From 57d5c1465a576e2d8bfc6496946aa24183c4456d Mon Sep 17 00:00:00 2001
From: Clyybber <darkmine956@gmail.com>
Date: Thu, 21 Jan 2021 15:25:00 +0100
Subject: [PATCH] Part-to-whole optimization (#16775)

---
 compiler/injectdestructors.nim | 31 +++++++++++-----------
 tests/arc/tmovebug.nim         | 47 +++++++++++++++++++++++++++++++++-
 2 files changed, 62 insertions(+), 16 deletions(-)

diff --git a/compiler/injectdestructors.nim b/compiler/injectdestructors.nim
index 48c501209..fc404f90f 100644
--- a/compiler/injectdestructors.nim
+++ b/compiler/injectdestructors.nim
@@ -21,16 +21,6 @@ import
 
 from trees import exprStructuralEquivalent, getRoot
 
-type
-  Scope = object  # well we do scope-based memory management. \
-    # a scope is comparable to an nkStmtListExpr like
-    # (try: statements; dest = y(); finally: destructors(); dest)
-    vars: seq[PSym]
-    wasMoved: seq[PNode]
-    final: seq[PNode] # finally section
-    needsTry: bool
-    parent: ptr Scope
-
 type
   Con = object
     owner: PSym
@@ -42,6 +32,15 @@ type
     uninitComputed: bool
     idgen: IdGenerator
 
+  Scope = object # we do scope-based memory management.
+    # a scope is comparable to an nkStmtListExpr like
+    # (try: statements; dest = y(); finally: destructors(); dest)
+    vars: seq[PSym]
+    wasMoved: seq[PNode]
+    final: seq[PNode] # finally section
+    needsTry: bool
+    parent: ptr Scope
+
   ProcessMode = enum
     normal
     consumed
@@ -1006,11 +1005,13 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, isDecl = false): PNod
       if isUnpackedTuple(ri[0]):
         # unpacking of tuple: take over the elements
         result = c.genSink(dest, p(ri, c, s, consumed), isDecl)
-      elif isAnalysableFieldAccess(ri, c.owner) and isLastRead(ri, c) and
-          not aliases(dest, ri):
-        # Rule 3: `=sink`(x, z); wasMoved(z)
-        var snk = c.genSink(dest, ri, isDecl)
-        result = newTree(nkStmtList, snk, c.genWasMoved(ri))
+      elif isAnalysableFieldAccess(ri, c.owner) and isLastRead(ri, c):
+        if not aliases(dest, ri):
+          # Rule 3: `=sink`(x, z); wasMoved(z)
+          var snk = c.genSink(dest, ri, isDecl)
+          result = newTree(nkStmtList, snk, c.genWasMoved(ri))
+        else:
+          result = c.genSink(dest, destructiveMoveVar(ri, c, s), isDecl)
       else:
         result = c.genCopy(dest, ri)
         result.add p(ri, c, s, consumed)
diff --git a/tests/arc/tmovebug.nim b/tests/arc/tmovebug.nim
index d43b3489e..71962e21d 100644
--- a/tests/arc/tmovebug.nim
+++ b/tests/arc/tmovebug.nim
@@ -92,6 +92,12 @@ copy
 destroy
 destroy
 destroy
+part-to-whole assigment:
+sink
+(children: @[])
+destroy
+copy
+destroy
 '''
 """
 
@@ -578,7 +584,7 @@ let w3 = newWrapper2(-1)
 echo $w3[]
 
 
-#---------------------------------------------------------------------
+#--------------------------------------------------------------------
 #self-assignments
 
 # Self-assignments that are not statically determinable will get
@@ -668,3 +674,42 @@ proc caseNotAConstant =
   echo s # @[(f: 2), (f: 2), (f: 3)]
 
 caseNotAConstant()
+
+
+#--------------------------------------------------------------------
+echo "part-to-whole assigment:"
+
+type
+  Tree = object
+    children: seq[Tree]
+
+  TreeDefaultHooks = object
+    children: seq[TreeDefaultHooks]
+
+proc `=destroy`(x: var Tree) = echo "destroy"
+proc `=sink`(x: var Tree, y: Tree) = echo "sink"
+proc `=copy`(x: var Tree, y: Tree) = echo "copy"
+
+proc partToWholeSeq =
+  var t = Tree(children: @[Tree()])
+  t = t.children[0] # This should be sunk, but with the special transform (tmp = t.children[0]; wasMoved(0); `=sink`(t, tmp))
+
+  var tc = TreeDefaultHooks(children: @[TreeDefaultHooks()])
+  tc = tc.children[0] # Ditto; if this were sunk with the normal transform (`=sink`(t, t.children[0]); wasMoved(t.children[0]))
+  echo tc             #        then it would crash because t.children[0] does not exist after the call to `=sink`
+
+partToWholeSeq()
+
+type List = object
+  next: ref List
+
+proc `=destroy`(x: var List) = echo "destroy"
+proc `=sink`(x: var List, y: List) = echo "sink"
+proc `=copy`(x: var List, y: List) = echo "copy"
+
+proc partToWholeUnownedRef =
+  var t = List(next: new List)
+  t = t.next[] # Copy because t.next is not an owned ref, and thus t.next[] cannot be moved
+
+partToWholeUnownedRef()
+