package standard library
go1.22.5 Latest Latest

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Published: Jul 2, 2024 License: BSD-3-Clause Imports: 2 Imported by: 0



package sys contains system- and configuration- and architecture-specific constants used by the runtime.



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const DefaultPhysPageSize = goarch.DefaultPhysPageSize

DefaultPhysPageSize is the default physical page size.

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const Int64Align = goarch.PtrSize

Int64Align is the required alignment for a 64-bit integer (4 on 32-bit systems, 8 on 64-bit).

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const MinFrameSize = goarch.MinFrameSize

MinFrameSize is the size of the system-reserved words at the bottom of a frame (just above the architectural stack pointer). It is zero on x86 and PtrSize on most non-x86 (LR-based) systems. On PowerPC it is larger, to cover three more reserved words: the compiler word, the link editor word, and the TOC save word.

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const PCQuantum = goarch.PCQuantum

PCQuantum is the minimal unit for a program counter (1 on x86, 4 on most other systems). The various PC tables record PC deltas pre-divided by PCQuantum.

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const StackAlign = goarch.StackAlign

StackAlign is the required alignment of the SP register. The stack must be at least word aligned, but some architectures require more.

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const StackGuardMultiplier = 1 + goos.IsAix + isRace

AIX requires a larger stack for syscalls. The race build also needs more stack. See issue 54291. This arithmetic must match that in cmd/internal/objabi/stack.go:stackGuardMultiplier.


This section is empty.


func Bswap32 added in go1.7

func Bswap32(x uint32) uint32

Bswap32 returns its input with byte order reversed 0x01020304 -> 0x04030201

func Bswap64 added in go1.7

func Bswap64(x uint64) uint64

Bswap64 returns its input with byte order reversed 0x0102030405060708 -> 0x0807060504030201

func LeadingZeros64 added in go1.14

func LeadingZeros64(x uint64) int

LeadingZeros64 returns the number of leading zero bits in x; the result is 64 for x == 0.

func LeadingZeros8 added in go1.14

func LeadingZeros8(x uint8) int

LeadingZeros8 returns the number of leading zero bits in x; the result is 8 for x == 0.

func Len64 added in go1.14

func Len64(x uint64) (n int)

Len64 returns the minimum number of bits required to represent x; the result is 0 for x == 0.

nosplit because this is used in src/runtime/histogram.go, which make run in sensitive contexts.

func Len8 added in go1.14

func Len8(x uint8) int

Len8 returns the minimum number of bits required to represent x; the result is 0 for x == 0.

func OnesCount64 added in go1.14

func OnesCount64(x uint64) int

OnesCount64 returns the number of one bits ("population count") in x.

func Prefetch added in go1.18

func Prefetch(addr uintptr)

Prefetch prefetches data from memory addr to cache

AMD64: Produce PREFETCHT0 instruction

ARM64: Produce PRFM instruction with PLDL1KEEP option

func PrefetchStreamed added in go1.18

func PrefetchStreamed(addr uintptr)

PrefetchStreamed prefetches data from memory addr, with a hint that this data is being streamed. That is, it is likely to be accessed very soon, but only once. If possible, this will avoid polluting the cache.

AMD64: Produce PREFETCHNTA instruction

ARM64: Produce PRFM instruction with PLDL1STRM option

func TrailingZeros32 added in go1.20

func TrailingZeros32(x uint32) int

TrailingZeros32 returns the number of trailing zero bits in x; the result is 32 for x == 0.

func TrailingZeros64 added in go1.14

func TrailingZeros64(x uint64) int

TrailingZeros64 returns the number of trailing zero bits in x; the result is 64 for x == 0.

func TrailingZeros8 added in go1.14

func TrailingZeros8(x uint8) int

TrailingZeros8 returns the number of trailing zero bits in x; the result is 8 for x == 0.


type NotInHeap added in go1.20

type NotInHeap struct {
	// contains filtered or unexported fields

NotInHeap is a type must never be allocated from the GC'd heap or on the stack, and is called not-in-heap.

Other types can embed NotInHeap to make it not-in-heap. Specifically, pointers to these types must always fail the `runtime.inheap` check. The type may be used for global variables, or for objects in unmanaged memory (e.g., allocated with `sysAlloc`, `persistentalloc`, r`fixalloc`, or from a manually-managed span).


1. `new(T)`, `make([]T)`, `append([]T, ...)` and implicit heap allocation of T are disallowed. (Though implicit allocations are disallowed in the runtime anyway.)

2. A pointer to a regular type (other than `unsafe.Pointer`) cannot be converted to a pointer to a not-in-heap type, even if they have the same underlying type.

3. Any type that containing a not-in-heap type is itself considered as not-in-heap.

- Structs and arrays are not-in-heap if their elements are not-in-heap. - Maps and channels contains no-in-heap types are disallowed.

4. Write barriers on pointers to not-in-heap types can be omitted.

The last point is the real benefit of NotInHeap. The runtime uses it for low-level internal structures to avoid memory barriers in the scheduler and the memory allocator where they are illegal or simply inefficient. This mechanism is reasonably safe and does not compromise the readability of the runtime.

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