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/*
* [The "BSD license"]
* Copyright (c) 2010 Terence Parr
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* 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.
*/
package org.antlr.analysis;
import org.antlr.misc.Utils;
import org.antlr.tool.Grammar;
import java.util.HashSet;
import java.util.Set;
/** A module to perform optimizations on DFAs.
*
* I could more easily (and more quickly) do some optimizations (such as
* PRUNE_EBNF_EXIT_BRANCHES) during DFA construction, but then it
* messes up the determinism checking. For example, it looks like
* loop exit branches are unreachable if you prune exit branches
* during DFA construction and before determinism checks.
*
* In general, ANTLR's NFA→DFA→codegen pipeline seems very robust
* to me which I attribute to a uniform and consistent set of data
* structures. Regardless of what I want to "say"/implement, I do so
* within the confines of, for example, a DFA. The code generator
* can then just generate code--it doesn't have to do much thinking.
* Putting optimizations in the code gen code really starts to make
* it a spagetti factory (uh oh, now I'm hungry!). The pipeline is
* very testable; each stage has well defined input/output pairs.
*
* ### Optimization: PRUNE_EBNF_EXIT_BRANCHES
*
* There is no need to test EBNF block exit branches. Not only is it
* an unneeded computation, but counter-intuitively, you actually get
* better errors. You can report an error at the missing or extra
* token rather than as soon as you've figured out you will fail.
*
* Imagine optional block "( DOT CLASS )? SEMI". ANTLR generates:
*
* int alt=0;
* if ( input.LA(1)==DOT ) {
* alt=1;
* }
* else if ( input.LA(1)==SEMI ) {
* alt=2;
* }
*
* Clearly, since Parser.match() will ultimately find the error, we
* do not want to report an error nor do we want to bother testing
* lookahead against what follows the (...)? We want to generate
* simply "should I enter the subrule?":
*
* int alt=2;
* if ( input.LA(1)==DOT ) {
* alt=1;
* }
*
* NOTE 1. Greedy loops cannot be optimized in this way. For example,
* "(greedy=false:'x'|.)* '\n'". You specifically need the exit branch
* to tell you when to terminate the loop as the same input actually
* predicts one of the alts (i.e., staying in the loop).
*
* NOTE 2. I do not optimize cyclic DFAs at the moment as it doesn't
* seem to work. ;) I'll have to investigate later to see what work I
* can do on cyclic DFAs to make them have fewer edges. Might have
* something to do with the EOT token.
*
* ### PRUNE_SUPERFLUOUS_EOT_EDGES
*
* When a token is a subset of another such as the following rules, ANTLR
* quietly assumes the first token to resolve the ambiguity.
*
* EQ : '=' ;
* ASSIGNOP : '=' | '+=' ;
*
* It can yield states that have only a single edge on EOT to an accept
* state. This is a waste and messes up my code generation. ;) If
* Tokens rule DFA goes
*
* s0 -'='-> s3 -EOT-> s5 (accept)
*
* then s5 should be pruned and s3 should be made an accept. Do NOT do this
* for keyword versus ID as the state with EOT edge emanating from it will
* also have another edge.
*
* ### Optimization: COLLAPSE_ALL_INCIDENT_EDGES
*
* Done during DFA construction. See method addTransition() in
* NFAToDFAConverter.
*
* ### Optimization: MERGE_STOP_STATES
*
* Done during DFA construction. See addDFAState() in NFAToDFAConverter.
*/
public class DFAOptimizer {
public static boolean PRUNE_EBNF_EXIT_BRANCHES = true;
public static boolean PRUNE_TOKENS_RULE_SUPERFLUOUS_EOT_EDGES = true;
public static boolean COLLAPSE_ALL_PARALLEL_EDGES = true;
public static boolean MERGE_STOP_STATES = true;
/** Used by DFA state machine generator to avoid infinite recursion
* resulting from cycles int the DFA. This is a set of int state #s.
* This is a side-effect of calling optimize; can't clear after use
* because code gen needs it.
