Remove some unnecessary initialization in seq operations (#22677)

* `PrepareSeqAdd`
* `add`
* `setLen`
* `grow`

Merge after #21842.

---------

Co-authored-by: ringabout <[email protected]>

lib/system/seqs_v2.nim

@@ -79,15 +79,42 @@ proc prepareSeqAdd(len: int; p: pointer; addlen, elemSize, elemAlign: int): poin
  var p = cast[ptr NimSeqPayloadBase](p)
  let oldCap = p.cap and not strlitFlag
  let newCap = max(resize(oldCap), len+addlen)
+ var q: ptr NimSeqPayloadBase
  if (p.cap and strlitFlag) == strlitFlag:
- var q = cast[ptr NimSeqPayloadBase](alignedAlloc0(headerSize + elemSize * newCap, elemAlign))
+ q = cast[ptr NimSeqPayloadBase](alignedAlloc(headerSize + elemSize * newCap, elemAlign))
+ copyMem(q +! headerSize, p +! headerSize, len * elemSize)
+ else:
+ let oldSize = headerSize + elemSize * oldCap
+ let newSize = headerSize + elemSize * newCap
+ q = cast[ptr NimSeqPayloadBase](alignedRealloc(p, oldSize, newSize, elemAlign))
+
+ zeroMem(q +! headerSize +! len * elemSize, addlen * elemSize)
+ q.cap = newCap
+ result = q
+
+proc prepareSeqAddUninit(len: int; p: pointer; addlen, elemSize, elemAlign: int): pointer {.
+ noSideEffect, tags: [], raises: [], compilerRtl.} =
+ {.noSideEffect.}:
+ let headerSize = align(sizeof(NimSeqPayloadBase), elemAlign)
+ if addlen <= 0:
+ result = p
+ elif p == nil:
+ result = newSeqPayloadUninit(len+addlen, elemSize, elemAlign)
+ else:
+ # Note: this means we cannot support things that have internal pointers as
+ # they get reallocated here. This needs to be documented clearly.
+ var p = cast[ptr NimSeqPayloadBase](p)
+ let oldCap = p.cap and not strlitFlag
+ let newCap = max(resize(oldCap), len+addlen)
+ if (p.cap and strlitFlag) == strlitFlag:
+ var q = cast[ptr NimSeqPayloadBase](alignedAlloc(headerSize + elemSize * newCap, elemAlign))
  copyMem(q +! headerSize, p +! headerSize, len * elemSize)
  q.cap = newCap
  result = q
  else:
  let oldSize = headerSize + elemSize * oldCap
  let newSize = headerSize + elemSize * newCap
- var q = cast[ptr NimSeqPayloadBase](alignedRealloc0(p, oldSize, newSize, elemAlign))
+ var q = cast[ptr NimSeqPayloadBase](alignedRealloc(p, oldSize, newSize, elemAlign))
  q.cap = newCap
  result = q
 
@@ -104,16 +131,17 @@ proc shrink*[T](x: var seq[T]; newLen: Natural) {.tags: [], raises: [].} =
  {.noSideEffect.}:
  cast[ptr NimSeqV2[T]](addr x).len = newLen
 
-proc grow*[T](x: var seq[T]; newLen: Natural; value: T) =
+proc grow*[T](x: var seq[T]; newLen: Natural; value: T) {.nodestroy.} =
  let oldLen = x.len
  #sysAssert newLen >= x.len, "invalid newLen parameter for 'grow'"
  if newLen <= oldLen: return
  var xu = cast[ptr NimSeqV2[T]](addr x)
  if xu.p == nil or (xu.p.cap and not strlitFlag) < newLen:
- xu.p = cast[typeof(xu.p)](prepareSeqAdd(oldLen, xu.p, newLen - oldLen, sizeof(T), alignof(T)))
+ xu.p = cast[typeof(xu.p)](prepareSeqAddUninit(oldLen, xu.p, newLen - oldLen, sizeof(T), alignof(T)))
  xu.len = newLen
  for i in oldLen .. newLen-1:
- xu.p.data[i] = value
+ wasMoved(xu.p.data[i])
+ `=copy`(xu.p.data[i], value)
 
 proc add*[T](x: var seq[T]; y: sink T) {.magic: "AppendSeqElem", noSideEffect, nodestroy.} =
  ## Generic proc for adding a data item `y` to a container `x`.
@@ -126,7 +154,7 @@ proc add*[T](x: var seq[T]; y: sink T) {.magic: "AppendSeqElem", noSideEffect, n
  let oldLen = x.len
  var xu = cast[ptr NimSeqV2[T]](addr x)
  if xu.p == nil or (xu.p.cap and not strlitFlag) < oldLen+1:
- xu.p = cast[typeof(xu.p)](prepareSeqAdd(oldLen, xu.p, 1, sizeof(T), alignof(T)))
+ xu.p = cast[typeof(xu.p)](prepareSeqAddUninit(oldLen, xu.p, 1, sizeof(T), alignof(T)))
  xu.len = oldLen+1
  # .nodestroy means `xu.p.data[oldLen] = value` is compiled into a
  # copyMem(). This is fine as know by construction that
@@ -143,7 +171,7 @@ proc setLen[T](s: var seq[T], newlen: Natural) {.nodestroy.} =
  if newlen <= oldLen: return
  var xu = cast[ptr NimSeqV2[T]](addr s)
  if xu.p == nil or (xu.p.cap and not strlitFlag) < newlen:
- xu.p = cast[typeof(xu.p)](prepareSeqAdd(oldLen, xu.p, newlen - oldLen, sizeof(T), alignof(T)))
+ xu.p = cast[typeof(xu.p)](prepareSeqAddUninit(oldLen, xu.p, newlen - oldLen, sizeof(T), alignof(T)))
  xu.len = newlen
  for i in oldLen..<newlen:
  xu.p.data[i] = default(T)