target-i386: Fix aflag logic for CODE64 and the 0x67 prefix

The code reorganization in commit 4a6fd938 broke handling of PREFIX_ADR.
While fixing this, tidy and comment the code so that it's more obvious
what's going on in setting both aflag and dflag.

The TARGET_X86_64 ifdef can be eliminated because CODE64 expands to the
constant zero when TARGET_X86_64 is undefined.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 1369855851-21400-1-git-send-email-rth@twiddle.net
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

target-i386/translate.c

@@ -4677,8 +4677,6 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
     }
     s->pc = pc_start;
     prefixes = 0;
-    aflag = s->code32;
-    dflag = s->code32;
     s->override = -1;
     rex_w = -1;
     rex_r = 0;
@@ -4801,23 +4799,25 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
     }
 
     /* Post-process prefixes.  */
-    if (prefixes & PREFIX_DATA) {
-        dflag ^= 1;
-    }
-    if (prefixes & PREFIX_ADR) {
-        aflag ^= 1;
-    }
-#ifdef TARGET_X86_64
     if (CODE64(s)) {
-        if (rex_w == 1) {
-            /* 0x66 is ignored if rex.w is set */
-            dflag = 2;
+        /* In 64-bit mode, the default data size is 32-bit.  Select 64-bit
+           data with rex_w, and 16-bit data with 0x66; rex_w takes precedence
+           over 0x66 if both are present.  */
+        dflag = (rex_w > 0 ? 2 : prefixes & PREFIX_DATA ? 0 : 1);
+        /* In 64-bit mode, 0x67 selects 32-bit addressing.  */
+        aflag = (prefixes & PREFIX_ADR ? 1 : 2);
+    } else {
+        /* In 16/32-bit mode, 0x66 selects the opposite data size.  */
+        dflag = s->code32;
+        if (prefixes & PREFIX_DATA) {
+            dflag ^= 1;
         }
-        if (!(prefixes & PREFIX_ADR)) {
-            aflag = 2;
+        /* In 16/32-bit mode, 0x67 selects the opposite addressing.  */
+        aflag = s->code32;
+        if (prefixes & PREFIX_ADR) {
+            aflag ^= 1;
         }
     }
-#endif
 
     s->prefix = prefixes;
     s->aflag = aflag;