cmd/compile: avoid assignment conversion in append(a, b...)

There's no need for a and b to match types. The typechecker already
ensured that a and b are both slices with the same base type, or
a and b are (possibly named) []byte and string.

The optimization to treat append(b, make([], ...)) as a zeroing
slice extension doesn't fire when there's a OCONVNOP wrapping the make.
Fixes #53888

Change-Id: Ied871ed0bbb8e4a4b35d280c71acbab8103691bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/418475
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>

4 files changed, 56 insertions, 8 deletions

diff --git a/src/cmd/compile/internal/noder/transform.go b/src/cmd/compile/internal/noder/transform.go
index ddbccf4ff4..50254e694a 100644
--- a/src/cmd/compile/internal/noder/transform.go
+++ b/src/cmd/compile/internal/noder/transform.go
@@ -730,11 +730,11 @@ func transformAppend(n *ir.CallExpr) ir.Node {
 	assert(t.IsSlice())
 
 	if n.IsDDD {
-		if t.Elem().IsKind(types.TUINT8) && args[1].Type().IsString() {
-			return n
-		}
-
-		args[1] = assignconvfn(args[1], t.Underlying())
+		// assignconvfn is of args[1] not required here, as the
+		// types of args[0] and args[1] don't need to match
+		// (They will both have an underlying type which are
+		// slices of indentical base types, or be []byte and string.)
+		// See issue 53888.
 		return n
 	}
 
diff --git a/src/cmd/compile/internal/test/issue53888_test.go b/src/cmd/compile/internal/test/issue53888_test.go
new file mode 100644
index 0000000000..9f50a82a3c
--- /dev/null
+++ b/src/cmd/compile/internal/test/issue53888_test.go
@@ -0,0 +1,44 @@
+// 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.
+
+//go:build !race
+
+package test
+
+import (
+	"testing"
+)
+
+func TestAppendOfMake(t *testing.T) {
+	for n := 32; n < 33; n++ { // avoid stack allocation of make()
+		b := make([]byte, n)
+		f := func() {
+			b = append(b[:0], make([]byte, n)...)
+		}
+		if n := testing.AllocsPerRun(10, f); n > 0 {
+			t.Errorf("got %f allocs, want 0", n)
+		}
+		type S []byte
+
+		s := make(S, n)
+		g := func() {
+			s = append(s[:0], make(S, n)...)
+		}
+		if n := testing.AllocsPerRun(10, g); n > 0 {
+			t.Errorf("got %f allocs, want 0", n)
+		}
+		h := func() {
+			s = append(s[:0], make([]byte, n)...)
+		}
+		if n := testing.AllocsPerRun(10, h); n > 0 {
+			t.Errorf("got %f allocs, want 0", n)
+		}
+		i := func() {
+			b = append(b[:0], make(S, n)...)
+		}
+		if n := testing.AllocsPerRun(10, i); n > 0 {
+			t.Errorf("got %f allocs, want 0", n)
+		}
+	}
+}
diff --git a/src/cmd/compile/internal/typecheck/func.go b/src/cmd/compile/internal/typecheck/func.go
index 0988ce8dc7..7790e65ca2 100644
--- a/src/cmd/compile/internal/typecheck/func.go
+++ b/src/cmd/compile/internal/typecheck/func.go
@@ -465,7 +465,11 @@ func tcAppend(n *ir.CallExpr) ir.Node {
 			return n
 		}
 
-		args[1] = AssignConv(args[1], t.Underlying(), "append")
+		// AssignConv is of args[1] not required here, as the
+		// types of args[0] and args[1] don't need to match
+		// (They will both have an underlying type which are
+		// slices of indentical base types, or be []byte and string.)
+		// See issue 53888.
 		return n
 	}
 
diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 9ca0adefd8..75f202fca0 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -194,7 +194,7 @@ func growslice(et *_type, old slice, cap int) slice {
 	}
 
 	if cap < old.cap {
-		panic(errorString("growslice: cap out of range"))
+		panic(errorString("growslice: len out of range"))
 	}
 
 	if et.size == 0 {
@@ -284,7 +284,7 @@ func growslice(et *_type, old slice, cap int) slice {
 	//   print(len(s), "\n")
 	// }
 	if overflow || capmem > maxAlloc {
-		panic(errorString("growslice: cap out of range"))
+		panic(errorString("growslice: len out of range"))
 	}
 
 	var p unsafe.Pointer