cmd/compile: reuse dead register before reusing register holding constant

For commuting ops, check whether the second argument is dead before
checking if the first argument is rematerializeable. Reusing the register
holding a dead value is always best.

Fixes #33580

Change-Id: I7372cfc03d514e6774d2d9cc727a3e6bf6ce2657
Reviewed-on: https://go-review.googlesource.com/c/go/+/199559
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -1328,28 +1328,26 @@ func (s *regAllocState) regalloc(f *Func) {
 					// arg0 is dead.  We can clobber its register.
 					goto ok
 				}
+				if opcodeTable[v.Op].commutative && !s.liveAfterCurrentInstruction(v.Args[1]) {
+					args[0], args[1] = args[1], args[0]
+					goto ok
+				}
 				if s.values[v.Args[0].ID].rematerializeable {
 					// We can rematerialize the input, don't worry about clobbering it.
 					goto ok
 				}
+				if opcodeTable[v.Op].commutative && s.values[v.Args[1].ID].rematerializeable {
+					args[0], args[1] = args[1], args[0]
+					goto ok
+				}
 				if countRegs(s.values[v.Args[0].ID].regs) >= 2 {
 					// we have at least 2 copies of arg0.  We can afford to clobber one.
 					goto ok
 				}
-				if opcodeTable[v.Op].commutative {
+				if opcodeTable[v.Op].commutative && countRegs(s.values[v.Args[1].ID].regs) >= 2 {
-					if !s.liveAfterCurrentInstruction(v.Args[1]) {
 					args[0], args[1] = args[1], args[0]
 					goto ok
 				}
-					if s.values[v.Args[1].ID].rematerializeable {
-						args[0], args[1] = args[1], args[0]
-						goto ok
-					}
-					if countRegs(s.values[v.Args[1].ID].regs) >= 2 {
-						args[0], args[1] = args[1], args[0]
-						goto ok
-					}
-				}
 
 				// We can't overwrite arg0 (or arg1, if commutative).  So we
 				// need to make a copy of an input so we have a register we can modify.

test/codegen/issue33580.go

@@ -0,0 +1,25 @@
+// 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.
+
+// Make sure we reuse large constant loads, if we can.
+// See issue 33580.
+
+package codegen
+
+const (
+	A = 7777777777777777
+	B = 8888888888888888
+)
+
+func f(x, y uint64) uint64 {
+	p := x & A
+	q := y & A
+	r := x & B
+	// amd64:-"MOVQ.*8888888888888888"
+	s := y & B
+
+	return p * q * r * s
+}