Re: [PATCH] tcg/i386: use 8-bit OR or XOR for unsigned 8-bit immediates

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

On 12/28/23 23:05, Paolo Bonzini wrote:
> In the case where OR or XOR has an 8-bit immediate between 128 and 255, we can
> operate on a low-byte register and shorten the output by two or three bytes
> (two if a prefix byte is needed for REX.B).
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   tcg/i386/tcg-target.c.inc | 7 +++++++
>   1 file changed, 7 insertions(+)

At least once upon a time the partial register stall like this was quite slow.  IIRC there 
have been improvements in the last couple of generations, but it's still slower.

Data to show this is worthwhile?


r~

> 
> diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
> index 1791b959738..a24a23f43b1 100644
> --- a/tcg/i386/tcg-target.c.inc
> +++ b/tcg/i386/tcg-target.c.inc
> @@ -244,6 +244,7 @@ static bool tcg_target_const_match(int64_t val, TCGType type, int ct, int vece)
>   #define P_VEXL          0x80000         /* Set VEX.L = 1 */
>   #define P_EVEX          0x100000        /* Requires EVEX encoding */
>   
> +#define OPC_ARITH_EbIb	(0x80)
>   #define OPC_ARITH_EvIz	(0x81)
>   #define OPC_ARITH_EvIb	(0x83)
>   #define OPC_ARITH_GvEv	(0x03)		/* ... plus (ARITH_FOO << 3) */
> @@ -1366,6 +1367,12 @@ static void tgen_arithi(TCGContext *s, int c, int r0,
>           tcg_out8(s, val);
>           return;
>       }
> +    if (val == (uint8_t)val && (c == ARITH_OR || c == ARITH_XOR) &&
> +        (r0 < 4 || TCG_TARGET_REG_BITS == 64)) {
> +        tcg_out_modrm(s, OPC_ARITH_EbIb + P_REXB_RM, c, r0);
> +        tcg_out8(s, val);
> +        return;
> +    }
>       if (rexw == 0 || val == (int32_t)val) {
>           tcg_out_modrm(s, OPC_ARITH_EvIz + rexw, c, r0);
>           tcg_out32(s, val);