bring back id table algorithm instead of std table [backport:2.2] (#24930)

refs #24929, partially reverts #23403

Instead of using `Table[ItemId, T]`, the old algorithm is brought back
into `TIdTable[T]` to prevent a performance regression. The inheritance
removal from #23403 still holds, only `ItemId`s are stored.

(cherry picked from commit 82553384d1)

compiler/ast.nim

@@ -796,6 +796,15 @@ type
 
   TPairSeq* = seq[TPair]
 
+  TIdPair*[T] = object
+    key*: ItemId
+    val*: T
+
+  TIdPairSeq*[T] = seq[TIdPair[T]]
+  TIdTable*[T] = object
+    counter*: int
+    data*: TIdPairSeq[T]
+
   TNodePair* = object
     h*: Hash                 # because it is expensive to compute!
     key*: PNode
@@ -940,9 +949,11 @@ proc getPIdent*(a: PNode): PIdent {.inline.} =
 const
   moduleShift = when defined(cpu32): 20 else: 24
 
-template id*(a: PType | PSym): int =
+template toId*(a: ItemId): int =
   let x = a
-  (x.itemId.module.int shl moduleShift) + x.itemId.item.int
+  (x.module.int shl moduleShift) + x.item.int
+
+template id*(a: PType | PSym): int = toId(a.itemId)
 
 type
   IdGenerator* = ref object # unfortunately, we really need the 'shared mutable' aspect here.
@@ -1269,6 +1280,11 @@ proc copyStrTable*(dest: var TStrTable, src: TStrTable) =
   setLen(dest.data, src.data.len)
   for i in 0..high(src.data): dest.data[i] = src.data[i]
 
+proc copyIdTable*[T](dest: var TIdTable[T], src: TIdTable[T]) =
+  dest.counter = src.counter
+  newSeq(dest.data, src.data.len)
+  for i in 0..high(src.data): dest.data[i] = src.data[i]
+
 proc copyObjectSet*(dest: var TObjectSet, src: TObjectSet) =
   dest.counter = src.counter
   setLen(dest.data, src.data.len)
@@ -1607,6 +1623,16 @@ proc initStrTable*(): TStrTable =
   result = TStrTable(counter: 0)
   newSeq(result.data, StartSize)
 
+proc initIdTable*[T](): TIdTable[T] =
+  result = TIdTable[T](counter: 0)
+  newSeq(result.data, StartSize)
+
+proc resetIdTable*[T](x: var TIdTable[T]) =
+  x.counter = 0
+  # clear and set to old initial size:
+  setLen(x.data, 0)
+  setLen(x.data, StartSize)
+
 proc initObjectSet*(): TObjectSet =
   result = TObjectSet(counter: 0)
   newSeq(result.data, StartSize)
@@ -2135,14 +2161,8 @@ proc isTrue*(n: PNode): bool =
     n.kind == nkIntLit and n.intVal != 0
 
 type
-  TypeMapping* = Table[ItemId, PType]
-  SymMapping* = Table[ItemId, PSym]
+  TypeMapping* = TIdTable[PType]
+  SymMapping* = TIdTable[PSym]
 
-template idTableGet*(tab: typed; key: PSym | PType): untyped = tab.getOrDefault(key.itemId)
-template idTablePut*(tab: typed; key, val: PSym | PType) = tab[key.itemId] = val
-
-template initSymMapping*(): Table[ItemId, PSym] = initTable[ItemId, PSym]()
-template initTypeMapping*(): Table[ItemId, PType] = initTable[ItemId, PType]()
-
-template resetIdTable*(tab: Table[ItemId, PSym]) = tab.clear()
-template resetIdTable*(tab: Table[ItemId, PType]) = tab.clear()
+template initSymMapping*(): SymMapping = initIdTable[PSym]()
+template initTypeMapping*(): TypeMapping = initIdTable[PType]()