Re: [PULL 02/38] tcg/optimize: Fix s_mask computation for shifts

On 8/18/26 20:01, Richard Henderson wrote:
> Skip s_mask computation for logical right shift.
> 
> Cc: [email protected]
> Fixes: 93a967fbb57 ("tcg/optimize: Propagate sign info for shifting")
> Reported-by: Jacob Young <[email protected]>
> Signed-off-by: Richard Henderson <[email protected]>
> ---
>   tcg/optimize.c                     | 11 ++++++++++-
>   tests/tcg/i386/test-i386-opt-shr.c | 21 +++++++++++++++++++++
>   2 files changed, 31 insertions(+), 1 deletion(-)
>   create mode 100644 tests/tcg/i386/test-i386-opt-shr.c
> 
> diff --git a/tcg/optimize.c b/tcg/optimize.c
> index ed2ff32ed2..d12babad88 100644
> --- a/tcg/optimize.c
> +++ b/tcg/optimize.c
> @@ -2656,8 +2656,17 @@ static bool fold_shift(OptContext *ctx, TCGOp *op)
>   
>           z_mask = do_constant_folding(op->opc, ctx->type, z_mask, sh);
>           o_mask = do_constant_folding(op->opc, ctx->type, o_mask, sh);
> -        s_mask = do_constant_folding(op->opc, ctx->type, s_mask, sh);
>   
> +        if (op->opc == INDEX_op_shr) {
> +            /*
> +             * Logical right shift will force the sign bit zero.
> +             * Don't bother computing s_mask and let fold_masks
> +             * recompute from z_mask.
> +             */
> +            return fold_masks_zo(ctx, op, z_mask, o_mask);
> +        }
> +
> +        s_mask = do_constant_folding(op->opc, ctx->type, s_mask, sh);

In 10.0.x, things are (were) a bit different.  Does this back-port look sane? --
https://gitlab.com/mjt0k/qemu/-/commit/77ad6491326b4d0b2907f0ed9d504d6f490e245c

Thanks,

/mjt