fixes #25007; implements setLenUninit for refc (#25022)

fixes #25007

```nim
proc setLengthSeqUninit(s: PGenericSeq, typ: PNimType, newLen: int, isTrivial: bool): PGenericSeq {.
    compilerRtl.} =
```

In this added function, only the line `zeroMem(dataPointer(result,
elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)` is
removed from `proc setLengthSeqV2` when enlarging a sequence.

JS and VM versions simply use `setLen`.

(cherry picked from commit 611b8bbf67)

compiler/ast.nim

@@ -500,6 +500,7 @@ type
     mAppendStrCh, mAppendStrStr, mAppendSeqElem,
     mInSet, mRepr, mExit,
     mSetLengthStr, mSetLengthSeq,
+    mSetLengthSeqUninit,
     mIsPartOf, mAstToStr, mParallel,
     mSwap, mIsNil, mArrToSeq, mOpenArrayToSeq,
     mNewString, mNewStringOfCap, mParseBiggestFloat,

compiler/ccgexprs.nim

@@ -1932,7 +1932,7 @@ proc isTrivialTypesToSnippet(t: PType): Rope =
   else:
     result = rope"NIM_TRUE"
 
-proc genSetLengthSeq(p: BProc, e: PNode, d: var TLoc) =
+proc genSetLengthSeq(p: BProc, e: PNode, d: var TLoc, noinit = false) =
   if optSeqDestructors in p.config.globalOptions:
     e[1] = makeAddr(e[1], p.module.idgen)
     genCall(p, e, d)
@@ -1952,11 +1952,13 @@ proc genSetLengthSeq(p: BProc, e: PNode, d: var TLoc) =
       genTypeInfoV1(p.module, t.skipTypes(abstractInst), e.info)])
 
   else:
-    const setLenPattern = "($3) #setLengthSeqV2($1, $4, $2, $5)"
+    const setLenPattern = "($3) #$6($1, $4, $2, $5)"
+    let name = if noinit: "setLengthSeqUninit" else: "setLengthSeqV2"
     call.snippet = ropecg(p.module, setLenPattern, [
       rdLoc(a), rdLoc(b), getTypeDesc(p.module, t),
       genTypeInfoV1(p.module, t.skipTypes(abstractInst), e.info),
-      isTrivialTypesToSnippet(t.skipTypes(abstractInst)[0])])
+      isTrivialTypesToSnippet(t.skipTypes(abstractInst)[0]),
+      name])
 
   genAssignment(p, a, call, {})
   gcUsage(p.config, e)
@@ -2579,6 +2581,7 @@ proc genMagicExpr(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
     unaryStmt(p, e, d, "if ($1) { #nimGCunref($1); }$n")
   of mSetLengthStr: genSetLengthStr(p, e, d)
   of mSetLengthSeq: genSetLengthSeq(p, e, d)
+  of mSetLengthSeqUninit: genSetLengthSeq(p, e, d, noinit = true)
   of mIncl, mExcl, mCard, mLtSet, mLeSet, mEqSet, mMulSet, mPlusSet, mMinusSet,
      mInSet, mXorSet:
     genSetOp(p, e, d, op)

compiler/condsyms.nim

@@ -173,3 +173,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasXorSet")
 
   defineSymbol("nimHasPreviewDuplicateModuleError")
+  defineSymbol("nimHasSetLengthSeqUninitMagic")

compiler/jsgen.nim

@@ -2439,7 +2439,7 @@ proc genMagic(p: PProc, n: PNode, r: var TCompRes) =
     binaryExpr(p, n, r, "mnewString",
       """if ($1.length < $2) { for (var i = $3.length; i < $4; ++i) $3.push(0); }
          else {$3.length = $4; }""")
-  of mSetLengthSeq:
+  of mSetLengthSeq, mSetLengthSeqUninit:
     var x, y: TCompRes = default(TCompRes)
     gen(p, n[1], x)
     gen(p, n[2], y)

compiler/semdata.nim

@@ -708,7 +708,7 @@ proc analyseIfAddressTakenInCall*(c: PContext, n: PNode, isConverter = false) =
     return
   const
     FakeVarParams = {mNew, mNewFinalize, mInc, ast.mDec, mIncl, mExcl,
-      mSetLengthStr, mSetLengthSeq, mAppendStrCh, mAppendStrStr, mSwap,
+      mSetLengthStr, mSetLengthSeq, mSetLengthSeqUninit, mAppendStrCh, mAppendStrStr, mSwap,
       mAppendSeqElem, mNewSeq, mShallowCopy, mDeepCopy, mMove, mWasMoved}
 
   template checkIfConverterCalled(c: PContext, n: PNode) =

compiler/semmagic.nim

@@ -667,7 +667,7 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
     result = semQuantifier(c, n)
   of mOld:
     result = semOld(c, n)
-  of mSetLengthSeq:
+  of mSetLengthSeq, mSetLengthSeqUninit:
     result = n
     let seqType = result[1].typ.skipTypes({tyPtr, tyRef, # in case we had auto-dereferencing
                                            tyVar, tyGenericInst, tyOwned, tySink,

compiler/vmgen.nim

@@ -1230,7 +1230,7 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; flags: TGenFlags = {}, m: TMag
     var tmp = c.genx(n[1])
     c.gABC(n, opcQuit, tmp)
     c.freeTemp(tmp)
-  of mSetLengthStr, mSetLengthSeq:
+  of mSetLengthStr, mSetLengthSeq, mSetLengthSeqUninit:
     unused(c, n, dest)
     var d = c.genx(n[1])
     var tmp = c.genx(n[2])

lib/system.nim

@@ -955,6 +955,22 @@ proc setLen*[T](s: var seq[T], newlen: Natural) {.
   ##   assert x == @[10]
   ##   ```
 
