cmd/compile: recognize when the result of append has a constant length

Fixes a performance regression due to CL 418554.

Fixes #56440

Change-Id: I6ff152e9b83084756363f49ee6b0844a7a284880
Reviewed-on: https://go-review.googlesource.com/c/go/+/445875
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

2 files changed, 19 insertions, 1 deletions

diff --git a/test/codegen/issue56440.go b/test/codegen/issue56440.go
new file mode 100644
index 0000000000..36b52ace03
--- /dev/null
+++ b/test/codegen/issue56440.go
@@ -0,0 +1,18 @@
+// asmcheck
+
+// Copyright 2022 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.
+
+// Check to make sure that we recognize when the length of an append
+// is constant. We check this by making sure that the constant length
+// is folded into a load offset.
+
+package p
+
+func f(x []int) int {
+	s := make([]int, 3)
+	s = append(s, 4, 5)
+	// amd64:`MOVQ\t40\(.*\),`
+	return x[len(s)]
+}
diff --git a/test/prove.go b/test/prove.go
index 1be257f206..7792b432f9 100644
--- a/test/prove.go
+++ b/test/prove.go
@@ -500,7 +500,7 @@ func f19() (e int64, err error) {
 	last := len(stack) - 1
 	e = stack[last]
 	// Buggy compiler prints "Disproved Leq64" for the next line.
-	stack = stack[:last] // ERROR "Proved IsSliceInBounds"
+	stack = stack[:last]
 	return e, nil
 }