github.com/shogo82148/std@v1.22.1-0.20240327122250-4e474527810c/runtime/internal/sys/nih.go (about)

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