*/
protected Set visited = new HashSet();
protected Grammar grammar;
public DFAOptimizer(Grammar grammar) {
this.grammar = grammar;
}
public void optimize() {
// optimize each DFA in this grammar
for (int decisionNumber=1;
decisionNumber<=grammar.getNumberOfDecisions();
decisionNumber++)
{
DFA dfa = grammar.getLookaheadDFA(decisionNumber);
optimize(dfa);
}
}
protected void optimize(DFA dfa) {
if ( dfa==null ) {
return; // nothing to do
}
/*
System.out.println("Optimize DFA "+dfa.decisionNFAStartState.decisionNumber+
" num states="+dfa.getNumberOfStates());
*/
//long start = System.currentTimeMillis();
if ( PRUNE_EBNF_EXIT_BRANCHES && dfa.canInlineDecision() ) {
visited.clear();
int decisionType =
dfa.getNFADecisionStartState().decisionStateType;
if ( dfa.isGreedy() &&
(decisionType==NFAState.OPTIONAL_BLOCK_START ||
decisionType==NFAState.LOOPBACK) )
{
optimizeExitBranches(dfa.startState);
}
}
// If the Tokens rule has syntactically ambiguous rules, try to prune
if ( PRUNE_TOKENS_RULE_SUPERFLUOUS_EOT_EDGES &&
dfa.isTokensRuleDecision() &&
dfa.probe.stateToSyntacticallyAmbiguousTokensRuleAltsMap.size()>0 )
{
visited.clear();
optimizeEOTBranches(dfa.startState);
}
/* ack...code gen needs this, cannot optimize
visited.clear();
unlinkUnneededStateData(dfa.startState);
*/
//long stop = System.currentTimeMillis();
//System.out.println("minimized in "+(int)(stop-start)+" ms");
}
protected void optimizeExitBranches(DFAState d) {
Integer sI = Utils.integer(d.stateNumber);
if ( visited.contains(sI) ) {
return; // already visited
}
visited.add(sI);
int nAlts = d.dfa.getNumberOfAlts();
for (int i = 0; i < d.getNumberOfTransitions(); i++) {
Transition edge = d.transition(i);
DFAState edgeTarget = ((DFAState)edge.target);
/*
System.out.println(d.stateNumber+"-"+
edge.label.toString(d.dfa.nfa.grammar)+"->"+
edgeTarget.stateNumber);
*/
// if target is an accept state and that alt is the exit alt
if ( edgeTarget.isAcceptState() &&
edgeTarget.getUniquelyPredictedAlt()==nAlts)
{
/*
System.out.println("ignoring transition "+i+" to max alt "+
d.dfa.getNumberOfAlts());
*/
d.removeTransition(i);
i--; // back up one so that i++ of loop iteration stays within bounds
}
optimizeExitBranches(edgeTarget);
}
}
protected void optimizeEOTBranches(DFAState d) {
Integer sI = Utils.integer(d.stateNumber);
if ( visited.contains(sI) ) {
return; // already visited
}
visited.add(sI);
for (int i = 0; i < d.getNumberOfTransitions(); i++) {
Transition edge = d.transition(i);
DFAState edgeTarget = ((DFAState)edge.target);
/*
System.out.println(d.stateNumber+"-"+
edge.label.toString(d.dfa.nfa.grammar)+"->"+
edgeTarget.stateNumber);
*/
// if only one edge coming out, it is EOT, and target is accept prune
if ( PRUNE_TOKENS_RULE_SUPERFLUOUS_EOT_EDGES &&
edgeTarget.isAcceptState() &&
d.getNumberOfTransitions()==1 &&
edge.label.isAtom() &&
edge.label.getAtom()==Label.EOT )
{
//System.out.println("state "+d+" can be pruned");
// remove the superfluous EOT edge
d.removeTransition(i);
d.setAcceptState(true); // make it an accept state
// force it to uniquely predict the originally predicted state
d.cachedUniquelyPredicatedAlt =
edgeTarget.getUniquelyPredictedAlt();
i--; // back up one so that i++ of loop iteration stays within bounds
}
optimizeEOTBranches(edgeTarget);
}
}
/** Walk DFA states, unlinking the nfa configs and whatever else I
* can to reduce memory footprint.
protected void unlinkUnneededStateData(DFAState d) {
Integer sI = Utils.integer(d.stateNumber);
if ( visited.contains(sI) ) {
return; // already visited
}
visited.add(sI);
d.nfaConfigurations = null;
for (int i = 0; i < d.getNumberOfTransitions(); i++) {
Transition edge = (Transition) d.transition(i);
DFAState edgeTarget = ((DFAState)edge.target);
unlinkUnneededStateData(edgeTarget);
}
}
*/
}