preparations for a new AST transformation pass with the aim of giving us reliable destructors

2026-01-02 11:12:37 +00:00 · 2017-10-09 20:44:55 +02:00
parent 6e337fe58a
commit cf01cb1935
2 changed files with 232 additions and 22 deletions
--- a/compiler/destroyer.nim
+++ b/compiler/destroyer.nim
@@ -0,0 +1,192 @@
+#
+#
+#           The Nim Compiler
+#        (c) Copyright 2017 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## Injects destructor calls into Nim code as well as
+## an optimizer that optimizes copies to moves. This is implemented as an
+## AST to AST transformation so that every backend benefits from it.
+
+## Rules for destructor injections:
+##
+## foo(bar(X(), Y()))
+## X and Y get destroyed after bar completes:
+##
+## foo( (tmpX = X(); tmpY = Y(); tmpBar = bar(tmpX, tmpY);
+##       destroy(tmpX); destroy(tmpY);
+##       tmpBar))
+## destroy(tmpBar)
+##
+## var x = f()
+## body
+##
+## is the same as:
+##
+##  var x;
+##  try:
+##    move(x, f())
+##  finally:
+##    destroy(x)
+##
+## But this really just an optimization that tries to avoid to
+## introduce too many temporaries, the 'destroy' is caused by
+## the 'f()' call. No! That is not true for 'result = f()'!
+##
+## x = y where y is read only once
+## is the same as:  move(x, y)
+##
+## We also need to keep in mind here that the number of reads is
+## control flow dependent:
+## let x = foo()
+## while true:
+##   y = x  # only one read, but the 2nd iteration will fail!
+## This also affects recursions! Only usages that do not cross
+## a loop boundary (scope) and are not used in function calls
+## are safe.
+##
+##
+## x = f() is the same as:  move(x, f())
+##
+## x = y
+## is the same as:  copy(x, y)
+##
+## Reassignment works under this scheme:
+## var x = f()
+## x = y
+##
+## is the same as:
+##
+##  var x;
+##  try:
+##    move(x, f())
+##    copy(x, y)
+##  finally:
+##    destroy(x)
+##
+##  result = f()  must not destroy 'result'!
+##
+## The produced temporaries clutter up the code and might lead to
+## inefficiencies. A better strategy is to collect all the temporaries
+## in a single object that we put into a single try-finally that
+## surrounds the proc body. This means the code stays quite efficient
+## when compiled to C.
+##
+## foo(bar(X(), Y()))
+## X and Y get destroyed after bar completes:
+##
+## var tmp: object
+## foo( (move tmp.x, X(); move tmp.y, Y(); tmp.bar = bar(tmpX, tmpY);
+##       tmp.bar))
+## destroy(tmp.bar)
+## destroy(tmp.x); destroy(tmp.y)
+
+
+import
+  intsets, ast, astalgo, msgs, renderer, magicsys, types, idents, trees,
+  strutils, options, dfa, lowerings
+
+template hasDestructor(t: PType): bool = tfHasAsgn in t.flags
+
+when false:
+  type
+    VarInfo = object
+      hasInitValue: bool
+      addrTaken: bool
+      assigned: int      # we don't care about the 'var' vs 'let'
+                        # distinction; it's an optimization pass
+      read: int
+      scope: int         # the scope the variable is declared in
+
+    Con = object
+      t: Table[int, VarInfo]
+      owner: PSym
+      scope: int
+
+  const
+    InterestingSyms = {skVar, skResult}
