fixes #17198, DFA failure on large case stmts (#17210)

This alters the DFA control flow graph generation for case statments.
Gotos are now generated as a chained link, this ensures that evaluation
of variant branches collapses as early as possible, without hitting the
2k call limit.

compiler/dfa.nim

@@ -455,6 +455,10 @@ proc genCase(c: var Con; n: PNode) =
   let isExhaustive = skipTypes(n[0].typ,
     abstractVarRange-{tyTypeDesc}).kind notin {tyFloat..tyFloat128, tyString}
 
+  # we generate endings as a set of chained gotos, this is a bit awkward but it
+  # ensures when recursively traversing the CFG for various analysis, we don't
+  # artificially extended the life of each branch (for the purposes of DFA)
+  # beyond the minimum amount.
   var endings: seq[TPosition] = @[]
   c.gen(n[0])
   for i in 1..<n.len:
@@ -462,13 +466,14 @@ proc genCase(c: var Con; n: PNode) =
     if it.len == 1 or (i == n.len-1 and isExhaustive):
       # treat the last branch as 'else' if this is an exhaustive case statement.
       c.gen(it.lastSon)
+      if endings.len != 0:
+          c.patch(endings[^1])
     else:
       forkT(it.lastSon):
         c.gen(it.lastSon)
+        if endings.len != 0:
+          c.patch(endings[^1])
         endings.add c.gotoI(it.lastSon)
-  for i in countdown(endings.high, 0):
-    let endPos = endings[i]
-    c.patch(endPos)
 
 proc genBlock(c: var Con; n: PNode) =
   withBlock(n[0].sym):

tests/destructor/t17198.nim

@@ -0,0 +1,32 @@
+discard """
+  cmd: '''nim c --gc:arc $file'''
+  output: '''
+other
+'''
+"""
+
+import std/macros
+
+macro bigCaseStmt(arg: untyped): untyped =
+  result = nnkCaseStmt.newTree(arg)
+
+  # try to change 2000 to a bigger value if it doesn't crash
+  for x in 0 ..< 2000:
+    result.add nnkOfBranch.newTree(newStrLitNode($x), newStrLitNode($x))
+
+  result.add nnkElse.newTree(newStrLitNode("other"))
+
+macro bigIfElseExpr(): untyped =
+  result = nnkIfExpr.newTree()
+
+  for x in 0 ..< 1000:
+    result.add nnkElifExpr.newTree(newLit(false), newStrLitNode($x))
+
+  result.add nnkElseExpr.newTree(newStrLitNode("other"))
+
+proc test(arg: string): string =
+  echo bigIfElseExpr()
+
+  result = bigCaseStmt(arg)
+
+discard test("test")