tcg/i386: Clear dest first in tcg_out_setcond if possible

Using XOR first is both smaller and more efficient,
though cannot be applied if it clobbers an input.

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

tcg/i386/tcg-target.c.inc

@@ -1532,6 +1532,7 @@ static void tcg_out_setcond(TCGContext *s, int rexw, TCGCond cond,
                             int const_arg2)
 {
     bool inv = false;
+    bool cleared;
 
     switch (cond) {
     case TCG_COND_NE:
@@ -1581,9 +1582,23 @@ static void tcg_out_setcond(TCGContext *s, int rexw, TCGCond cond,
         break;
     }
 
+    /*
+     * If dest does not overlap the inputs, clearing it first is preferred.
+     * The XOR breaks any false dependency for the low-byte write to dest,
+     * and is also one byte smaller than MOVZBL.
+     */
+    cleared = false;
+    if (dest != arg1 && (const_arg2 || dest != arg2)) {
+        tgen_arithr(s, ARITH_XOR, dest, dest);
+        cleared = true;
+    }
+
     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);
+
+    if (!cleared) {
+        tcg_out_ext8u(s, dest, dest);
+    }
 }
 
 #if TCG_TARGET_REG_BITS == 32