From 3e44393152947d55e02a7a71356473b89fe48f29 Mon Sep 17 00:00:00 2001
From: clubby789 <jamie@hill-daniel.co.uk>
Date: Thu, 31 Dec 2020 11:40:09 +0000
Subject: [PATCH] x64: optimize mov <reg>, 0 (#7733)

---
 vlib/v/gen/x64/gen.v | 74 +++++++++++++++++++++++++++++++-------------
 1 file changed, 53 insertions(+), 21 deletions(-)

diff --git a/vlib/v/gen/x64/gen.v b/vlib/v/gen/x64/gen.v
index ee5f8ee648..8e1935703d 100644
--- a/vlib/v/gen/x64/gen.v
+++ b/vlib/v/gen/x64/gen.v
@@ -514,30 +514,62 @@ pub fn (mut g Gen) gen_exit() {
 }
 
 fn (mut g Gen) mov(reg Register, val int) {
-	match reg {
-		.eax, .rax {
-			g.write8(0xb8)
+	if val == 0 {
+		// Optimise to xor reg, reg when val is 0
+		match reg {
+			.eax, .rax {
+				g.write8(0x31)
+				g.write8(0xc0)
+			}
+			.edi, .rdi {
+				g.write8(0x31)
+				g.write8(0xff)
+			}
+			.edx {
+				g.write8(0x31)
+				g.write8(0xd2)
+			}
+			.rsi {
+				g.write8(0x48)
+				g.write8(0x31)
+				g.write8(0xf6)
+			}
+			.r12 {
+				g.write8(0x4d)
+				g.write8(0x31)
+				g.write8(0xe4)
+			}
+			else {
+				panic('unhandled mov $reg')
+			}
 		}
-		.edi, .rdi {
-			g.write8(0xbf)
-		}
-		.edx {
-			g.write8(0xba)
-		}
-		.rsi {
-			g.write8(0x48)
-			g.write8(0xbe)
-		}
-		.r12 {
-			g.write8(0x41)
-			g.write8(0xbc) // r11 is 0xbb etc
-		}
-		else {
-			panic('unhandled mov $reg')
+		g.println('xor $reg, $reg')
+	} else {
+		match reg {
+			.eax, .rax {
+				g.write8(0xb8)
+			}
+			.edi, .rdi {
+				g.write8(0xbf)
+			}
+			.edx {
+				g.write8(0xba)
+			}
+			.rsi {
+				g.write8(0x48)
+				g.write8(0xbe)
+			}
+			.r12 {
+				g.write8(0x41)
+				g.write8(0xbc) // r11 is 0xbb etc
+			}
+			else {
+				panic('unhandled mov $reg')
+			}
 		}
+		g.write32(val)
+		g.println('mov $reg, $val')
 	}
-	g.write32(val)
-	g.println('mov $reg, $val')
 }
 
 fn (mut g Gen) mov_reg(a Register, b Register) {