ssa: add builtin append

internal/runtime/slice.go

@@ -0,0 +1,40 @@
+// Copyright 2009 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 runtime
+
+// nextslicecap computes the next appropriate slice length.
+func nextslicecap(newLen, oldCap int) int {
+	newcap := oldCap
+	doublecap := newcap + newcap
+	if newLen > doublecap {
+		return newLen
+	}
+
+	const threshold = 256
+	if oldCap < threshold {
+		return doublecap
+	}
+	for {
+		// Transition from growing 2x for small slices
+		// to growing 1.25x for large slices. This formula
+		// gives a smooth-ish transition between the two.
+		newcap += (newcap + 3*threshold) >> 2
+
+		// We need to check `newcap >= newLen` and whether `newcap` overflowed.
+		// newLen is guaranteed to be larger than zero, hence
+		// when newcap overflows then `uint(newcap) > uint(newLen)`.
+		// This allows to check for both with the same comparison.
+		if uint(newcap) >= uint(newLen) {
+			break
+		}
+	}
+
+	// Set newcap to the requested cap when
+	// the newcap calculation overflowed.
+	if newcap <= 0 {
+		return newLen
+	}
+	return newcap
+}