implement LR optimisation in Python3 + align naming + DFAState ctor bugs

runtime/Python3/src/antlr4/ParserInterpreter.py

@@ -71,7 +71,7 @@ class ParserInterpreter(Parser):
         for state in atn.states:
             if not isinstance(state, StarLoopEntryState):
                 continue
-            if state.precedenceRuleDecision:
+            if state.isPrecedenceDecision:
                 self.pushRecursionContextStates.add(state.stateNumber)
         # get atn simulator that knows how to do predictions
         self._interp = ParserATNSimulator(self, atn, self.decisionToDFA, self.sharedContextCache)

runtime/Python3/src/antlr4/atn/ATNDeserializer.py

@@ -368,7 +368,7 @@ class ATNDeserializer (object):
 
     #
     # Analyze the {@link StarLoopEntryState} states in the specified ATN to set
-    # the {@link StarLoopEntryState#precedenceRuleDecision} field to the
+    # the {@link StarLoopEntryState#isPrecedenceDecision} field to the
     # correct value.
     #
     # @param atn The ATN.
@@ -387,7 +387,7 @@ class ATNDeserializer (object):
                 if isinstance(maybeLoopEndState, LoopEndState):
                     if maybeLoopEndState.epsilonOnlyTransitions and \
                             isinstance(maybeLoopEndState.transitions[0].target, RuleStopState):
-                        state.precedenceRuleDecision = True
+                        state.isPrecedenceDecision = True
 
     def verifyATN(self, atn:ATN):
         if not self.deserializationOptions.verifyATN:

runtime/Python3/src/antlr4/atn/ATNSimulator.py

@@ -37,7 +37,7 @@ from antlr4.dfa.DFAState import DFAState
 class ATNSimulator(object):
 
     # Must distinguish between missing edge and edge we know leads nowhere#/
-    ERROR = DFAState(ATNConfigSet())
+    ERROR = DFAState(configs=ATNConfigSet())
     ERROR.stateNumber = 0x7FFFFFFF
 
     # The context cache maps all PredictionContext objects that are ==

runtime/Python3/src/antlr4/atn/ATNState.py

@@ -261,7 +261,7 @@ class StarLoopEntryState(DecisionState):
         self.stateType = self.STAR_LOOP_ENTRY
         self.loopBackState = None
         # Indicates whether this state can benefit from a precedence DFA during SLL decision making.
-        self.precedenceRuleDecision = None
+        self.isPrecedenceDecision = None
 
 # Mark the end of a * or + loop.
 class LoopEndState(ATNState):

runtime/Python3/src/antlr4/atn/ParserATNSimulator.py

@@ -346,15 +346,6 @@ class ParserATNSimulator(ATNSimulator):
                                        " exec LA(1)==" + self.getLookaheadName(input) +
                                        ", outerContext=" + outerContext.toString(self.parser.literalNames, None))
 
-                # If this is not a precedence DFA, we check the ATN start state
-                # to determine if this ATN start state is the decision for the
-                # closure block that determines whether a precedence rule
-                # should continue or complete.
-                #
-                if not dfa.precedenceDfa and isinstance(dfa.atnStartState, StarLoopEntryState):
-                    if dfa.atnStartState.precedenceRuleDecision:
-                        dfa.setPrecedenceDfa(True)
-
                 fullCtx = False
                 s0_closure = self.computeStartState(dfa.atnStartState, ParserRuleContext.EMPTY, fullCtx)
 
@@ -365,6 +356,7 @@ class ParserATNSimulator(ATNSimulator):
                     # appropriate start state for the precedence level rather
                     # than simply setting DFA.s0.
                     #
+                    dfa.s0.configs = s0_closure # not used for prediction but useful to know start configs anyway
                     s0_closure = self.applyPrecedenceFilter(s0_closure)
                     s0 = self.addDFAState(dfa, DFAState(configs=s0_closure))
                     dfa.setPrecedenceStartState(self.parser.getPrecedence(), s0)
@@ -1173,7 +1165,13 @@ class ParserATNSimulator(ATNSimulator):
             # make sure to not return here, because EOF transitions can act as
             # both epsilon transitions and non-epsilon transitions.
 
