Commit 52c02c2d authored by Robert Griesemer's avatar Robert Griesemer

Added section on package unsafe:

- contains a (proposed) constant Maxalign
- contains some text re" alignment guarantees

R=ken,r
DELTA=97  (94 added, 1 deleted, 2 changed)
OCL=24791
CL=24871
parent 0c4010a7
......@@ -3,7 +3,7 @@ The Go Programming Language Specification (DRAFT)
Robert Griesemer, Rob Pike, Ken Thompson
(February 6, 2009)
(February 11, 2009)
----
......@@ -254,6 +254,10 @@ Contents
Program initialization and execution
Systems considerations
Package unsafe
Size and alignment guarantees
----
......@@ -753,7 +757,7 @@ The following identifiers are predeclared:
All basic types:
bool, byte, uint8, uint16, uint32, uint64, int8, int16, int32, int64,
float32, float64, float80, string
float32, float64, string
A set of platform-specific convenience types:
......@@ -1102,7 +1106,6 @@ The following list enumerates all platform-independent numeric types:
float32 the set of all valid IEEE-754 32-bit floating point numbers
float64 the set of all valid IEEE-754 64-bit floating point numbers
float80 the set of all valid IEEE-754 80-bit floating point numbers
Integer types are represented in the usual binary format; the value of
an n-bit integer is n bits wide. A negative signed integer is represented
......@@ -3453,3 +3456,93 @@ Program execution begins by initializing the main package and then
invoking main.main().
When main.main() returns, the program exits.
----
Systems considerations
----
Package unsafe
----
The special package "unsafe", known to the compiler, provides facilities
for low-level programming including operations that violate the Go type
system. A package using "unsafe" must be vetted manually for type safety.
The package "unsafe" provides (at least) the following package interface:
package unsafe
const Maxalign
type Pointer *any
func Alignof(variable any) int
func Offsetof(selector any) int
func Sizeof(variable any) int
The pseudo type "any" stands for any Go type; "any" is not a type generally
available in Go programs.
Any pointer type as well as values of type "uintptr" can be converted into
an "unsafe.Pointer" and vice versa.
The function "Sizeof" takes an expression denoting a variable of any type
and returns the size of the variable in bytes.
The function "Offsetof" takes a selector (§Selectors) denoting a struct
field of any type and returns the field offset in bytes relative to the
struct address. Specifically, the following condition is satisfied for
a struct "s" with field "f":
uintptr(unsafe.Pointer(&s)) + uintptr(unsafe.Offsetof(s.f)) ==
uintptr(unsafe.Pointer(&s.f))
Computer architectures may impose restrictions on the memory addresses accessed
directly by machine instructions. A common such restriction is the requirement
for such addresses to be ``aligned''; that is, addresses must be a multiple
of a factor, the ``alignment''. The alignment depends on the type of datum
accessed.
The function "Alignof" takes an expression denoting a variable of any type
and returns the alignment of the variable in bytes. The following alignment
condition is satisfied for a variable "x":
uintptr(unsafe.Pointer(&x)) % uintptr(unsafe.Alignof(x)) == 0
The maximum alignment is given by the constant "unsafe.Maxalign".
It usually corresponds to the value of "unsafe.Sizeof(x)" for
a variable "x" of the largest arithmetic type (8 for a float64), but may
be smaller on systems that have less stringent alignment restrictions
or are space constrained.
Size and alignment guarantees
----
For the arithmetic types (§Arithmetic types), a Go compiler guarantees the
following sizes:
type size in bytes
byte, uint8, int8 1
uint16, int16 2
uint32, int32, float32 4
uint64, int64, float64 8
A Go compiler guarantees the following minimal alignment properties:
1) For a variable "x" of any type: "1 <= unsafe.Alignof(x) <= unsafe.Maxalign".
2) For a variable "x" of arithmetic type: "unsafe.Alignof(x)" is the smaller
of "unsafe.Sizeof(x)" and "unsafe.Maxalign", but at least 1.
3) For a variable "x" of struct type: "unsafe.Alignof(x)" is the largest of
all the values "unsafe.Alignof(x.f)" for each field "f" of x, but at least 1.
4) For a variable "x" of array type: "unsafe.Alignof(x)" is the same as
unsafe.Alignof(x[0]), but at least 1.
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