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05903f036e
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. Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Message-id: 20201019151301.2046-7-peter.maydell@linaro.org |
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