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>
diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
index 4d7b745..3f3c114 100644
--- a/tcg/i386/tcg-target.c.inc
+++ b/tcg/i386/tcg-target.c.inc
@@ -1532,6 +1532,7 @@
int const_arg2)
{
bool inv = false;
+ bool cleared;
switch (cond) {
case TCG_COND_NE:
@@ -1581,9 +1582,23 @@
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
