An optimizer for ARC (#14962)

* WIP: an optimizer for ARC
* do not optimize away destructors in 'finally' if unstructured control flow is involved
* optimized the optimizer
* minor code cleanup
* first steps to .cursor inference
* cursor inference: big steps to a working solution
* baby steps
* better .cursor inference
* new feature: expandArc for easy inspection of the AST after ARC transformations
* added topt_cursor test
* adapt tests
* cleanups, make tests green
* optimize common traversal patterns
* moved test case
* fixes .cursor inference so that npeg compiles once again
* cursor inference: more bugfixes

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/arc/tcomputedgoto.nim

@@ -1,15 +1,7 @@
 discard """
   cmd: '''nim c --newruntime $file'''
-  output: '''copied
-copied
-2
-destroyed
-destroyed
-copied
-copied
-2
-destroyed
-destroyed'''
+  output: '''2
+2'''
 """
 
 type

tests/arc/topt_cursor.nim

@@ -0,0 +1,35 @@
+discard """
+  output: '''("string here", 80)'''
+  cmd: '''nim c --gc:arc --expandArc:main --hint:Performance:off $file'''
+  nimout: '''--expandArc: main
+
+var
+  :tmpD
+  :tmpD_1
+  :tmpD_2
+try:
+  var x = ("hi", 5)
+  x = if cond:
+    :tmpD = ("different", 54)
+    :tmpD else:
+    :tmpD_1 = ("string here", 80)
+    :tmpD_1
+  echo [
+    :tmpD_2 = `$`(x)
+    :tmpD_2]
+finally:
+  `=destroy`(:tmpD_2)
+-- end of expandArc ------------------------'''
+"""
+
+proc main(cond: bool) =
+  var x = ("hi", 5) # goal: computed as cursor
+
+  x = if cond:
+        ("different", 54)
+      else:
+        ("string here", 80)
+
+  echo x
+
+main(false)

tests/arc/topt_no_cursor.nim

@@ -0,0 +1,37 @@
+discard """
+  output: '''(repo: "", package: "meo", ext: "")'''
+  cmd: '''nim c --gc:arc --expandArc:newTarget --hint:Performance:off $file'''
+  nimout: '''--expandArc: newTarget
+
+var
+  splat
+  :tmp
+  :tmp_1
+  :tmp_2
+splat = splitFile(path)
+:tmp = splat.dir
+wasMoved(splat.dir)
+:tmp_1 = splat.name
+wasMoved(splat.name)
+:tmp_2 = splat.ext
+wasMoved(splat.ext)
+result = (
+  let blitTmp = :tmp
+  blitTmp,
+  let blitTmp_1 = :tmp_1
+  blitTmp_1,
+  let blitTmp_2 = :tmp_2
+  blitTmp_2)
+`=destroy`(splat)
+-- end of expandArc ------------------------'''
+"""
+
+import os
+
+type Target = tuple[repo, package, ext: string]
+
+proc newTarget*(path: string): Target =
+  let splat = path.splitFile
+  result = (repo: splat.dir, package: splat.name, ext: splat.ext)
+
+echo newTarget("meo")

tests/arc/topt_refcursors.nim

@@ -0,0 +1,39 @@
+discard """
+  output: ''''''
+  cmd: '''nim c --gc:arc --expandArc:traverse --hint:Performance:off $file'''
+  nimout: '''--expandArc: traverse
+
+var it = root
+block :tmp:
+  while (
+    not (it == nil)):
+    echo [it.s]
+    it = it.ri
+var jt = root
+block :tmp_1:
+  while (
+    not (jt == nil)):
+    let ri_1 = jt.ri
+    echo [jt.s]
+    jt = ri_1
+-- end of expandArc ------------------------'''
+"""
+
+type
+  Node = ref object
+    le, ri: Node
+    s: string
+
+proc traverse(root: Node) =
+  var it = root
+  while it != nil:
+    echo it.s
+    it = it.ri
+
+  var jt = root
+  while jt != nil:
+    let ri = jt.ri
+    echo jt.s
+    jt = ri
+
+traverse(nil)

tests/arc/topt_wasmoved_destroy_pairs.nim

@@ -0,0 +1,92 @@
+discard """
+  output: ''''''
+  cmd: '''nim c --gc:arc --expandArc:main --expandArc:tfor --hint:Performance:off $file'''
+  nimout: '''--expandArc: main
+
+var
+  a
+  b
+  x
+x = f()
+if cond:
+  add(a):
+    let blitTmp = x
+    blitTmp
+else:
+  add(b):
+    let blitTmp_1 = x
+    blitTmp_1
+`=destroy`(b)
+`=destroy`(a)
+-- end of expandArc ------------------------
+--expandArc: tfor
+
+var
+  a
+  b
+  x
+try:
+  x = f()
+  block :tmp:
+    var i
+    var i_1 = 0
+    block :tmp_1:
+      while i_1 < 4:
+        var :tmpD
+        i = i_1
+        if i == 2:
+          return
+        add(a):
+          wasMoved(:tmpD)
+          `=`(:tmpD, x)
+          :tmpD
+        inc i_1, 1
+  if cond:
+    add(a):
+      let blitTmp = x
+      wasMoved(x)
+      blitTmp
+  else:
+    add(b):
+      let blitTmp_1 = x
+      wasMoved(x)
+      blitTmp_1
+finally:
+  `=destroy`(x)
+  `=destroy_1`(b)
+  `=destroy_1`(a)
+-- end of expandArc ------------------------'''
+"""
+
+proc f(): seq[int] =
+  @[1, 2, 3]
+
+proc main(cond: bool) =
+  var a, b: seq[seq[int]]
+  var x = f()
+  if cond:
+    a.add x
+  else:
+    b.add x
+
+# all paths move 'x' so no wasMoved(x); destroy(x) pair should be left in the
+# AST.
+
+main(false)
+
+
+proc tfor(cond: bool) =
+  var a, b: seq[seq[int]]
+
+  var x = f()
+
+  for i in 0 ..< 4:
+    if i == 2: return
+    a.add x
+
+  if cond:
+    a.add x
+  else:
+    b.add x
+
+tfor(false)

