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Re: [Qemu-arm] [Qemu-devel] Expensive emulation of CPU condition flags

From: Richard Henderson
Subject: Re: [Qemu-arm] [Qemu-devel] Expensive emulation of CPU condition flags
Date: Fri, 1 Jul 2016 15:34:15 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 06/30/2016 11:13 AM, Shuang Zhai wrote:
We wonder if there exists any optimization, e.g., directly mapping the
frontend flags to that of the backend? Any suggestions are appreciated.

Directly mapping frontend to backend flags is a non-starter, since not all backends have those flags.

There are alternate methods of emulating condition codes. As an example, target-i386 and target-sparc store two values and an "operation code" value. The latter indicates how to treat the former. This allows for the full computation of the flags to be delayed, and for the host compare-and-branch to be less complicated.

See also my design for an improved m68k condition code scheme:


You're welcome to experiment with target-arm. If you can create a scheme that performs better than the current, we'd like to hear about it.


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