Re: [Qemu-devel] [PATCH 08/20] target/arm: Don't warn about exception return with PC low bit set for v8M

[Richard Henderson <richard.henderson@linaro.org>]

On 09/22/2017 10:59 AM, Peter Maydell wrote:
> +        if ((env->regs[15] & 1) && !arm_feature(env, ARM_FEATURE_V8)) {
>              qemu_log_mask(LOG_GUEST_ERROR,
>                            "M profile return from interrupt with misaligned "
> -                          "PC is UNPREDICTABLE\n");
> -            /* Actual hardware seems to ignore the lsbit, and there are several
> -             * RTOSes out there which incorrectly assume the r15 in the stack
> -             * frame should be a Thumb-style "lsbit indicates ARM/Thumb" value.
> -             */
> -            env->regs[15] &= ~1U;
> +                          "PC is UNPREDICTABLE on v7M\n");
>          }
> +        env->regs[15] &= ~1U;

If you're going to always test regs[15] & 1, you might as well use that and
avoid an unlikely(?) writeback.

    if (env->regs[15] & 1) {
        if (!arm_feature(env, ARM_FEATURE_V8)) {
            qemu_log_mask(...);
        }
        env->regs[15] &= ~1U;
    }

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


r~