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Source file src/runtime/slice.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	package runtime
     6	
     7	import (
     8		"unsafe"
     9	)
    10	
    11	type slice struct {
    12		array unsafe.Pointer
    13		len   int
    14		cap   int
    15	}
    16	
    17	// maxElems is a lookup table containing the maximum capacity for a slice.
    18	// The index is the size of the slice element.
    19	var maxElems = [...]uintptr{
    20		^uintptr(0),
    21		_MaxMem / 1, _MaxMem / 2, _MaxMem / 3, _MaxMem / 4,
    22		_MaxMem / 5, _MaxMem / 6, _MaxMem / 7, _MaxMem / 8,
    23		_MaxMem / 9, _MaxMem / 10, _MaxMem / 11, _MaxMem / 12,
    24		_MaxMem / 13, _MaxMem / 14, _MaxMem / 15, _MaxMem / 16,
    25		_MaxMem / 17, _MaxMem / 18, _MaxMem / 19, _MaxMem / 20,
    26		_MaxMem / 21, _MaxMem / 22, _MaxMem / 23, _MaxMem / 24,
    27		_MaxMem / 25, _MaxMem / 26, _MaxMem / 27, _MaxMem / 28,
    28		_MaxMem / 29, _MaxMem / 30, _MaxMem / 31, _MaxMem / 32,
    29	}
    30	
    31	// maxSliceCap returns the maximum capacity for a slice.
    32	func maxSliceCap(elemsize uintptr) uintptr {
    33		if elemsize < uintptr(len(maxElems)) {
    34			return maxElems[elemsize]
    35		}
    36		return _MaxMem / elemsize
    37	}
    38	
    39	func makeslice(et *_type, len, cap int) slice {
    40		// NOTE: The len > maxElements check here is not strictly necessary,
    41		// but it produces a 'len out of range' error instead of a 'cap out of range' error
    42		// when someone does make([]T, bignumber). 'cap out of range' is true too,
    43		// but since the cap is only being supplied implicitly, saying len is clearer.
    44		// See issue 4085.
    45		maxElements := maxSliceCap(et.size)
    46		if len < 0 || uintptr(len) > maxElements {
    47			panic(errorString("makeslice: len out of range"))
    48		}
    49	
    50		if cap < len || uintptr(cap) > maxElements {
    51			panic(errorString("makeslice: cap out of range"))
    52		}
    53	
    54		p := mallocgc(et.size*uintptr(cap), et, true)
    55		return slice{p, len, cap}
    56	}
    57	
    58	func makeslice64(et *_type, len64, cap64 int64) slice {
    59		len := int(len64)
    60		if int64(len) != len64 {
    61			panic(errorString("makeslice: len out of range"))
    62		}
    63	
    64		cap := int(cap64)
    65		if int64(cap) != cap64 {
    66			panic(errorString("makeslice: cap out of range"))
    67		}
    68	
    69		return makeslice(et, len, cap)
    70	}
    71	
    72	// growslice handles slice growth during append.
    73	// It is passed the slice element type, the old slice, and the desired new minimum capacity,
    74	// and it returns a new slice with at least that capacity, with the old data
    75	// copied into it.
    76	// The new slice's length is set to the old slice's length,
    77	// NOT to the new requested capacity.
    78	// This is for codegen convenience. The old slice's length is used immediately
    79	// to calculate where to write new values during an append.
    80	// TODO: When the old backend is gone, reconsider this decision.
    81	// The SSA backend might prefer the new length or to return only ptr/cap and save stack space.
    82	func growslice(et *_type, old slice, cap int) slice {
    83		if raceenabled {
    84			callerpc := getcallerpc(unsafe.Pointer(&et))
    85			racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
    86		}
    87		if msanenabled {
    88			msanread(old.array, uintptr(old.len*int(et.size)))
    89		}
    90	
    91		if et.size == 0 {
    92			if cap < old.cap {
    93				panic(errorString("growslice: cap out of range"))
    94			}
    95			// append should not create a slice with nil pointer but non-zero len.
    96			// We assume that append doesn't need to preserve old.array in this case.
