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Improve WithMemoryCapacityFromMax (#317).

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This commit is contained in:
Nuno Cruces
2025-09-03 08:03:59 +01:00
committed by GitHub
parent 07fec784e1
commit 54bb94ce58
2 changed files with 33 additions and 21 deletions
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26
internal/alloc/alloc_unix.go
View File

@@ -9,24 +9,31 @@ import (
"golang.org/x/sys/unix"
)
func NewMemory(_, max uint64) experimental.LinearMemory {
func NewMemory(cap, max uint64) experimental.LinearMemory {
// Round up to the page size.
rnd := uint64(unix.Getpagesize() - 1)
max = (max + rnd) &^ rnd
res := (max + rnd) &^ rnd
if max > math.MaxInt {
// This ensures int(max) overflows to a negative value,
if res > math.MaxInt {
// This ensures int(res) overflows to a negative value,
// and unix.Mmap returns EINVAL.
max = math.MaxUint64
res = math.MaxUint64
}
// Reserve max bytes of address space, to ensure we won't need to move it.
com := res
prot := unix.PROT_READ | unix.PROT_WRITE
if cap < max { // Commit memory only if cap=max.
com = 0
prot = unix.PROT_NONE
}
// Reserve res bytes of address space, to ensure we won't need to move it.
// A protected, private, anonymous mapping should not commit memory.
b, err := unix.Mmap(-1, 0, int(max), unix.PROT_NONE, unix.MAP_PRIVATE|unix.MAP_ANON)
b, err := unix.Mmap(-1, 0, int(res), prot, unix.MAP_PRIVATE|unix.MAP_ANON)
if err != nil {
panic(err)
}
return &mmappedMemory{buf: b[:0]}
return &mmappedMemory{buf: b[:com]}
}
// The slice covers the entire mmapped memory:
@@ -52,8 +59,7 @@ func (m *mmappedMemory) Reallocate(size uint64) []byte {
return nil
}
// Update committed memory.
m.buf = m.buf[:new]
m.buf = m.buf[:new] // Update committed memory.
}
// Limit returned capacity because bytes beyond
// len(m.buf) have not yet been committed.

28
internal/alloc/alloc_windows.go
View File

@@ -9,20 +9,26 @@ import (
"golang.org/x/sys/windows"
)
func NewMemory(_, max uint64) experimental.LinearMemory {
func NewMemory(cap, max uint64) experimental.LinearMemory {
// Round up to the page size.
rnd := uint64(windows.Getpagesize() - 1)
max = (max + rnd) &^ rnd
res := (max + rnd) &^ rnd
if max > math.MaxInt {
// This ensures uintptr(max) overflows to a large value,
if res > math.MaxInt {
// This ensures uintptr(res) overflows to a large value,
// and windows.VirtualAlloc returns an error.
max = math.MaxUint64
res = math.MaxUint64
}
// Reserve max bytes of address space, to ensure we won't need to move it.
// This does not commit memory.
r, err := windows.VirtualAlloc(0, uintptr(max), windows.MEM_RESERVE, windows.PAGE_READWRITE)
com := res
kind := windows.MEM_COMMIT
if cap < max { // Commit memory only if cap=max.
com = 0
kind = windows.MEM_RESERVE
}
// Reserve res bytes of address space, to ensure we won't need to move it.
r, err := windows.VirtualAlloc(0, uintptr(res), uint32(kind), windows.PAGE_READWRITE)
if err != nil {
panic(err)
}
@@ -30,8 +36,9 @@ func NewMemory(_, max uint64) experimental.LinearMemory {
mem := virtualMemory{addr: r}
// SliceHeader, although deprecated, avoids a go vet warning.
sh := (*reflect.SliceHeader)(unsafe.Pointer(&mem.buf))
sh.Cap = int(max)
sh.Data = r
sh.Len = int(com)
sh.Cap = int(res)
return &mem
}
@@ -59,8 +66,7 @@ func (m *virtualMemory) Reallocate(size uint64) []byte {
return nil
}
// Update committed memory.
m.buf = m.buf[:new]
m.buf = m.buf[:new] // Update committed memory.
}
// Limit returned capacity because bytes beyond
// len(m.buf) have not yet been committed.