math/big: add tests for allocation during multiply

Test that big.Int.Mul reusing the same target is not allocating
temporary garbage during its computation. That code is going
to be modified in an upcoming CL.

Change-Id: I3ed55c06da030282233c29cd7af2a04f395dc7a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/652056
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>

src/math/big/escape_test.go

@@ -0,0 +1,53 @@
+// Copyright 2025 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 big
+
+import (
+	"internal/testenv"
+	"math/rand"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+func TestEscape(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	// The multiplication routines create many temporary Int values,
+	// expecting them to be stack-allocated. Make sure none escape to the heap.
+	out, err := exec.Command("go", "build", "-gcflags=-m").CombinedOutput()
+	if err != nil {
+		t.Fatalf("go build -gcflags=-m: %v\n%s", err, out)
+	}
+	for line := range strings.Lines(string(out)) {
+		if strings.Contains(line, "natmul.go") && strings.Contains(line, "Int") && strings.Contains(line, "escapes") {
+			t.Error(strings.TrimSpace(line))
+		}
+	}
+}
+
+func TestMulAlloc(t *testing.T) {
+	r := rand.New(rand.NewSource(1234))
+	sizes := []int{karatsubaThreshold / 2, karatsubaThreshold}
+	for _, size := range sizes {
+		x := randInt(r, uint(size))
+		y := randInt(r, uint(size))
+		z := &Int{abs: make(nat, 2*uint(size))}
+		if n := testing.AllocsPerRun(10, func() { z.Mul(x, y) }); n >= 1 {
+			t.Errorf("Mul(len %d, len %d) allocates %.2f objects", size, size, n)
+		}
+	}
+}
+
+func TestSqrAlloc(t *testing.T) {
+	r := rand.New(rand.NewSource(1234))
+	sizes := []int{basicSqrThreshold / 2, basicSqrThreshold, karatsubaSqrThreshold}
+	for _, size := range sizes {
+		x := randInt(r, uint(size))
+		z := &Int{abs: make(nat, 2*uint(size))}
+		if n := testing.AllocsPerRun(10, func() { z.Mul(x, x) }); n >= 1 {
+			t.Errorf("Mul(len %d with itself) allocates %.2f objects", size, n)
+		}
+	}
+}