Source file src/runtime/internal/sys/nih.go
1 // Copyright 2014 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package sys 6 7 // NOTE: keep in sync with cmd/compile/internal/types.CalcSize 8 // to make the compiler recognize this as an intrinsic type. 9 type nih struct{} 10 11 // NotInHeap is a type must never be allocated from the GC'd heap or on the stack, 12 // and is called not-in-heap. 13 // 14 // Other types can embed NotInHeap to make it not-in-heap. Specifically, pointers 15 // to these types must always fail the `runtime.inheap` check. The type may be used 16 // for global variables, or for objects in unmanaged memory (e.g., allocated with 17 // `sysAlloc`, `persistentalloc`, r`fixalloc`, or from a manually-managed span). 18 // 19 // Specifically: 20 // 21 // 1. `new(T)`, `make([]T)`, `append([]T, ...)` and implicit heap 22 // allocation of T are disallowed. (Though implicit allocations are 23 // disallowed in the runtime anyway.) 24 // 25 // 2. A pointer to a regular type (other than `unsafe.Pointer`) cannot be 26 // converted to a pointer to a not-in-heap type, even if they have the 27 // same underlying type. 28 // 29 // 3. Any type that containing a not-in-heap type is itself considered as not-in-heap. 30 // 31 // - Structs and arrays are not-in-heap if their elements are not-in-heap. 32 // - Maps and channels contains no-in-heap types are disallowed. 33 // 34 // 4. Write barriers on pointers to not-in-heap types can be omitted. 35 // 36 // The last point is the real benefit of NotInHeap. The runtime uses 37 // it for low-level internal structures to avoid memory barriers in the 38 // scheduler and the memory allocator where they are illegal or simply 39 // inefficient. This mechanism is reasonably safe and does not compromise 40 // the readability of the runtime. 41 type NotInHeap struct{ _ nih } 42