When ADCX is followed by ADOX or vice versa, the second instruction's
carry comes from EFLAGS.  This 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>
---
 target/i386/tcg/emit.c.inc       | 20 +++++----
 tests/tcg/i386/Makefile.target   |  6 ++-
 tests/tcg/i386/test-i386-adcox.c | 75 ++++++++++++++++++++++++++++++++
 3 files changed, 91 insertions(+), 10 deletions(-)
 create mode 100644 tests/tcg/i386/test-i386-adcox.c

diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc
index e33688f672a2..5a1d3803f901 100644
--- a/target/i386/tcg/emit.c.inc
+++ b/target/i386/tcg/emit.c.inc
@@ -866,6 +866,7 @@ 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;
@@ -873,14 +874,8 @@ 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);
         }
@@ -904,7 +899,14 @@ 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)
diff --git a/tests/tcg/i386/Makefile.target b/tests/tcg/i386/Makefile.target
index 3273aa8061f8..ac443995447f 100644
--- a/tests/tcg/i386/Makefile.target
+++ b/tests/tcg/i386/Makefile.target
@@ -14,7 +14,7 @@ config-cc.mak: Makefile
 I386_SRCS=$(notdir $(wildcard $(I386_SRC)/*.c))
 ALL_X86_TESTS=$(I386_SRCS:.c=)
 SKIP_I386_TESTS=test-i386-ssse3 test-avx test-3dnow test-mmx
-X86_64_TESTS:=$(filter test-i386-bmi2 $(SKIP_I386_TESTS), $(ALL_X86_TESTS))
+X86_64_TESTS:=$(filter test-i386-adcox test-i386-bmi2 $(SKIP_I386_TESTS), $(ALL_X86_TESTS))
 
 test-i386-sse-exceptions: CFLAGS += -msse4.1 -mfpmath=sse
 run-test-i386-sse-exceptions: QEMU_OPTS += -cpu max
@@ -28,6 +28,10 @@ test-i386-bmi2: CFLAGS=-O2
 run-test-i386-bmi2: QEMU_OPTS += -cpu max
 run-plugin-test-i386-bmi2-%: QEMU_OPTS += -cpu max
 
+test-i386-adcox: CFLAGS=-O2
+run-test-i386-adcox: QEMU_OPTS += -cpu max
+run-plugin-test-i386-adcox-%: QEMU_OPTS += -cpu max
+
 #
 # hello-i386 is a barebones app
 #
diff --git a/tests/tcg/i386/test-i386-adcox.c b/tests/tcg/i386/test-i386-adcox.c
new file mode 100644
index 000000000000..16169efff823
--- /dev/null
+++ b/tests/tcg/i386/test-i386-adcox.c
@@ -0,0 +1,75 @@
+/* See if various BMI2 instructions give expected results */
+#include <assert.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#define CC_C 1
+#define CC_O (1 << 11)
+
+#ifdef __x86_64__
+#define REG uint64_t
+#else
+#define REG uint32_t
+#endif
+
+void test_adox_adcx(uint32_t in_c, uint32_t in_o, REG adcx_operand, REG adox_operand)
+{
+    REG flags;
+    REG out_adcx, out_adox;
+
+    asm("pushf; pop %0" : "=r"(flags));
+    flags &= ~(CC_C | CC_O);
+    flags |= (in_c ? CC_C : 0);
+    flags |= (in_o ? CC_O : 0);
+
+    out_adcx = adcx_operand;
+    out_adox = adox_operand;
+    asm("push %0; popf;"
+        "adox %3, %2;"
+        "adcx %3, %1;"
+        "pushf; pop %0"
+        : "+r" (flags), "+r" (out_adcx), "+r" (out_adox)
+        : "r" ((REG)-1), "0" (flags), "1" (out_adcx), "2" (out_adox));
+
+    assert(out_adcx == in_c + adcx_operand - 1);
+    assert(out_adox == in_o + adox_operand - 1);
+    assert(!!(flags & CC_C) == (in_c || adcx_operand));
+    assert(!!(flags & CC_O) == (in_o || adox_operand));
+}
+
+void test_adcx_adox(uint32_t in_c, uint32_t in_o, REG adcx_operand, REG adox_operand)
+{
+    REG flags;
+    REG out_adcx, out_adox;
+
+    asm("pushf; pop %0" : "=r"(flags));
+    flags &= ~(CC_C | CC_O);
+    flags |= (in_c ? CC_C : 0);
+    flags |= (in_o ? CC_O : 0);
+
+    out_adcx = adcx_operand;
+    out_adox = adox_operand;
+    asm("push %0; popf;"
+        "adcx %3, %1;"
+        "adox %3, %2;"
+        "pushf; pop %0"
+        : "+r" (flags), "+r" (out_adcx), "+r" (out_adox)
+        : "r" ((REG)-1), "0" (flags), "1" (out_adcx), "2" (out_adox));
+
+    assert(out_adcx == in_c + adcx_operand - 1);
+    assert(out_adox == in_o + adox_operand - 1);
+    assert(!!(flags & CC_C) == (in_c || adcx_operand));
+    assert(!!(flags & CC_O) == (in_o || adox_operand));
+}
+
+int main(int argc, char *argv[]) {
+    /* try all combinations of input CF, input OF, CF from op1+op2,  OF from op2+op1 */
+    int i;
+    for (i = 0; i <= 15; i++) {
+        printf("%d\n", i);
+        test_adcx_adox(!!(i & 1), !!(i & 2), !!(i & 4), !!(i & 8));
+        test_adox_adcx(!!(i & 1), !!(i & 2), !!(i & 4), !!(i & 8));
+    }
+    return 0;
+}
+
-- 
2.38.1

On 1/30/23 22:54, Paolo Bonzini wrote:
> When ADCX is followed by ADOX or vice versa, the second instruction's
> carry comes from EFLAGS.  This 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>
> ---
>   target/i386/tcg/emit.c.inc       | 20 +++++----
>   tests/tcg/i386/Makefile.target   |  6 ++-
>   tests/tcg/i386/test-i386-adcox.c | 75 ++++++++++++++++++++++++++++++++
>   3 files changed, 91 insertions(+), 10 deletions(-)
>   create mode 100644 tests/tcg/i386/test-i386-adcox.c

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

r~