+when defined(nimHasSetLengthSeqUninitMagic):
+  func setLenUninit*[T](s: var seq[T], newlen: Natural) {.magic: "SetLengthSeqUninit", nodestroy.} =
+    ## Sets the length of seq `s` to `newlen`. `T` may be any sequence type.
+    ## New slots will not be initialized.
+    ##
+    ## If the current length is greater than the new length,
+    ## `s` will be truncated.
+    ##   ```nim
+    ##   var x = @[10, 20]
+    ##   x.setLenUninit(5)
+    ##   x[4] = 50
+    ##   assert x[4] == 50Add commentMore actions
+    ##   x.setLenUninit(1)
+    ##   assert x == @[10]
+    ##   ```
+
 proc setLen*(s: var string, newlen: Natural) {.
   magic: "SetLengthStr", noSideEffect.}
   ## Sets the length of string `s` to `newlen`.

lib/system/seqs_v2.nim

@@ -196,7 +196,7 @@ func capacity*[T](self: seq[T]): int {.inline.} =
   let sek = cast[ptr NimSeqV2[T]](unsafeAddr self)
   result = if sek.p != nil: sek.p.cap and not strlitFlag else: 0
 
-func setLenUninit*[T](s: var seq[T], newlen: Natural) {.nodestroy.} =
+func setLenUninit[T](s: var seq[T], newlen: Natural) {.nodestroy.} =
   ## Sets the length of seq `s` to `newlen`. `T` may be any sequence type.
   ## New slots will not be initialized.
   ##

lib/system/sysstr.nim

@@ -300,6 +300,46 @@ proc setLengthSeq(seq: PGenericSeq, elemSize, elemAlign, newLen: int): PGenericS
     zeroMem(dataPointer(result, elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)
   result.len = newLen
 
+proc setLengthSeqUninit(s: PGenericSeq, typ: PNimType, newLen: int, isTrivial: bool): PGenericSeq {.
+    compilerRtl.} =
+  sysAssert typ.kind == tySequence, "setLengthSeqUninit: type is not a seq"
+  if s == nil:
+    if newLen == 0:
+      result = s
+    else:
+      result = cast[PGenericSeq](newSeq(typ, newLen))
+  else:
+    let elemSize = typ.base.size
+    let elemAlign = typ.base.align
+    if s.space < newLen:
+      let r = max(resize(s.space), newLen)
+      result = cast[PGenericSeq](newSeq(typ, r))
+      copyMem(dataPointer(result, elemAlign), dataPointer(s, elemAlign), s.len * elemSize)
+      # since we steal the content from 's', it's crucial to set s's len to 0.
+      s.len = 0
+    elif newLen < s.len:
+      result = s
+      # we need to decref here, otherwise the GC leaks!
+      when not defined(boehmGC) and not defined(nogc) and
+          not defined(gcMarkAndSweep) and not defined(gogc) and
+          not defined(gcRegions):
+        if ntfNoRefs notin typ.base.flags:
+          for i in newLen..result.len-1:
+            forAllChildrenAux(dataPointer(result, elemAlign, elemSize, i),
+                              extGetCellType(result).base, waZctDecRef)
+
+      # XXX: zeroing out the memory can still result in crashes if a wiped-out
+      # cell is aliased by another pointer (ie proc parameter or a let variable).
+      # This is a tough problem, because even if we don't zeroMem here, in the
+      # presence of user defined destructors, the user will expect the cell to be
+      # "destroyed" thus creating the same problem. We can destroy the cell in the
+      # finalizer of the sequence, but this makes destruction non-deterministic.
+      if not isTrivial: # optimization for trivial types
+        zeroMem(dataPointer(result, elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)
+    else:
+      result = s
+    result.len = newLen
+
 proc setLengthSeqV2(s: PGenericSeq, typ: PNimType, newLen: int, isTrivial: bool): PGenericSeq {.
     compilerRtl.} =
   sysAssert typ.kind == tySequence, "setLengthSeqV2: type is not a seq"

tests/stdlib/tsystem.nim

@@ -236,6 +236,13 @@ proc bar2() =
     doAssert cstring(nil) <= cstring(nil)
     doAssert cstring("") <= cstring("")
 
+  var x = @[10, 20]
+  x.setLenUninit(5)
+  x[4] = 50
+  doAssert x[4] == 50
+  x.setLenUninit(1)
+  doAssert x == @[10]
+
 static: bar2()
 bar2()