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spec: add unsafe.Add and unsafe.Slice

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Updates #19367.
Updates #40481.

Change-Id: I578066ad68d2cd6bea50df1a534cf799e4404a7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/312212
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
This commit is contained in:
Matthew Dempsky 2021-04-21 00:12:02 -07:00
parent 74059685fd
commit 1da05eb0ce
2 changed files with 54 additions and 1 deletions
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32
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of Apr 20, 2021",
"Subtitle": "Version of Apr 21, 2021",
"Path": "/ref/spec"
}-->
@ -6711,6 +6711,10 @@ type Pointer *ArbitraryType
func Alignof(variable ArbitraryType) uintptr
func Offsetof(selector ArbitraryType) uintptr
func Sizeof(variable ArbitraryType) uintptr
type IntegerType int // shorthand for an integer type; it is not a real type
func Add(ptr Pointer, len IntegerType) Pointer
func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
</pre>
<p>
@ -6767,6 +6771,32 @@ Calls to <code>Alignof</code>, <code>Offsetof</code>, and
<code>Sizeof</code> are compile-time constant expressions of type <code>uintptr</code>.
</p>
<p>
The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
if it is an untyped constant it is given type <code>int</code>.
The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
</p>
<p>
The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
and whose length and capacity are <code>len</code>:
</p>
<pre>
(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
</pre>
<p>
The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
if it is an untyped constant it is given type <code>int</code>.
If <code>ptr</code> is <code>nil</code> or <code>len</code> is negative at run time,
a <a href="#Run_time_panics">run-time panic</a> occurs.
</p>
<h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
<p>

23
src/unsafe/unsafe.go
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@ -14,6 +14,10 @@ package unsafe
// part of the unsafe package. It represents the type of an arbitrary Go expression.
type ArbitraryType int
// IntegerType is here for the purposes of documentation only and is not actually
// part of the unsafe package. It represents any arbitrary integer type.
type IntegerType int
// Pointer represents a pointer to an arbitrary type. There are four special operations
// available for type Pointer that are not available for other types:
// - A pointer value of any type can be converted to a Pointer.
@ -203,3 +207,22 @@ func Offsetof(x ArbitraryType) uintptr
// value returned by reflect.TypeOf(s.f).FieldAlign().
// The return value of Alignof is a Go constant.
func Alignof(x ArbitraryType) uintptr
// The function Add adds len to ptr and returns the updated pointer
// Pointer(uintptr(ptr) + uintptr(len)).
// The len argument must be of integer type or an untyped constant.
// A constant len argument must be representable by a value of type int;
// if it is an untyped constant it is given type int.
// The rules for valid uses of Pointer still apply.
func Add(ptr Pointer, len IntegerType) Pointer
// The function Slice returns a slice whose underlying array starts at ptr
// and whose length and capacity are len:
//
// (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
//
// The len argument must be of integer type or an untyped constant.
// A constant len argument must be non-negative and representable by a value of type int;
// if it is an untyped constant it is given type int.
// If ptr is nil or len is negative at run time, a run-time panic occurs.
func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType