New "ping-pong" DFA (#14322)
* New ping-pong analysis * Add testcase for #13456 * Remove debugging leftover * Unquote "unstructured controlflow" * Fix typo * Fix exponential complexity in edge cases * Add sanity testcase * Fix
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Clyybber
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3 changed files with 163 additions and 73 deletions
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@ -35,11 +35,11 @@ from patterns import sameTrees
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type
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InstrKind* = enum
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goto, fork, join, def, use
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goto, fork, def, use
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Instr* = object
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n*: PNode # contains the def/use location.
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case kind*: InstrKind
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of goto, fork, join: dest*: int
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of goto, fork: dest*: int
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else: discard
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ControlFlowGraph* = seq[Instr]
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@ -47,8 +47,6 @@ type
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TPosition = distinct int
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TBlock = object
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joins: seq[Instr]
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forks: seq[TPosition]
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case isTryBlock: bool
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of false:
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label: PSym
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@ -72,7 +70,7 @@ proc codeListing(c: ControlFlowGraph, result: var string, start=0; last = -1) =
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var jumpTargets = initIntSet()
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let last = if last < 0: c.len-1 else: min(last, c.len-1)
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for i in start..last:
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if c[i].kind in {goto, fork, join}:
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if c[i].kind in {goto, fork}:
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jumpTargets.incl(i+c[i].dest)
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var i = start
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while i <= last:
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@ -83,7 +81,7 @@ proc codeListing(c: ControlFlowGraph, result: var string, start=0; last = -1) =
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case c[i].kind
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of def, use:
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result.add renderTree(c[i].n)
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of goto, fork, join:
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of goto, fork:
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result.add "L"
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result.addInt c[i].dest+i
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result.add("\t#")
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@ -102,7 +100,6 @@ proc echoCfg*(c: ControlFlowGraph; start=0; last = -1) {.deprecated.} =
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proc forkI(c: var Con; n: PNode): TPosition =
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result = TPosition(c.code.len)
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c.code.add Instr(n: n, kind: fork, dest: 0)
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c.blocks[^1].forks.add result
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proc gotoI(c: var Con; n: PNode): TPosition =
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result = TPosition(c.code.len)
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@ -110,6 +107,24 @@ proc gotoI(c: var Con; n: PNode): TPosition =
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#[
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Join is no more
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===============
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Instead of generating join instructions we adapt our traversal of the CFG.
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When encountering a fork we split into two paths, we follow the path
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starting at "pc + 1" until it encounters the joinpoint: "pc + forkInstr.dest".
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If we encounter gotos that would jump further than the current joinpoint,
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as can happen with gotos generated by unstructured controlflow such as break, raise or return,
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we simply suspend following the current path, and follow the other path until the new joinpoint
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which is simply the instruction pointer returned to us by the now suspended path.
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If the path we are following now, also encounters a goto that exceeds the joinpoint
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we repeat the process; suspending the current path and evaluating the other one with a new joinpoint.
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If we eventually reach a common joinpoint we join the two paths.
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This new "ping-pong" approach has the obvious advantage of not requiring join instructions, as such
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cutting down on the CFG size but is also mandatory for correctly handling complicated cases
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of unstructured controlflow.
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Design of join
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==============
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@ -258,10 +273,6 @@ duplicate the 'join' instructions on breaks and return exits!
