fixes #19728; setLen slow when shrinking seq due to zero-filling of released area (#24683)

fixes #19728

don't zero-filling memory for "trivial types" without destructor in
refc. I tested locally with internal apis.

compiler/ccgexprs.nim

@@ -2198,6 +2198,13 @@ proc genArrayLen(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
     else: putIntoDest(p, d, e, cIntValue(lengthOrd(p.config, typ)))
   else: internalError(p.config, e.info, "genArrayLen()")
 
+proc isTrivialTypesToSnippet(t: PType): Snippet =
+  if containsGarbageCollectedRef(t) or
+                     hasDestructor(t):
+    result = NimFalse
+  else:
+    result = NimTrue
+
 proc genSetLengthSeq(p: BProc, e: PNode, d: var TLoc) =
   if optSeqDestructors in p.config.globalOptions:
     e[1] = makeAddr(e[1], p.module.idgen)
@@ -2220,7 +2227,8 @@ proc genSetLengthSeq(p: BProc, e: PNode, d: var TLoc) =
     pExpr = cIfExpr(ra, cAddr(derefField(ra, "Sup")), NimNil)
   else:
     pExpr = ra
-  call.snippet = cCast(rt, cgCall(p, "setLengthSeqV2", pExpr, rti, rb))
+  call.snippet = cCast(rt, cgCall(p, "setLengthSeqV2", pExpr, rti, rb,
+          isTrivialTypesToSnippet(t.skipTypes(abstractInst)[0])))
 
   genAssignment(p, a, call, {})
   gcUsage(p.config, e)

lib/system/sysstr.nim

@@ -300,7 +300,7 @@ proc setLengthSeq(seq: PGenericSeq, elemSize, elemAlign, newLen: int): PGenericS
     zeroMem(dataPointer(result, elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)
   result.len = newLen
 
-proc setLengthSeqV2(s: PGenericSeq, typ: PNimType, newLen: int): PGenericSeq {.
+proc setLengthSeqV2(s: PGenericSeq, typ: PNimType, newLen: int, isTrivial: bool): PGenericSeq {.
     compilerRtl.} =
   sysAssert typ.kind == tySequence, "setLengthSeqV2: type is not a seq"
   if s == nil:
@@ -334,7 +334,8 @@ proc setLengthSeqV2(s: PGenericSeq, typ: PNimType, newLen: int): PGenericSeq {.
       # 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.
-      zeroMem(dataPointer(result, elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)
+      if not isTrivial: # optimization for trivial types
+        zeroMem(dataPointer(result, elemAlign, elemSize, newLen), (result.len-%newLen) *% elemSize)
     else:
       result = s
       zeroMem(dataPointer(result, elemAlign, elemSize, result.len), (newLen-%result.len) *% elemSize)

tests/stdlib/tmisc_issues.nim

@@ -37,3 +37,30 @@ block: # bug #16771
   a.foo b
   doAssert a.n == 42
   doAssert b.n == 1
+
+block: # bug #24683
+  block:
+    var v = newSeq[int](100)
+    v[99]= 444
+    v.setLen(5)
+    v.setLen(100)
+    doAssert v[99] == 0
+
+  when not defined(js):
+    block:
+      var 
+        x = @[1, 2, 3, 4, 45, 56, 67, 999, 88, 777]
+
+      x.setLen(0) # zero-fills 1mb of released data
+
+      type
+        TGenericSeq = object
+          len, reserved: int
+        PGenericSeq = ptr TGenericSeq
+
+      when defined(gcRefc):
+        cast[PGenericSeq](x).len = 10
+      else:
+        cast[ptr int](addr x)[] = 10
+
+      doAssert x == @[1, 2, 3, 4, 45, 56, 67, 999, 88, 777]