+
+  proc collectData(c: var Con; n: PNode)
+
+  proc collectDef(c: var Con; n: PNode; hasInitValue: bool) =
+    if n.kind == nkSym:
+      c.t[n.sym.id] = VarInfo(hasInitValue: hasInitValue,
+                              addrTaken: false, assigned: 0, read: 0,
+                              scope: scope)
+
+  proc collectVarSection(c: var Con; n: PNode) =
+    for a in n:
+      if a.kind == nkCommentStmt: continue
+      if a.kind == nkVarTuple:
+        collectData(c, a.lastSon)
+        for i in 0 .. a.len-3: collectDef(c, a[i], a.lastSon != nil)
+      else:
+        collectData(c, a.lastSon)
+        if a.lastSon.kind != nkEmpty:
+          collectDef(c, a.sons[0], a.lastSon != nil)
+
+  proc collectData(c: var Con; n: PNode) =
+    case n.kind
+    of nkAsgn, nkFastAsgn:
+      if n[0].kind == nkSym and (let s = n[0].sym; s.owner == c.owner and
+                                s.kind in InterestingSyms):
+        inc c.t[s.id].assigned
+      collectData(c, n[1])
+    of nkSym:
+      if (let s = n[0].sym; s.owner == c.owner and
+          s.kind in InterestingSyms):
+        inc c.t[s.id].read
+    of nkAddr, nkHiddenAddr:
+      var n = n[0]
+      while n.kind == nkBracketExpr: n = n[0]
+      if (let s = n[0].sym; s.owner == c.owner and
+          s.kind in InterestingSyms):
+        c.t[s.id].addrTaken = true
+
+    of nkCallKinds:
+      if n.sons[0].kind == nkSym:
+        let s = n.sons[0].sym
+        if s.magic != mNone:
+          genMagic(c, n, s.magic)
+        else:
+          genCall(c, n)
+      else:
+        genCall(c, n)
+    of nkCharLit..nkNilLit, nkIdent: discard
+    of nkDotExpr, nkCheckedFieldExpr, nkBracketExpr,
+        nkDerefExpr, nkHiddenDeref:
+      collectData(c, n[0])
+    of nkIfStmt, nkIfExpr: genIf(c, n)
+    of nkWhenStmt:
+      # This is "when nimvm" node. Chose the first branch.
+      collectData(c, n.sons[0].sons[1])
+    of nkCaseStmt: genCase(c, n)
+    of nkWhileStmt: genWhile(c, n)
+    of nkBlockExpr, nkBlockStmt: genBlock(c, n)
+    of nkReturnStmt: genReturn(c, n)
+    of nkRaiseStmt: genRaise(c, n)
+    of nkBreakStmt: genBreak(c, n)
+    of nkTryStmt: genTry(c, n)
+    of nkStmtList, nkStmtListExpr, nkChckRangeF, nkChckRange64, nkChckRange,
+        nkBracket, nkCurly, nkPar, nkClosure, nkObjConstr:
+      for x in n: collectData(c, x)
+    of nkPragmaBlock: collectData(c, n.lastSon)
+    of nkDiscardStmt: collectData(c, n.sons[0])
+    of nkHiddenStdConv, nkHiddenSubConv, nkConv, nkExprColonExpr, nkExprEqExpr,
+        nkCast:
+      collectData(c, n.sons[1])
+    of nkObjDownConv, nkStringToCString, nkCStringToString:
+      collectData(c, n.sons[0])
+    of nkVarSection, nkLetSection: collectVarSection(c, n)
+    else: discard
+
+proc injectDestructorCalls*(owner: PSym; n: PNode;
+                            disableExceptions = false): PNode =
+  when false:
+    var c = Con(t: initTable[int, VarInfo](), owner: owner)
+    collectData(c, n)
+  var allTemps = createObj(owner, n.info)
+
--- a/compiler/dfa.nim
+++ b/compiler/dfa.nim
@@ -26,13 +26,19 @@
 import ast, astalgo, types, intsets, tables, msgs