    97			return slice{unsafe.Pointer(&zerobase), old.len, cap}
    98		}
    99	
   100		newcap := old.cap
   101		doublecap := newcap + newcap
   102		if cap > doublecap {
   103			newcap = cap
   104		} else {
   105			if old.len < 1024 {
   106				newcap = doublecap
   107			} else {
   108				for newcap < cap {
   109					newcap += newcap / 4
   110				}
   111			}
   112		}
   113	
   114		var lenmem, newlenmem, capmem uintptr
   115		const ptrSize = unsafe.Sizeof((*byte)(nil))
   116		switch et.size {
   117		case 1:
   118			lenmem = uintptr(old.len)
   119			newlenmem = uintptr(cap)
   120			capmem = roundupsize(uintptr(newcap))
   121			newcap = int(capmem)
   122		case ptrSize:
   123			lenmem = uintptr(old.len) * ptrSize
   124			newlenmem = uintptr(cap) * ptrSize
   125			capmem = roundupsize(uintptr(newcap) * ptrSize)
   126			newcap = int(capmem / ptrSize)
   127		default:
   128			lenmem = uintptr(old.len) * et.size
   129			newlenmem = uintptr(cap) * et.size
   130			capmem = roundupsize(uintptr(newcap) * et.size)
   131			newcap = int(capmem / et.size)
   132		}
   133	
   134		if cap < old.cap || uintptr(newcap) > maxSliceCap(et.size) {
   135			panic(errorString("growslice: cap out of range"))
   136		}
   137	
   138		var p unsafe.Pointer
   139		if et.kind&kindNoPointers != 0 {
   140			p = mallocgc(capmem, nil, false)
   141			memmove(p, old.array, lenmem)
   142			// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
   143			// Only clear the part that will not be overwritten.
   144			memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
   145		} else {
   146			// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
   147			p = mallocgc(capmem, et, true)
   148			if !writeBarrier.enabled {
   149				memmove(p, old.array, lenmem)
   150			} else {
   151				for i := uintptr(0); i < lenmem; i += et.size {
   152					typedmemmove(et, add(p, i), add(old.array, i))
   153				}
   154			}
   155		}
   156	
   157		return slice{p, old.len, newcap}
   158	}
   159	
   160	func slicecopy(to, fm slice, width uintptr) int {
   161		if fm.len == 0 || to.len == 0 {
   162			return 0
   163		}
   164	
   165		n := fm.len
   166		if to.len < n {
   167			n = to.len
   168		}
   169	
   170		if width == 0 {
   171			return n
   172		}
   173	
   174		if raceenabled {
   175			callerpc := getcallerpc(unsafe.Pointer(&to))
   176			pc := funcPC(slicecopy)
   177			racewriterangepc(to.array, uintptr(n*int(width)), callerpc, pc)
   178			racereadrangepc(fm.array, uintptr(n*int(width)), callerpc, pc)
   179		}
   180		if msanenabled {
   181			msanwrite(to.array, uintptr(n*int(width)))
   182			msanread(fm.array, uintptr(n*int(width)))
   183		}
   184	
   185		size := uintptr(n) * width
   186		if size == 1 { // common case worth about 2x to do here
   187			// TODO: is this still worth it with new memmove impl?
   188			*(*byte)(to.array) = *(*byte)(fm.array) // known to be a byte pointer
   189		} else {
   190			memmove(to.array, fm.array, size)
   191		}
   192		return n
   193	}
   194	
   195	func slicestringcopy(to []byte, fm string) int {
   196		if len(fm) == 0 || len(to) == 0 {
   197			return 0
   198		}
   199	
   200		n := len(fm)
   201		if len(to) < n {
   202			n = len(to)
   203		}
   204	
   205		if raceenabled {
   206			callerpc := getcallerpc(unsafe.Pointer(&to))
   207			pc := funcPC(slicestringcopy)
   208			racewriterangepc(unsafe.Pointer(&to[0]), uintptr(n), callerpc, pc)
   209		}
   210		if msanenabled {
   211			msanwrite(unsafe.Pointer(&to[0]), uintptr(n))
   212		}
   213	
   214		memmove(unsafe.Pointer(&to[0]), stringStructOf(&fm).str, uintptr(n))
   215		return n
   216	}
   217	

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