tcg/i386: Use CMP+SBB in tcg_out_setcond

Use the carry bit to optimize some forms of setcond.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 50 insertions, 0 deletions

diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
index 1542afd94d..4d7b745a52 100644
--- a/tcg/i386/tcg-target.c.inc
+++ b/tcg/i386/tcg-target.c.inc
@@ -1531,6 +1531,56 @@ static void tcg_out_setcond(TCGContext *s, int rexw, TCGCond cond,
                             TCGArg dest, TCGArg arg1, TCGArg arg2,
                             int const_arg2)
 {
+    bool inv = false;
+
+    switch (cond) {
+    case TCG_COND_NE:
+        inv = true;
+        /* fall through */
+    case TCG_COND_EQ:
+        /* If arg2 is 0, convert to LTU/GEU vs 1. */
+        if (const_arg2 && arg2 == 0) {
+            arg2 = 1;
+            goto do_ltu;
+        }
+        break;
+
+    case TCG_COND_LEU:
+        inv = true;
+        /* fall through */
+    case TCG_COND_GTU:
+        /* If arg2 is a register, swap for LTU/GEU. */
+        if (!const_arg2) {
+            TCGReg t = arg1;
+            arg1 = arg2;
+            arg2 = t;
+            goto do_ltu;
+        }
+        break;
+
+    case TCG_COND_GEU:
+        inv = true;
+        /* fall through */
+    case TCG_COND_LTU:
+    do_ltu:
+        /*
+         * Relying on the carry bit, use SBB to produce -1 if LTU, 0 if GEU.
+         * We can then use NEG or INC to produce the desired result.
+         * This is always smaller than the SETCC expansion.
+         */
+        tcg_out_cmp(s, arg1, arg2, const_arg2, rexw);
+        tgen_arithr(s, ARITH_SBB, dest, dest);              /* T:-1 F:0 */
+        if (inv) {
+            tgen_arithi(s, ARITH_ADD, dest, 1, 0);          /* T:0  F:1 */
+        } else {
+            tcg_out_modrm(s, OPC_GRP3_Ev, EXT3_NEG, dest);  /* T:1  F:0 */
+        }
+        return;
+
+    default:
+        break;
+    }
+
     tcg_out_cmp(s, arg1, arg2, const_arg2, rexw);
     tcg_out_modrm(s, OPC_SETCC | tcg_cond_to_jcc[cond], 0, dest);
     tcg_out_ext8u(s, dest, dest);