cmd/compile: use ADDQ instead of LEAQ when we can

The address calculations in the example end up doing x << 4 + y + 0.
Before this CL we use a SHLQ+LEAQ. Since the constant offset is 0,
we can use SHLQ+ADDQ instead.

Change-Id: Ia048c4fdbb3a42121c7e1ab707961062e8247fca
Reviewed-on: https://go-review.googlesource.com/c/go/+/209959
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1513,6 +1513,9 @@
 (TESTWconst [-1] x) && x.Op != OpAMD64MOVLconst -> (TESTW x x)
 (TESTBconst [-1] x) && x.Op != OpAMD64MOVLconst -> (TESTB x x)
 
+// Convert LEAQ1 back to ADDQ if we can
+(LEAQ1 [0] x y) && v.Aux == nil -> (ADDQ x y)
+
 // Combining byte loads into larger (unaligned) loads.
 // There are many ways these combinations could occur.  This is
 // designed to match the way encoding/binary.LittleEndian does it.

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -9829,6 +9829,23 @@ func rewriteValueAMD64_OpAMD64LEAQ1(v *Value) bool {
 		}
 		break
 	}
+	// match: (LEAQ1 [0] x y)
+	// cond: v.Aux == nil
+	// result: (ADDQ x y)
+	for {
+		if v.AuxInt != 0 {
+			break
+		}
+		x := v_0
+		y := v_1
+		if !(v.Aux == nil) {
+			break
+		}
+		v.reset(OpAMD64ADDQ)
+		v.AddArg(x)
+		v.AddArg(y)
+		return true
+	}
 	return false
 }
 func rewriteValueAMD64_OpAMD64LEAQ2(v *Value) bool {

test/codegen/addrcalc.go

@@ -0,0 +1,14 @@
+// asmcheck
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codegen
+
+// Make sure we use ADDQ instead of LEAQ when we can.
+
+func f(p *[4][2]int, x int) *int {
+	// amd64:"ADDQ",-"LEAQ"
+	return &p[x][0]
+}