 type
-  InstrKind = enum
-    goto, fork, def, use
-  Instr = object
-    n: PNode
-    case kind: InstrKind
-    of def, use: sym: PSym
-    of goto, fork: dest: int
+  InstrKind* = enum
+    goto, fork, def, use,
+    useWithinCall # this strange special case is used to get more
+                  # move optimizations out of regular code
+                  # XXX This is still overly pessimistic in
+                  # call(let x = foo; bar(x))
+  Instr* = object
+    n*: PNode
+    case kind*: InstrKind
+    of def, use, useWithinCall: sym*: PSym
+    of goto, fork: dest*: int
+
+  ControlFlowGraph* = seq[Instr]

  TPosition = distinct int
  TBlock = object
@@ -43,40 +49,42 @@ type
    undef, value, valueOrUndef

  Con = object
-    code: seq[Instr]
+    code: ControlFlowGraph
+    inCall: int
    blocks: seq[TBlock]

 proc debugInfo(info: TLineInfo): string =
  result = info.toFilename & ":" & $info.line

-proc codeListing(c: Con, result: var string, start=0; last = -1) =
+proc codeListing(c: ControlFlowGraph, result: var string, start=0; last = -1) =
  # for debugging purposes
  # first iteration: compute all necessary labels:
  var jumpTargets = initIntSet()
-  let last = if last < 0: c.code.len-1 else: min(last, c.code.len-1)
+  let last = if last < 0: c.len-1 else: min(last, c.len-1)
  for i in start..last:
-    if c.code[i].kind in {goto, fork}:
-      jumpTargets.incl(i+c.code[i].dest)
+    if c[i].kind in {goto, fork}:
+      jumpTargets.incl(i+c[i].dest)
  var i = start
  while i <= last:
    if i in jumpTargets: result.add("L" & $i & ":\n")
    result.add "\t"
-    result.add $c.code[i].kind
+    result.add $c[i].kind
    result.add "\t"
-    case c.code[i].kind
-    of def, use:
-      result.add c.code[i].sym.name.s
+    case c[i].kind
+    of def, use, useWithinCall:
+      result.add c[i].sym.name.s
    of goto, fork:
      result.add "L"
-      result.add c.code[i].dest+i
+      result.add c[i].dest+i
    result.add("\t#")
-    result.add(debugInfo(c.code[i].n.info))
+    result.add(debugInfo(c[i].n.info))
    result.add("\n")
    inc i
  if i in jumpTargets: result.add("L" & $i & ": End\n")


-proc echoCfg*(c: Con; start=0; last = -1) {.deprecated.} =
+proc echoCfg*(c: ControlFlowGraph; start=0; last = -1) {.deprecated.} =
+  ## echos the ControlFlowGraph for debugging purposes.
  var buf = ""
  codeListing(c, buf, start, last)
  echo buf
@@ -243,7 +251,10 @@ proc genUse(c: var Con; n: PNode) =
                   nkAddr, nkHiddenAddr}:
    n = n[0]
  if n.kind == nkSym and n.sym.kind in InterestingSyms:
-    c.code.add Instr(n: n, kind: use, sym: n.sym)
+    if c.inCall > 0:
+      c.code.add Instr(n: n, kind: useWithinCall, sym: n.sym)
+    else:
+      c.code.add Instr(n: n, kind: use, sym: n.sym)

 proc genDef(c: var Con; n: PNode) =
  if n.kind == nkSym and n.sym.kind in InterestingSyms:
@@ -253,10 +264,12 @@ proc genCall(c: var Con; n: PNode) =
  gen(c, n[0])
  var t = n[0].typ
  if t != nil: t = t.skipTypes(abstractInst)
+  inc c.inCall
  for i in 1..<n.len:
    gen(c, n[i])
    if t != nil and i < t.len and t.sons[i].kind == tyVar:
      genDef(c, n[i])
+  dec c.inCall

 proc genMagic(c: var Con; n: PNode; m: TMagic) =
  case m
@@ -356,7 +369,7 @@ proc dfa(code: seq[Instr]) =
      var sid = -1
      case code[pc].kind
      of goto, fork: discard
-      of use:
+      of use, useWithinCall:
        let sym = code[pc].sym
        if s[pc].contains(sym.id):
          localError(code[pc].n.info, "variable read before initialized: " & sym.name.s)
@@ -417,5 +430,10 @@ proc dfa(code: seq[Instr]) =
 proc dataflowAnalysis*(s: PSym; body: PNode) =
  var c = Con(code: @[], blocks: @[])
  gen(c, body)
-  echoCfg(c)
+  #echoCfg(c.code)
  dfa(c.code)
+
+proc constructCfg*(s: PSym; body: PNode): ControlFlowGraph =
+  ## constructs a control flow graph for ``body``.
+  var c = Con(code: @[], blocks: @[])
+  shallowCopy(result, c.code)