isLastRead regression fix (#10463)

* fixes #10462

* add a test

compiler/destroyer.nim

@@ -139,7 +139,7 @@ proc isLastRead(s: PSym; c: var Con; pc, comesFrom: int): int =
     of def:
       if c.g[pc].sym == s:
         # the path lead to a redefinition of 's' --> abandon it.
-        return pc
+        return high(int) 
       inc pc
     of use:
       if c.g[pc].sym == s:
@@ -150,14 +150,16 @@ proc isLastRead(s: PSym; c: var Con; pc, comesFrom: int): int =
       pc = pc + c.g[pc].dest
     of fork:
       # every branch must lead to the last read of the location:
-      let variantA = isLastRead(s, c, pc+1, pc)
+      var variantA = isLastRead(s, c, pc+1, pc)
       if variantA < 0: return -1
       let variantB = isLastRead(s, c, pc + c.g[pc].dest, pc)
       if variantB < 0: return -1
-      pc = variantA+1
+      elif variantA == high(int): 
+        variantA = variantB
+      pc = variantA
     of InstrKind.join:
       let dest = pc + c.g[pc].dest
-      if dest == comesFrom: return pc
+      if dest == comesFrom: return pc + 1
       inc pc
   return pc
 

compiler/dfa.nim

@@ -74,7 +74,7 @@ proc codeListing(c: ControlFlowGraph, result: var string, start=0; last = -1) =
   while i <= last:
     if i in jumpTargets: result.add("L" & $i & ":\n")
     result.add "\t"
-    result.add $c[i].kind
+    result.add ($i & " " & $c[i].kind)
     result.add "\t"
     case c[i].kind
     of def, use:
@@ -540,13 +540,17 @@ proc genTry(c: var Con; n: PNode) =
     c.gen(fin.sons[0])
   doAssert(c.forks.len == oldLen)
 
+template genNoReturn(c: var Con; n: PNode) =
+  # leave the graph
+  c.code.add Instr(n: n, kind: goto, dest: high(int) - c.code.len)
+
 proc genRaise(c: var Con; n: PNode) =
   genJoins(c, n)
   gen(c, n.sons[0])
   if c.inTryStmt > 0:
     c.tryStmtFixups.add c.gotoI(n)
   else:
-    c.code.add Instr(n: n, kind: goto, dest: high(int) - c.code.len)
+    genNoReturn(c, n)
 
 proc genImplicitReturn(c: var Con) =
   if c.owner.kind in {skProc, skFunc, skMethod, skIterator, skConverter} and resultPos < c.owner.ast.len:
@@ -558,7 +562,7 @@ proc genReturn(c: var Con; n: PNode) =
     gen(c, n.sons[0])
   else:
     genImplicitReturn(c)
-  c.code.add Instr(n: n, kind: goto, dest: high(int) - c.code.len)
+  genNoReturn(c, n)
 
 const
   InterestingSyms = {skVar, skResult, skLet, skParam}
@@ -612,9 +616,6 @@ proc genMagic(c: var Con; n: PNode; m: TMagic) =
   of mNew, mNewFinalize:
     genDef(c, n[1])
     for i in 2..<n.len: gen(c, n[i])
-  of mExit:
-    genCall(c, n)
-    c.code.add Instr(n: n, kind: goto, dest: high(int) - c.code.len)
   else:
     genCall(c, n)
 
@@ -639,6 +640,8 @@ proc gen(c: var Con; n: PNode) =
         genMagic(c, n, s.magic)
       else:
         genCall(c, n)
+      if sfNoReturn in n.sons[0].sym.flags:
+        genNoReturn(c, n)
     else:
       genCall(c, n)
   of nkCharLit..nkNilLit: discard

tests/destructor/tmove_objconstr.nim

@@ -166,3 +166,12 @@ seq4 =
 var ii = 1
 let arr2 = [newMySeq(2, 5.0), if i > 1: newMySeq(3, 1.0) else: newMySeq(0, 0.0)]
 var seqOfSeq2 = @[newMySeq(2, 5.0), newMySeq(3, 1.0)]
+
+
+## issue #10462
+proc myfuncLoop(x: int): MySeqNonCopyable =
+  for i in 0..<x:
+    var cc = newMySeq(i, 5.0)
+    result = cc
+
+discard myfuncLoop(3)