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 | 13 +++++++++++++
 target/i386/tcg/translate.c | 10 +++++++---
 3 files changed, 21 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..1b83775a914 100644
--- a/target/i386/tcg/cc_helper.c
+++ b/target/i386/tcg/cc_helper.c
@@ -95,6 +95,19 @@ 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)
+{
+    if (CC_OP_HAS_EFLAGS(op)) {
+        return ~src1 & CC_Z;
+    } else {
+        MemOp size = cc_op_size(op);
+        target_ulong mask = MAKE_64BIT_MASK(0, 8 << size);
+
+        return dst & mask;
+    }
+}
+
 target_ulong helper_cc_compute_all(target_ulong dst, target_ulong src1,
                                    target_ulong src2, int op)
 {
diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index 1a9a2fe709e..5e326ab1aff 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -1008,15 +1008,19 @@ static CCPrepare gen_prepare_eflags_o(DisasContext *s, TCGv reg)
 static CCPrepare gen_prepare_eflags_z(DisasContext *s, TCGv reg)
 {
     switch (s->cc_op) {
-    case CC_OP_DYNAMIC:
-        gen_compute_eflags(s);
-        /* FALLTHRU */
     case CC_OP_EFLAGS:
     case CC_OP_ADCX:
     case CC_OP_ADOX:
     case CC_OP_ADCOX:
         return (CCPrepare) { .cond = TCG_COND_TSTNE, .reg = cpu_cc_src,
                              .imm = CC_Z };
+    case CC_OP_DYNAMIC:
+        gen_update_cc_op(s);
+        if (!reg) {
+            reg = tcg_temp_new();
+        }
+        gen_helper_cc_compute_nz(reg, cpu_cc_dst, cpu_cc_src, cpu_cc_op);
+        return (CCPrepare) { .cond = TCG_COND_EQ, .reg = reg, .imm = 0 };
     default:
         {
             MemOp size = cc_op_size(s->cc_op);
-- 
2.47.0

On 10/28/24 15:18, 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 | 13 +++++++++++++
>   target/i386/tcg/translate.c | 10 +++++++---
>   3 files changed, 21 insertions(+), 3 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~