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]#
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proc joinI(c: var Con; fromFork: TPosition; n: PNode) =
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let dist = fromFork.int - c.code.len
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c.code.add Instr(n: n, kind: join, dest: dist)
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proc genLabel(c: Con): TPosition =
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result = TPosition(c.code.len)
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@ -289,10 +300,6 @@ proc popBlock(c: var Con; oldLen: int) =
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exits.add c.gotoI(newNode(nkEmpty))
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for e in exits:
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c.patch e
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for j in c.blocks[oldLen].joins:
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var patchedJ = j
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patchedJ.dest -= c.code.len
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c.code.add patchedJ
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c.blocks.setLen(oldLen)
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template withBlock(labl: PSym; body: untyped) {.dirty.} =
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@ -353,7 +360,6 @@ when true:
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c.gen(n[1])
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else:
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withBlock(nil):
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let oldForksLen = c.blocks[^1].forks.len
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var endings: array[3, TPosition]
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for i in 0..2:
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c.gen(n[0])
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@ -362,8 +368,6 @@ when true:
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for i in countdown(endings.high, 0):
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let endPos = endings[i]
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c.patch(endPos)
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c.joinI(c.blocks[^1].forks.pop(), n)
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doAssert(c.blocks[^1].forks.len == oldForksLen)
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else:
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@ -374,7 +378,6 @@ else:
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# body
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# jmp lab1
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# lab2:
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let oldForksLen = c.blocks[^1].forks.len
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let lab1 = c.genLabel
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withBlock(nil):
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if isTrue(n[0]):
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@ -386,15 +389,11 @@ else:
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c.gen(n[1])
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c.jmpBack(n, lab1)
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c.patch(lab2)
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setLen(c.blocks[^1].forks, oldForksLen)
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template forkT(n, body) =
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let oldLen = c.blocks[^1].forks.len
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let lab1 = c.forkI(n)
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body
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c.patch(lab1)
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c.joinI(lab1, n)
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setLen(c.blocks[^1].forks, oldLen)
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proc genIf(c: var Con, n: PNode) =
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#[
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@ -433,7 +432,6 @@ proc genIf(c: var Con, n: PNode) =
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join F1
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]#
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let oldLen = c.blocks[^1].forks.len
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var endings: seq[TPosition] = @[]
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for i in 0..<n.len:
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var it = n[i]
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@ -446,8 +444,6 @@ proc genIf(c: var Con, n: PNode) =
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for i in countdown(endings.high, 0):
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let endPos = endings[i]
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c.patch(endPos)
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c.joinI(c.blocks[^1].forks.pop(), n)
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doAssert(c.blocks[^1].forks.len == oldLen)
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proc genAndOr(c: var Con; n: PNode) =
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# asgn dest, a
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@ -474,7 +470,6 @@ proc genCase(c: var Con; n: PNode) =
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abstractVarRange-{tyTypeDesc}).kind notin {tyFloat..tyFloat128, tyString}
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var endings: seq[TPosition] = @[]
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let oldLen = c.blocks[^1].forks.len
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c.gen(n[0])
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for i in 1..<n.len:
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let it = n[i]
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@ -491,8 +486,6 @@ proc genCase(c: var Con; n: PNode) =
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for i in countdown(endings.high, 0):
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let endPos = endings[i]
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c.patch(endPos)
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c.joinI(c.blocks[^1].forks.pop(), n)
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doAssert(c.blocks[^1].forks.len == oldLen)
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proc genBlock(c: var Con; n: PNode) =
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withBlock(n[0].sym):
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@ -511,10 +504,6 @@ proc genBreakOrRaiseAux(c: var Con, i: int, n: PNode) =
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c.blocks[i].breakFixups.add (lab1, trailingFinales)
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for b in countdown(c.blocks.high, i):
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for f in countdown(c.blocks[b].forks.high, 0):
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c.blocks[i].joins.add Instr(n: n, kind: join, dest: c.blocks[b].forks[f].int)
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proc genBreak(c: var Con; n: PNode) =
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if n[0].kind == nkSym:
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#echo cast[int](n[0].sym)
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@ -530,7 +519,6 @@ proc genBreak(c: var Con; n: PNode) =
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return
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proc genTry(c: var Con; n: PNode) =
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let oldForksLen = c.blocks[^1].forks.len
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var endings: seq[TPosition] = @[]
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let oldLen = c.blocks.len
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@ -543,10 +531,6 @@ proc genTry(c: var Con; n: PNode) =
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for f in c.blocks[oldLen].raiseFixups:
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c.patch(f)
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for j in c.blocks[oldLen].joins:
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var patchedJ = j
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patchedJ.dest -= c.code.len
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c.code.add patchedJ
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c.blocks.setLen oldLen
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@ -561,7 +545,6 @@ proc genTry(c: var Con; n: PNode) =
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for i in countdown(endings.high, 0):
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let endPos = endings[i]
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c.patch(endPos)
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c.joinI(c.blocks[^1].forks.pop(), n)
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# join the 'elsePos' forkI instruction:
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#c.joinI(c.blocks[^1].forks.pop(), n)
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@ -569,7 +552,6 @@ proc genTry(c: var Con; n: PNode) =
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let fin = lastSon(n)
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if fin.kind == nkFinally:
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c.gen(fin[0])
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doAssert(c.blocks[^1].forks.len == oldForksLen)
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template genNoReturn(c: var Con; n: PNode) =
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# leave the graph
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@ -751,7 +733,6 @@ proc genCall(c: var Con; n: PNode) =
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genBreakOrRaiseAux(c, i, n)
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break
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c.patch(endGoto)
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c.joinI(c.blocks[^1].forks.pop(), n)
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dec c.inCall
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proc genMagic(c: var Con; n: PNode; m: TMagic) =
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