strings: prevent copyCheck from forcing Builder to escape and allocate

All credit and blame goes to Ian for this suggestion, copied from the
runtime.

Fixes #23382
Updates #7921

Change-Id: I3d5a9ee4ab730c87e0f3feff3e7fceff9bcf9e18
Reviewed-on: https://go-review.googlesource.com/86976
Reviewed-by: Ian Lance Taylor <[email protected]>

src/strings/builder.go

@@ -17,9 +17,26 @@ type Builder struct {
  buf []byte
 }
 
+// noescape hides a pointer from escape analysis. noescape is
+// the identity function but escape analysis doesn't think the
+// output depends on the input. noescape is inlined and currently
+// compiles down to zero instructions.
+// USE CAREFULLY!
+// This was copied from the runtime; see issues 23382 and 7921.
+//go:nosplit
+func noescape(p unsafe.Pointer) unsafe.Pointer {
+ x := uintptr(p)
+ return unsafe.Pointer(x ^ 0)
+}
+
 func (b *Builder) copyCheck() {
  if b.addr == nil {
- b.addr = b
+ // This hack works around a failing of Go's escape analysis
+ // that was causing b to escape and be heap allocated.
+ // See issue 23382.
+ // TODO: once issue 7921 is fixed, this should be reverted to
+ // just "b.addr = b".
+ b.addr = (*Builder)(noescape(unsafe.Pointer(b)))
  } else if b.addr != b {
  panic("strings: illegal use of non-zero Builder copied by value")
  }

src/strings/builder_test.go

@@ -180,6 +180,18 @@ func TestBuilderAllocs(t *testing.T) {
  if allocs > 0 {
  t.Fatalf("got %d alloc(s); want 0", allocs)
  }
+
+ // Issue 23382; verify that copyCheck doesn't force the
+ // Builder to escape and be heap allocated.
+ n := testing.AllocsPerRun(10000, func() {
+ var b Builder
+ b.Grow(5)
+ b.WriteString("abcde")
+ _ = b.String()
+ })
+ if n != 1 {
+ t.Errorf("Builder allocs = %v; want 1", n)
+ }
 }
 
 func numAllocs(fn func()) uint64 {