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Re: [PATCH 06/10] target/arm: Implement v8.1M branch-future insns (as NO

From: Richard Henderson
Subject: Re: [PATCH 06/10] target/arm: Implement v8.1M branch-future insns (as NOPs)
Date: Tue, 13 Oct 2020 09:58:29 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 10/12/20 8:37 AM, Peter Maydell wrote:
> v8.1M implements a new 'branch future' feature, which is a
> set of instructions that request the CPU to perform a branch
> "in the future", when it reaches a particular execution address.
> In hardware, the expected implementation is that the information
> about the branch location and destination is cached and then
> acted upon when execution reaches the specified address.
> However the architecture permits an implementation to discard
> this cached information at any point, and so guest code must
> always include a normal branch insn at the branch point as
> a fallback. In particular, an implementation is specifically
> permitted to treat all BF insns as NOPs (which is equivalent
> to discarding the cached information immediately).
> For QEMU, implementing this caching of branch information
> would be complicated and would not improve the speed of
> execution at all, so we make the IMPDEF choice to implement
> all BF insns as NOPs.
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


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