target/i386: fix ADOX followed by ADCX

When ADCX is followed by ADOX or vice versa, the second instruction's
carry comes from EFLAGS and the condition codes use the CC_OP_ADCOX
operation.  Retrieving the carry from EFLAGS is handled by this bit
of gen_ADCOX:

        tcg_gen_extract_tl(carry_in, cpu_cc_src,
            ctz32(cc_op == CC_OP_ADCX ? CC_C : CC_O), 1);

Unfortunately, in this case cc_op has been overwritten by the previous
"if" statement to CC_OP_ADCOX.  This works by chance when the first
instruction is ADCX; however, if the first instruction is ADOX,
ADCX will incorrectly take its carry from OF instead of CF.

Fix by moving the computation of the new cc_op at the end of the function.
The included exhaustive test case fails without this patch and passes
afterwards.

Because ADCX/ADOX need not be invoked through the VEX prefix, this
regression bisects to commit 16fc5726a6e2 ("target/i386: reimplement
0x0f 0x38, add AVX", 2022-10-18).  However, the mistake happened a
little earlier, when BMI instructions were rewritten using the new
decoder framework.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1471
Reported-by: Paul Jolly <https://gitlab.com/myitcv>
Fixes: 1d0b926150e5 ("target/i386: move scalar 0F 38 and 0F 3A instruction to new decoder", 2022-10-18)
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 11 insertions, 9 deletions

diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc
index 4d7702c..0d7c6e8 100644
--- a/target/i386/tcg/emit.c.inc
+++ b/target/i386/tcg/emit.c.inc
@@ -1015,6 +1015,7 @@ VSIB_AVX(VPGATHERQ, vpgatherq)
 
 static void gen_ADCOX(DisasContext *s, CPUX86State *env, MemOp ot, int cc_op)
 {
+    int opposite_cc_op;
     TCGv carry_in = NULL;
     TCGv carry_out = (cc_op == CC_OP_ADCX ? cpu_cc_dst : cpu_cc_src2);
     TCGv zero;
@@ -1022,14 +1023,8 @@ static void gen_ADCOX(DisasContext *s, CPUX86State *env, MemOp ot, int cc_op)
     if (cc_op == s->cc_op || s->cc_op == CC_OP_ADCOX) {
         /* Re-use the carry-out from a previous round.  */
         carry_in = carry_out;
-        cc_op = s->cc_op;
-    } else if (s->cc_op == CC_OP_ADCX || s->cc_op == CC_OP_ADOX) {
-        /* Merge with the carry-out from the opposite instruction.  */
-        cc_op = CC_OP_ADCOX;
-    }
-
-    /* If we don't have a carry-in, get it out of EFLAGS.  */
-    if (!carry_in) {
+    } else {
+        /* We don't have a carry-in, get it out of EFLAGS.  */
         if (s->cc_op != CC_OP_ADCX && s->cc_op != CC_OP_ADOX) {
             gen_compute_eflags(s);
         }
@@ -1053,7 +1048,14 @@ static void gen_ADCOX(DisasContext *s, CPUX86State *env, MemOp ot, int cc_op)
         tcg_gen_add2_tl(s->T0, carry_out, s->T0, carry_out, s->T1, zero);
         break;
     }
-    set_cc_op(s, cc_op);
+
+    opposite_cc_op = cc_op == CC_OP_ADCX ? CC_OP_ADOX : CC_OP_ADCX;
+    if (s->cc_op == CC_OP_ADCOX || s->cc_op == opposite_cc_op) {
+        /* Merge with the carry-out from the opposite instruction.  */
+        set_cc_op(s, CC_OP_ADCOX);
+    } else {
+        set_cc_op(s, cc_op);
+    }
 }
 
 static void gen_ADCX(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)