Re: [PATCH] tcg/aarch64: Fix constant subtraction in tcg_out_addsub2

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

On Sun, 28 Feb 2021 at 23:55, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> An hppa guest executing
>
> 0x000000000000e05c:  ldil L%10000,r4
> 0x000000000000e060:  ldo 0(r4),r4
> 0x000000000000e064:  sub r3,r4,sp
>
> produces
>
>  ---- 000000000000e064 000000000000e068
>  sub2_i32 tmp0,tmp4,r3,$0x1,$0x10000,$0x0
>
> after folding and constant propagation.  Then we hit
>
> tcg-target.c.inc:640: tcg_out_insn_3401: Assertion `aimm <= 0xfff' failed.
>
> because aimm is in fact -16, but unsigned.
>
> The ((bl < 0) ^ sub) condition which negates bl is incorrect and will
> always lead to this abort.  If the constant is positive, sub will make
> it negative; if the constant is negative, sub will keep it negative.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
> -static inline void tcg_out_addsub2(TCGContext *s, TCGType ext, TCGReg rl,
> -                                   TCGReg rh, TCGReg al, TCGReg ah,
> -                                   tcg_target_long bl, tcg_target_long bh,
> -                                   bool const_bl, bool const_bh, bool sub)
> +static void tcg_out_addsub2(TCGContext *s, TCGType ext, TCGReg rl,
> +                            TCGReg rh, TCGReg al, TCGReg ah,
> +                            tcg_target_long bl, tcg_target_long bh,
> +                            bool const_bl, bool const_bh, bool sub)
>  {
>      TCGReg orig_rl = rl;
>      AArch64Insn insn;

Seems like an unrelated change ?

> @@ -1423,11 +1423,13 @@ static inline void tcg_out_addsub2(TCGContext *s, TCGType ext, TCGReg rl,
>      }
>
>      if (const_bl) {
> -        insn = I3401_ADDSI;
> -        if ((bl < 0) ^ sub) {
> -            insn = I3401_SUBSI;
> +        if (bl < 0) {
>              bl = -bl;
> +            insn = sub ? I3401_ADDSI : I3401_SUBSI;
> +        } else {
> +            insn = sub ? I3401_SUBSI : I3401_ADDSI;
>          }
> +
>          if (unlikely(al == TCG_REG_XZR)) {
>              /* ??? We want to allow al to be zero for the benefit of
>                 negation via subtraction.  However, that leaves open the
> --
> 2.25.1

Otherwise
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM