of doing a lot of logic/arithmetic operations for which the TCG
instruction set if not really rich and for which the compiler can do
a lot better. When the instruction is emulated mostly by load/store
ops, there isn't much possible optimizations left.
- Your patch calls the slow version of the _mmu functions, which doesn't
use the QEMU TLB. It is therefore normal to expect a slow down.
Unfortunately the fast version of _mmu functions are not really usable
from an helper, or at least they need a lot of code to convert an mmu
index to the right function. It is something I have on my TODO list
for some months, haven't found time to look at it.
--
Aurelien Jarno GPG: 1024D/F1BCDB73
address@hidden http://www.aurel32.net
- Re: [Qemu-devel] [PATCH] target-arm: Move VLD/VST multiple into helper functions,
Aurelien Jarno <=