[Qemu-devel] [PATCH] target-i386: Preserve the Z bit for bt/bts/btr/btc

[Richard Henderson <rth@twiddle.net>]

Older Intel manuals (pre-2010) and current AMD manuals describe Z as
undefined, but newer Intel manuals describe Z as unchanged.

Cc: qemu-stable@nongnu.org
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 target-i386/translate.c | 40 +++++++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/target-i386/translate.c b/target-i386/translate.c
index 02625e3..032b0fd 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -6708,41 +6708,63 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
         }
     bt_op:
         tcg_gen_andi_tl(cpu_T[1], cpu_T[1], (1 << (3 + ot)) - 1);
+        tcg_gen_shr_tl(cpu_tmp4, cpu_T[0], cpu_T[1]);
         switch(op) {
         case 0:
-            tcg_gen_shr_tl(cpu_cc_src, cpu_T[0], cpu_T[1]);
-            tcg_gen_movi_tl(cpu_cc_dst, 0);
             break;
         case 1:
-            tcg_gen_shr_tl(cpu_tmp4, cpu_T[0], cpu_T[1]);
             tcg_gen_movi_tl(cpu_tmp0, 1);
             tcg_gen_shl_tl(cpu_tmp0, cpu_tmp0, cpu_T[1]);
             tcg_gen_or_tl(cpu_T[0], cpu_T[0], cpu_tmp0);
             break;
         case 2:
-            tcg_gen_shr_tl(cpu_tmp4, cpu_T[0], cpu_T[1]);
             tcg_gen_movi_tl(cpu_tmp0, 1);
             tcg_gen_shl_tl(cpu_tmp0, cpu_tmp0, cpu_T[1]);
-            tcg_gen_not_tl(cpu_tmp0, cpu_tmp0);
-            tcg_gen_and_tl(cpu_T[0], cpu_T[0], cpu_tmp0);
+            tcg_gen_andc_tl(cpu_T[0], cpu_T[0], cpu_tmp0);
             break;
         default:
         case 3:
-            tcg_gen_shr_tl(cpu_tmp4, cpu_T[0], cpu_T[1]);
             tcg_gen_movi_tl(cpu_tmp0, 1);
             tcg_gen_shl_tl(cpu_tmp0, cpu_tmp0, cpu_T[1]);
             tcg_gen_xor_tl(cpu_T[0], cpu_T[0], cpu_tmp0);
             break;
         }
-        set_cc_op(s, CC_OP_SARB + ot);
         if (op != 0) {
             if (mod != 3) {
                 gen_op_st_v(s, ot, cpu_T[0], cpu_A0);
             } else {
                 gen_op_mov_reg_v(ot, rm, cpu_T[0]);
             }
+        }
+
+        /* Delay all CC updates until after the store above.  Note that
+           C is the result of the test, Z is unchanged, and the others
+           are all undefined.  */
+        switch (s->cc_op) {
+        case CC_OP_MULB ... CC_OP_MULQ:
+        case CC_OP_ADDB ... CC_OP_ADDQ:
+        case CC_OP_ADCB ... CC_OP_ADCQ:
+        case CC_OP_SUBB ... CC_OP_SUBQ:
+        case CC_OP_SBBB ... CC_OP_SBBQ:
+        case CC_OP_LOGICB ... CC_OP_LOGICQ:
+        case CC_OP_INCB ... CC_OP_INCQ:
+        case CC_OP_DECB ... CC_OP_DECQ:
+        case CC_OP_SHLB ... CC_OP_SHLQ:
+        case CC_OP_SARB ... CC_OP_SARQ:
+        case CC_OP_BMILGB ... CC_OP_BMILGQ:
+            /* Z was going to be computed from the non-zero status of CC_DST.
+               We can get that same Z value (and the new C value) by leaving
+               CC_DST alone, setting CC_SRC, and using a CC_OP_SAR of the
+               same width.  */
             tcg_gen_mov_tl(cpu_cc_src, cpu_tmp4);
-            tcg_gen_movi_tl(cpu_cc_dst, 0);
+            set_cc_op(s, ((s->cc_op - CC_OP_MULB) & 3) + CC_OP_SARB);
+            break;
+        default:
+            /* Otherwise, generate EFLAGS and replace the C bit.  */
+            gen_compute_eflags(s);
+            tcg_gen_deposit_tl(cpu_cc_src, cpu_cc_src, cpu_tmp4,
+                               ctz32(CC_C), 1);
+            break;
         }
         break;
     case 0x1bc: /* bsf / tzcnt */
-- 
1.9.0