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|
package json
import (
"fmt"
"reflect"
"sync/atomic"
"unsafe"
)
const (
maxAcceptableTypeAddrRange = 1024 * 1024 * 2 // 2 Mib
)
var (
cachedDecoder []decoder
cachedDecoderMap unsafe.Pointer // map[uintptr]decoder
baseTypeAddr uintptr
maxTypeAddr uintptr
typeAddrShift uintptr
)
//go:linkname typelinks reflect.typelinks
func typelinks() ([]unsafe.Pointer, [][]int32)
//go:linkname rtypeOff reflect.rtypeOff
func rtypeOff(unsafe.Pointer, int32) unsafe.Pointer
func setupCodec() error {
sections, offsets := typelinks()
if len(sections) != 1 {
return fmt.Errorf("failed to get sections")
}
if len(offsets) != 1 {
return fmt.Errorf("failed to get offsets")
}
section := sections[0]
offset := offsets[0]
var (
min uintptr = uintptr(^uint(0))
max uintptr = 0
isAligned64 = true
isAligned32 = true
)
for i := 0; i < len(offset); i++ {
typ := (*rtype)(rtypeOff(section, offset[i]))
addr := uintptr(unsafe.Pointer(typ))
if min > addr {
min = addr
}
if max < addr {
max = addr
}
if typ.Kind() == reflect.Ptr {
addr = uintptr(unsafe.Pointer(typ.Elem()))
if min > addr {
min = addr
}
if max < addr {
max = addr
}
}
// check every address is aligned from the base address
isAligned64 = isAligned64 && (addr-min)&63 == 0
isAligned32 = isAligned32 && (addr-min)&31 == 0
}
addrRange := max - min
if addrRange == 0 {
return fmt.Errorf("failed to get address range of types")
}
if isAligned64 {
typeAddrShift = 6
} else if isAligned32 {
typeAddrShift = 5
}
cacheSize := addrRange >> typeAddrShift
if cacheSize > maxAcceptableTypeAddrRange {
return fmt.Errorf("too big address range %d", addrRange)
}
cachedDecoder = make([]decoder, cacheSize)
baseTypeAddr = min
maxTypeAddr = max
return nil
}
func init() {
_ = setupCodec()
}
func loadDecoderMap() map[uintptr]decoder {
p := atomic.LoadPointer(&cachedDecoderMap)
return *(*map[uintptr]decoder)(unsafe.Pointer(&p))
}
func storeDecoder(typ uintptr, dec decoder, m map[uintptr]decoder) {
newDecoderMap := make(map[uintptr]decoder, len(m)+1)
newDecoderMap[typ] = dec
for k, v := range m {
newDecoderMap[k] = v
}
atomic.StorePointer(&cachedDecoderMap, *(*unsafe.Pointer)(unsafe.Pointer(&newDecoderMap)))
}
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