Re: [PATCH v3 06/14] tcg/arm: Support unaligned access for softmmu

[Peter Maydell <peter.maydell@linaro.org>]

On Wed, 18 Aug 2021 at 22:32, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> From armv6, the architecture supports unaligned accesses.
> All we need to do is perform the correct alignment check
> in tcg_out_tlb_read and not use LDRD/STRD when the access
> is not aligned.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> @@ -1578,27 +1576,32 @@ static TCGReg tcg_out_tlb_read(TCGContext *s, TCGReg addrlo, TCGReg addrhi,
>
>      /*
>       * Check alignment, check comparators.
> -     * Do this in no more than 3 insns.  Use MOVW for v7, if possible,
> +     * Do this in 2-4 insns.  Use MOVW for v7, if possible,
>       * to reduce the number of sequential conditional instructions.
>       * Almost all guests have at least 4k pages, which means that we need
>       * to clear at least 9 bits even for an 8-byte memory, which means it
>       * isn't worth checking for an immediate operand for BIC.
>       */
> +    /* For unaligned accesses, test the page of the last byte. */
> +    t_addr = addrlo;
> +    if (a_mask < s_mask) {
> +        t_addr = TCG_REG_R0;
> +        tcg_out_dat_imm(s, COND_AL, ARITH_ADD, t_addr,
> +                        addrlo, s_mask - a_mask);
> +    }

I don't understand what this comment means or why we're doing the
addition. If we know we need to check eg whether the address is 2-aligned,
why aren't we just checking whether it's 2-aligned ? Could you
expand on the explanation a bit?


thanks
-- PMM