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>

1 files changed, 25 insertions, 0 deletions

diff --git a/test/codegen/issue33580.go b/test/codegen/issue33580.go
new file mode 100644
index 0000000000..1ded944c33
--- /dev/null
+++ b/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
+}