tcg/i386: convert add/sub of 128 to sub/add of -128

Extend the existing conditional that generates INC/DEC, to also swap an
ADD for a SUB and vice versa when the immediate is 128.  This facilitates
using OPC_ARITH_EvIb instead of OPC_ARITH_EvIz.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20231228120514.70205-1-pbonzini@redhat.com>
[rth: Use a switch on C]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
index a83f8aab30446df4f60cf584a37e615e790048c0..29e80af78bb17a7702584abbd391d133f15ced45 100644 (file)
--- a/tcg/i386/tcg-target.c.inc
+++ b/tcg/i386/tcg-target.c.inc
@@ -1316,23 +1316,41 @@ static void tgen_arithi(TCGContext *s, int c, int r0,
         c &= 7;
     }
 
-    /* ??? While INC is 2 bytes shorter than ADDL $1, they also induce
-       partial flags update stalls on Pentium4 and are not recommended
-       by current Intel optimization manuals.  */
-    if (!cf && (c == ARITH_ADD || c == ARITH_SUB) && (val == 1 || val == -1)) {
-        int is_inc = (c == ARITH_ADD) ^ (val < 0);
-        if (TCG_TARGET_REG_BITS == 64) {
-            /* The single-byte increment encodings are re-tasked as the
-               REX prefixes.  Use the MODRM encoding.  */
-            tcg_out_modrm(s, OPC_GRP5 + rexw,
-                          (is_inc ? EXT5_INC_Ev : EXT5_DEC_Ev), r0);
-        } else {
-            tcg_out8(s, (is_inc ? OPC_INC_r32 : OPC_DEC_r32) + r0);
+    switch (c) {
+    case ARITH_ADD:
+    case ARITH_SUB:
+        if (!cf) {
+            /*
+             * ??? While INC is 2 bytes shorter than ADDL $1, they also induce
+             * partial flags update stalls on Pentium4 and are not recommended
+             * by current Intel optimization manuals.
+             */
+            if (val == 1 || val == -1) {
+                int is_inc = (c == ARITH_ADD) ^ (val < 0);
+                if (TCG_TARGET_REG_BITS == 64) {
+                    /*
+                     * The single-byte increment encodings are re-tasked
+                     * as the REX prefixes.  Use the MODRM encoding.
+                     */
+                    tcg_out_modrm(s, OPC_GRP5 + rexw,
+                                  (is_inc ? EXT5_INC_Ev : EXT5_DEC_Ev), r0);
+                } else {
+                    tcg_out8(s, (is_inc ? OPC_INC_r32 : OPC_DEC_r32) + r0);
+                }
+                return;
+            }
+            if (val == 128) {
+                /*
+                 * Facilitate using an 8-bit immediate.  Carry is inverted
+                 * by this transformation, so do it only if cf == 0.
+                 */
+                c ^= ARITH_ADD ^ ARITH_SUB;
+                val = -128;
+            }
         }
-        return;
-    }
+        break;
 
-    if (c == ARITH_AND) {
+    case ARITH_AND:
         if (TCG_TARGET_REG_BITS == 64) {
             if (val == 0xffffffffu) {
                 tcg_out_ext32u(s, r0, r0);
@@ -1351,6 +1369,7 @@ static void tgen_arithi(TCGContext *s, int c, int r0,
             tcg_out_ext16u(s, r0, r0);
             return;
         }
+        break;
     }
 
     if (val == (int8_t)val) {