Re: [PATCH 07/14] target/i386: optimize computation of ZF from CC_OP_DYNAMIC

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

On 10/20/24 08:53, Paolo Bonzini wrote:
> Most uses of CC_OP_DYNAMIC are for CMP/JB/JE or similar sequences.
> We can optimize many of them to avoid computation of the flags.
> This eliminates both TCG ops to set up the new cc_op, and helper
> instructions because evaluating just ZF is much cheaper.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   target/i386/helper.h        |  1 +
>   target/i386/tcg/cc_helper.c | 20 ++++++++++++++++++++
>   target/i386/tcg/translate.c | 10 +++++++---
>   3 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/target/i386/helper.h b/target/i386/helper.h
> index eeb8df56eaa..3f67098f11f 100644
> --- a/target/i386/helper.h
> +++ b/target/i386/helper.h
> @@ -1,5 +1,6 @@
>   DEF_HELPER_FLAGS_4(cc_compute_all, TCG_CALL_NO_RWG_SE, tl, tl, tl, tl, int)
>   DEF_HELPER_FLAGS_4(cc_compute_c, TCG_CALL_NO_RWG_SE, tl, tl, tl, tl, int)
> +DEF_HELPER_FLAGS_3(cc_compute_nz, TCG_CALL_NO_RWG_SE, tl, tl, tl, int)
>   
>   DEF_HELPER_3(write_eflags, void, env, tl, i32)
>   DEF_HELPER_1(read_eflags, tl, env)
> diff --git a/target/i386/tcg/cc_helper.c b/target/i386/tcg/cc_helper.c
> index 40583c04cf9..c24e6a14c07 100644
> --- a/target/i386/tcg/cc_helper.c
> +++ b/target/i386/tcg/cc_helper.c
> @@ -95,6 +95,26 @@ static target_ulong compute_all_adcox(target_ulong dst, target_ulong src1,
>       return (src1 & ~(CC_C | CC_O)) | (dst * CC_C) | (src2 * CC_O);
>   }
>   
> +target_ulong helper_cc_compute_nz(target_ulong dst, target_ulong src1,
> +                                  int op)
> +{
> +    target_ulong mask;
> +
> +    if (CC_OP_HAS_EFLAGS(op)) {
> +        return ~src1 & CC_Z;
> +    } else {
> +        MemOp size = cc_op_size(op);
> +
> +        if (size == MO_TL) {
> +            /* Avoid shift count overflow when computing the mask below.  */
> +            return dst;
> +        }
> +
> +        mask = (1ull << (8 << size)) - 1;

FWIW, MAKE_64BIT_MASK(0, 8 << size) does not have the overflow problem.


r~