Source File
intrinsics_common.go
Belonging Package
runtime/internal/sys
// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package sys
// Copied from math/bits to avoid dependence.
const len8tab = "" +
"\x00\x01\x02\x02\x03\x03\x03\x03\x04\x04\x04\x04\x04\x04\x04\x04" +
"\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05\x05" +
"\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06" +
"\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06\x06" +
"\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07" +
"\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07" +
"\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07" +
"\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07\x07" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08"
// 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.
//
//go:nosplit
func ( uint64) ( int) {
if >= 1<<32 {
>>= 32
= 32
}
if >= 1<<16 {
>>= 16
+= 16
}
if >= 1<<8 {
>>= 8
+= 8
}
return + int(len8tab[])
}
// --- OnesCount ---
const m0 = 0x5555555555555555 // 01010101 ...
const m1 = 0x3333333333333333 // 00110011 ...
const m2 = 0x0f0f0f0f0f0f0f0f // 00001111 ...
// OnesCount64 returns the number of one bits ("population count") in x.
func ( uint64) int {
// Implementation: Parallel summing of adjacent bits.
// See "Hacker's Delight", Chap. 5: Counting Bits.
// The following pattern shows the general approach:
//
// x = x>>1&(m0&m) + x&(m0&m)
// x = x>>2&(m1&m) + x&(m1&m)
// x = x>>4&(m2&m) + x&(m2&m)
// x = x>>8&(m3&m) + x&(m3&m)
// x = x>>16&(m4&m) + x&(m4&m)
// x = x>>32&(m5&m) + x&(m5&m)
// return int(x)
//
// Masking (& operations) can be left away when there's no
// danger that a field's sum will carry over into the next
// field: Since the result cannot be > 64, 8 bits is enough
// and we can ignore the masks for the shifts by 8 and up.
// Per "Hacker's Delight", the first line can be simplified
// more, but it saves at best one instruction, so we leave
// it alone for clarity.
const = 1<<64 - 1
= >>1&(m0&) + &(m0&)
= >>2&(m1&) + &(m1&)
= (>>4 + ) & (m2 & )
+= >> 8
+= >> 16
+= >> 32
return int() & (1<<7 - 1)
}
// LeadingZeros64 returns the number of leading zero bits in x; the result is 64 for x == 0.
func ( uint64) int { return 64 - Len64() }
// LeadingZeros8 returns the number of leading zero bits in x; the result is 8 for x == 0.
func ( uint8) int { return 8 - Len8() }
// Len8 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
func ( uint8) int {
return int(len8tab[])
}
// Prefetch prefetches data from memory addr to cache
//
// AMD64: Produce PREFETCHT0 instruction
//
// ARM64: Produce PRFM instruction with PLDL1KEEP option
func ( 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 ( uintptr) {}
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