Re: [PATCH 20/20] target/i386/tcg: optimize CCMP

[Zhao Liu <zhao1.liu@intel.com>]

On Tue, Aug 25, 2026 at 02:29:21PM +0200, Paolo Bonzini wrote:
> Date: Tue, 25 Aug 2026 14:29:21 +0200
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH 20/20] target/i386/tcg: optimize CCMP
> X-Mailer: git-send-email 2.55.0
> 
> If possible, modify conditionally the input arguments to generate the
> flags requested by dfv; this way CCMP can use CC_OP_SUB* instead of having
> to compute the carry-out vector by hand.  This happens relatively often
> for dfv=0, and also for dfv=sf for code produced by clang.
> 
> Of the combinations that cannot be optimized, both GCC and clang generate
> dfv=zf.  GCC also generates OS and OSZ.
> 
> Do not bother doing this for CTEST; the savings are modest because it
> does not need complex code to compute the carry-out vector (CC_SRC
> is always 0).  In addition, trivially replacing the arguments to the AND
> would only support dfv=0 (produced by -2 & 2) or dfv=S (produced by -2 &
> -2), because TEST cannot produce any value where CF or OF are not zero.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  target/i386/tcg/emit.c.inc | 56 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 51 insertions(+), 5 deletions(-)
> 
> diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc
> index 1144953abb1..7641b2fc104 100644
> --- a/target/i386/tcg/emit.c.inc
> +++ b/target/i386/tcg/emit.c.inc
> @@ -1749,6 +1749,53 @@ static void gen_dfv_movcond(DisasContext *s, X86DecodedInsn *decode, int cond, u
>                         decode->cc_dst, tcg_constant_tl(dst));
>  }
>  
> +static bool gen_ccmp_movcond(DisasContext *s, int cond, uint32_t dfv, MemOp ot)
> +{
> +    target_ulong op0, op1, max_int;
> +    if (cond == CCMP_T) {
> +        return true;
> +    }

should we return false for CCMP_F? Or...

> +}
> +
>  static void gen_SUB(DisasContext *s, X86DecodedInsn *decode);
>  static void gen_CMP(DisasContext *s, X86DecodedInsn *decode)
>  {
> @@ -1757,16 +1804,15 @@ static void gen_CMP(DisasContext *s, X86DecodedInsn *decode)
>      MemOp ot = decode->op[1].ot;
>      TCGv cout;
>  
> -    switch (cond) {
> -    case CCMP_T:
> +    if (gen_ccmp_movcond(s, cond, dfv, ot)) {
> +        /* Including CCMP_T, aka "normal" CMP.  */
>          gen_SUB(s, decode);
>          return;
> -    case CCMP_F:
> +    }
> +    if (cond == CCMP_F) {
>          decode->cc_op = CC_OP_EFLAGS;
>          decode->cc_src = tcg_constant_tl(dfv);
>          return;
> -    default:
> -        break;
>      }

...check the CCMP_F and return first:

if (cond == CCMP_F) {
    ...
    return;
}

if (gen_ccmp_movcond(s, cond, dfv, ot)) {
   ...
   return;
}

Thanks,
Zhao