fixes #25687; optimizes seq assignment for orc

compiler/liftdestructors.nim

@@ -620,11 +620,34 @@ proc checkSelfAssignment(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   cond.typ = getSysType(c.g, c.info, tyBool)
   body.add genIf(c, cond, newTreeI(nkReturnStmt, c.info, newNodeI(nkEmpty, c.info)))
 
+proc genBulkCopySeq(c: var TLiftCtx; t: PType; body, x, y: PNode) =
+  ## Generates a call to nimCopySeqPayload for bulk memcpy of seq data.
+  let elemType = t.elementType
+  let sym = magicsys.getCompilerProc(c.g, "nimCopySeqPayload")
+  if sym == nil:
+    localError(c.g.config, c.info, "system module needs: nimCopySeqPayload")
+    return
+  var sizeOf = genBuiltin(c, mSizeOf, "sizeof", newNodeIT(nkType, c.info, elemType))
+  sizeOf.typ = getSysType(c.g, c.info, tyInt)
+  var alignOf = genBuiltin(c, mAlignOf, "alignof", newNodeIT(nkType, c.info, elemType))
+  alignOf.typ = getSysType(c.g, c.info, tyInt)
+  let call = newNodeI(nkCall, c.info)
+  call.add newSymNode(sym)
+  call.add newTreeIT(nkAddr, c.info, makePtrType(c.fn, x.typ, c.idgen), x)
+  call.add newTreeIT(nkAddr, c.info, makePtrType(c.fn, y.typ, c.idgen), y)
+  call.add sizeOf
+  call.add alignOf
+  call.typ = sym.typ.returnType
+  body.add call
+
 proc fillSeqOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   case c.kind
   of attachedDup:
     body.add setLenSeqCall(c, t, x, y)
-    forallElements(c, t, body, x, y)
+    if supportsCopyMem(t.elementType):
+      genBulkCopySeq(c, t, body, x, y)
+    else:
+      forallElements(c, t, body, x, y)
   of attachedAsgn, attachedDeepCopy:
     # we generate:
     # if x.p == y.p:
@@ -633,9 +656,13 @@ proc fillSeqOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
     # var i = 0
     # while i < y.len: dest[i] = y[i]; inc(i)
     # This is usually more efficient than a destroy/create pair.
+    # For trivially copyable types, use bulk copyMem instead of element loop.
     checkSelfAssignment(c, t, body, x, y)
     body.add setLenSeqCall(c, t, x, y)
-    forallElements(c, t, body, x, y)
+    if supportsCopyMem(t.elementType):
+      genBulkCopySeq(c, t, body, x, y)
+    else:
+      forallElements(c, t, body, x, y)
   of attachedSink:
     let moveCall = genBuiltin(c, mMove, "move", x)
     moveCall.add y

compiler/semmagic.nim

@@ -232,10 +232,7 @@ proc evalTypeTrait(c: PContext; traitCall: PNode, operand: PType, context: PSym)
   of "stripGenericParams":
     result = uninstantiate(operand).toNode(traitCall.info)
   of "supportsCopyMem":
-    let t = operand.skipTypes({tyVar, tyLent, tyGenericInst, tyAlias, tySink, tyInferred})
-    let complexObj = containsGarbageCollectedRef(t) or
-                     hasDestructor(t)
-    result = newIntNodeT(toInt128(ord(not complexObj)), traitCall, c.idgen, c.graph)
+    result = newIntNodeT(toInt128(ord(supportsCopyMem(operand))), traitCall, c.idgen, c.graph)
   of "canFormCycles":
     result = newIntNodeT(toInt128(ord(types.canFormAcycle(c.graph, operand))), traitCall, c.idgen, c.graph)
   of "hasDefaultValue":

compiler/types.nim

@@ -1779,3 +1779,7 @@ proc reduceToBase*(f: PType): PType =
     result = f.elementType
   else:
     result = f
+
+proc supportsCopyMem*(t: PType): bool =
+  let t = t.skipTypes({tyVar, tyLent, tyGenericInst, tyAlias, tySink, tyInferred})
+  result = not containsGarbageCollectedRef(t) and not hasDestructor(t)

lib/system/seqs_v2.nim

@@ -272,6 +272,16 @@ proc newSeq[T](s: var seq[T], len: Natural) =
 proc sameSeqPayload(x: pointer, y: pointer): bool {.compilerRtl, inl.} =
   result = cast[ptr NimRawSeq](x)[].p == cast[ptr NimRawSeq](y)[].p
 
+proc nimCopySeqPayload(dest: pointer, src: pointer, elemSize: int, elemAlign: int) {.compilerRtl, inline.} =
+  ## Bulk-copies the payload data from src seq to dest seq using copyMem.
+  ## Only valid for trivially copyable element types (no GC refs, no destructors).
+  ## Caller must have already ensured dest has the correct length and capacity
+  ## (e.g. via setLen).
+  let d = cast[ptr NimRawSeq](dest)
+  let s = cast[ptr NimRawSeq](src)
+  if s.len > 0:
+    let headerSize = align(sizeof(NimSeqPayloadBase), elemAlign)
+    copyMem(d.p +! headerSize, s.p +! headerSize, s.len * elemSize)
 
 func capacity*[T](self: seq[T]): int {.inline.} =
   ## Returns the current capacity of the seq.