tests/arc/trtree.nim

@@ -138,16 +138,16 @@ proc search*[M, D: Dim; RT, LT](t: RTree[M, D, RT, LT]; b: Box[D, RT]): seq[LT]
 # a R*TREE proc
 proc chooseSubtree[M, D: Dim; RT, LT](t: RTree[M, D, RT, LT]; b: Box[D, RT]; level: int): H[M, D, RT, LT] =
   assert level >= 0
-  var n = t.root
-  while n.level > level:
-    let nn = Node[M, D, RT, LT](n)
+  var it = t.root
+  while it.level > level:
+    let nn = Node[M, D, RT, LT](it)
     var i0 = 0 # selected index
     var minLoss = type(b[0].a).high
-    if n.level == 1: # childreen are leaves -- determine the minimum overlap costs
-      for i in 0 ..< n.numEntries:
+    if it.level == 1: # childreen are leaves -- determine the minimum overlap costs
+      for i in 0 ..< it.numEntries:
         let nx = union(nn.a[i].b, b)
         var loss = 0
-        for j in 0 ..< n.numEntries:
+        for j in 0 ..< it.numEntries:
           if i == j: continue
           loss += (overlap(nx, nn.a[j].b) - overlap(nn.a[i].b, nn.a[j].b)) # overlap (i, j) == (j, i), so maybe cache that?
         var rep = loss < minLoss
@@ -163,7 +163,7 @@ proc chooseSubtree[M, D: Dim; RT, LT](t: RTree[M, D, RT, LT]; b: Box[D, RT]; lev
           i0 = i
           minLoss = loss
     else:
-      for i in 0 ..< n.numEntries:
+      for i in 0 ..< it.numEntries:
         let loss = enlargement(nn.a[i].b, b)
         var rep = loss < minLoss
         if loss == minLoss:
@@ -174,8 +174,8 @@ proc chooseSubtree[M, D: Dim; RT, LT](t: RTree[M, D, RT, LT]; b: Box[D, RT]; lev
         if rep:
           i0 = i
           minLoss = loss
-    n = nn.a[i0].n
-  return n
+    it = nn.a[i0].n
+  return it
 
 proc pickSeeds[M, D: Dim; RT, LT](t: RTree[M, D, RT, LT]; n: Node[M, D, RT, LT] | Leaf[M, D, RT, LT]; bx: Box[D, RT]): (int, int) =
   var i0, j0: int

tests/destructor/tdestructor3.nim

@@ -1,5 +1,6 @@
 discard """
-  output: '''assign
+  output: '''
+assign
 destroy
 destroy
 5
@@ -104,12 +105,12 @@ test()
 #------------------------------------------------------------
 # Issue #12883
 
-type 
+type
   TopObject = object
     internal: UniquePtr[int]
 
 proc deleteTop(p: ptr TopObject) =
-  if p != nil:    
+  if p != nil:
     `=destroy`(p[]) # !!! this operation used to leak the integer
     deallocshared(p)
 
@@ -117,12 +118,12 @@ proc createTop(): ptr TopObject =
   result = cast[ptr TopObject](allocShared0(sizeof(TopObject)))
   result.internal = newUniquePtr(1)
 
-proc test2() = 
+proc test2() =
   let x = createTop()
   echo $x.internal
   deleteTop(x)
 
-echo "---------------"  
+echo "---------------"
 echo "app begin"
 test2()
 echo "app end"

tests/destructor/tmisc_destructors.nim

@@ -21,11 +21,13 @@ proc `=sink`(dest: var Foo, src: Foo) =
 proc `=`(dest: var Foo, src: Foo) =
   assign_counter.inc
 
+proc createFoo(): Foo = Foo(boo: 0)
+
 proc test(): auto =
-  var a, b: Foo
+  var a, b = createFoo()
   return (a, b, Foo(boo: 5))
 
-var (a, b, _) = test()
+var (ag, bg, _) = test()
 
 doAssert assign_counter == 0
 doAssert sink_counter == 0

tests/destructor/tuse_result_prevents_sinks.nim

@@ -17,15 +17,20 @@ proc `=sink`(self: var Foo; other: Foo) =
 
 proc `=destroy`(self: var Foo) = discard
 
+template preventCursorInference(x) =
+  let p = unsafeAddr(x)
+
 proc test(): Foo =
   result = Foo()
   let temp = result
+  preventCursorInference temp
   doAssert temp.i > 0
   return result
 
 proc testB(): Foo =
   result = Foo()
   let temp = result
+  preventCursorInference temp
   doAssert temp.i > 0
 
 discard test()