+        first = True
         for t in p.transitions:
+            if first:
+                first = False
+                if self.canDropLoopEntryEdgeInLeftRecursiveRule(config):
+                    continue
+
             continueCollecting = collectPredicates and not isinstance(t, ActionTransition)
             c = self.getEpsilonTarget(config, t, continueCollecting, depth == 0, fullCtx, treatEofAsEpsilon)
             if c is not None:
@@ -1210,6 +1208,161 @@ class ParserATNSimulator(ATNSimulator):
 
                 self.closureCheckingStopState(c, configs, closureBusy, continueCollecting, fullCtx, newDepth, treatEofAsEpsilon)
 
+
+
+    # Implements first-edge (loop entry) elimination as an optimization
+    #  during closure operations.  See antlr/antlr4#1398.
+    #
+    # The optimization is to avoid adding the loop entry config when
+    # the exit path can only lead back to the same
+    # StarLoopEntryState after popping context at the rule end state
+    # (traversing only epsilon edges, so we're still in closure, in
+    # this same rule).
+    #
+    # We need to detect any state that can reach loop entry on
+    # epsilon w/o exiting rule. We don't have to look at FOLLOW
+    # links, just ensure that all stack tops for config refer to key
+    # states in LR rule.
+    #
+    # To verify we are in the right situation we must first check
+    # closure is at a StarLoopEntryState generated during LR removal.
+    # Then we check that each stack top of context is a return state
+    # from one of these cases:
+    #
+    #   1. 'not' expr, '(' type ')' expr. The return state points at loop entry state
+    #   2. expr op expr. The return state is the block end of internal block of (...)*
+    #   3. 'between' expr 'and' expr. The return state of 2nd expr reference.
+    #      That state points at block end of internal block of (...)*.
+    #   4. expr '?' expr ':' expr. The return state points at block end,
+    #      which points at loop entry state.
+    #
+    # If any is true for each stack top, then closure does not add a
+    # config to the current config set for edge[0], the loop entry branch.
+    #
+    #  Conditions fail if any context for the current config is:
+    #
+    #   a. empty (we'd fall out of expr to do a global FOLLOW which could
+    #      even be to some weird spot in expr) or,
+    #   b. lies outside of expr or,
+    #   c. lies within expr but at a state not the BlockEndState
+    #   generated during LR removal
+    #
+    # Do we need to evaluate predicates ever in closure for this case?
+    #
+    # No. Predicates, including precedence predicates, are only
+    # evaluated when computing a DFA start state. I.e., only before
+    # the lookahead (but not parser) consumes a token.
+    #
+    # There are no epsilon edges allowed in LR rule alt blocks or in
+    # the "primary" part (ID here). If closure is in
+    # StarLoopEntryState any lookahead operation will have consumed a
+    # token as there are no epsilon-paths that lead to
+    # StarLoopEntryState. We do not have to evaluate predicates
+    # therefore if we are in the generated StarLoopEntryState of a LR
+    # rule. Note that when making a prediction starting at that
+    # decision point, decision d=2, compute-start-state performs
+    # closure starting at edges[0], edges[1] emanating from
+    # StarLoopEntryState. That means it is not performing closure on
+    # StarLoopEntryState during compute-start-state.
+    #
+    # How do we know this always gives same prediction answer?
+    #
+    # Without predicates, loop entry and exit paths are ambiguous
+    # upon remaining input +b (in, say, a+b). Either paths lead to
+    # valid parses. Closure can lead to consuming + immediately or by
+    # falling out of this call to expr back into expr and loop back
+    # again to StarLoopEntryState to match +b. In this special case,
+    # we choose the more efficient path, which is to take the bypass
+    # path.
+    #
+    # The lookahead language has not changed because closure chooses
+    # one path over the other. Both paths lead to consuming the same
+    # remaining input during a lookahead operation. If the next token
+    # is an operator, lookahead will enter the choice block with
+    # operators. If it is not, lookahead will exit expr. Same as if
+    # closure had chosen to enter the choice block immediately.
+    #
+    # Closure is examining one config (some loopentrystate, some alt,
+    # context) which means it is considering exactly one alt. Closure
+    # always copies the same alt to any derived configs.
+    #
+    # How do we know this optimization doesn't mess up precedence in
+    # our parse trees?
+    #
+    # Looking through expr from left edge of stat only has to confirm
+    # that an input, say, a+b+c; begins with any valid interpretation
+    # of an expression. The precedence actually doesn't matter when
+    # making a decision in stat seeing through expr. It is only when
+    # parsing rule expr that we must use the precedence to get the
+    # right interpretation and, hence, parse tree.
+    #
+    # @since 4.6
+    #
+    def canDropLoopEntryEdgeInLeftRecursiveRule(self, config):
+        # return False
+        p = config.state
+        # First check to see if we are in StarLoopEntryState generated during
+        # left-recursion elimination. For efficiency, also check if
+        # the context has an empty stack case. If so, it would mean
+        # global FOLLOW so we can't perform optimization
+        # Are we the special loop entry/exit state? or SLL wildcard
+        if p.stateType != ATNState.STAR_LOOP_ENTRY  \
+                or not p.isPrecedenceDecision       \
+                or config.context.isEmpty()         \
+                or config.context.hasEmptyPath():
+            return False
+
+        # Require all return states to return back to the same rule
+        # that p is in.
+        numCtxs = len(config.context)
+        for i in range(0, numCtxs):  # for each stack context
+            returnState = self.atn.states[config.context.getReturnState(i)]
+            if returnState.ruleIndex != p.ruleIndex:
+                return False
+
+        decisionStartState = p.transitions[0].target
+        blockEndStateNum = decisionStartState.endState.stateNumber
+        blockEndState = self.atn.states[blockEndStateNum]
+
+        # Verify that the top of each stack context leads to loop entry/exit
+        # state through epsilon edges and w/o leaving rule.
+        for i in range(0, numCtxs):  # for each stack context
+            returnStateNumber = config.context.getReturnState(i)
+            returnState = self.atn.states[returnStateNumber]
+            # all states must have single outgoing epsilon edge
+            if len(returnState.transitions) != 1 or not returnState.transitions[0].isEpsilon:
+                return False
+
+            # Look for prefix op case like 'not expr', (' type ')' expr
+            returnStateTarget = returnState.transitions[0].target
+            if returnState.stateType == ATNState.BLOCK_END and returnStateTarget is p:
+                continue
+
+            # Look for 'expr op expr' or case where expr's return state is block end
+            # of (...)* internal block; the block end points to loop back
+            # which points to p but we don't need to check that
+            if returnState is blockEndState:
+                continue
+
+            # Look for ternary expr ? expr : expr. The return state points at block end,
+            # which points at loop entry state
+            if returnStateTarget is blockEndState:
+                continue
+
+            # Look for complex prefix 'between expr and expr' case where 2nd expr's
+            # return state points at block end state of (...)* internal block
+            if returnStateTarget.stateType == ATNState.BLOCK_END \
+                and len(returnStateTarget.transitions) == 1 \
+                and returnStateTarget.transitions[0].isEpsilon \
+                and returnStateTarget.transitions[0].target is p:
+                    continue
+
+            # anything else ain't conforming
+            return False
+
+        return True
+
+
     def getRuleName(self, index:int):
         if self.parser is not None and index>=0:
             return self.parser.ruleNames[index]

runtime/Python3/src/antlr4/dfa/DFA.py

@@ -27,6 +27,7 @@
 #  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 #  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 #  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+from antlr4.atn.ATNState import StarLoopEntryState
 
 from antlr4.atn.ATNConfigSet import ATNConfigSet
 from antlr4.atn.ATNState import DecisionState
@@ -49,6 +50,15 @@ class DFA(object):
         # {@link #setPrecedenceDfa}.
         self.precedenceDfa = False
 
+        if isinstance(atnStartState, StarLoopEntryState):
+            if atnStartState.isPrecedenceDecision:
+                self.precedenceDfa = True
+                precedenceState = DFAState(configs=ATNConfigSet())
+                precedenceState.edges = []
+                precedenceState.isAcceptState = False
+                precedenceState.requiresFullContext = False
+                self.s0 = precedenceState
+
 
     # Get the start state for a specific precedence value.
     #
@@ -112,7 +122,7 @@ class DFA(object):
         if self.precedenceDfa != precedenceDfa:
             self._states = dict()
             if precedenceDfa:
-                precedenceState = DFAState(ATNConfigSet())
+                precedenceState = DFAState(configs=ATNConfigSet())
                 precedenceState.edges = []
                 precedenceState.isAcceptState = False
                 precedenceState.requiresFullContext = False