performance - ANTLR 4 - How to reduce prediction time for IF statements with optional END-IF (COBOL grammar) -


i've been working on cobol grammar , found generated parser runs slow programs having nested if statements.

i think it's better @ sample code first, explain 1 of cobol's "annoying" feature - 1 dot (full stop) can close level of unclosed nested if statements. here is:

if condition-1     s1     if condition-2         s2         (more nested if conditions)            if condition-n                sn   (0~n "end-if"s ) .

the text between parenthesis pseudo code. key point "end-if"s optional before dot closes so-called "sentence" in cobol. allow "end-if"s optional, i've written grammar way:

sentence:   statement_list dot   ; statement_list:   statement+   ; statement:   if_statement   | ...   //other statements   ; if_statement:   if condition then? statement_list   (else statement_list)?   end_if?   ;

this grammar turn out slow style of code sample above. when debugging found parser busy predicting inside if_statement. expected there ambiguities in grammar. sample code, "if condition-2 ..." can recognized either child or brother of "if condition-1 ...", because there can omitted "end-if" right after s1. inner "if condition-n" can have n choices. similarly, every end-if before dot have alternatives on "if" pair if total number not n.

i tried cut off alternatives adding semantic predicates, below:

statement_list:   statement+  {_input.la(1)==end_if || _input.la(1)==else || _input.la(1)==dot}?   ;   if_statement:   if condition then? statement_list    ( {_input.la(1)!=else}? | else statement_list)    ( {_input.la(1)!=end-if}? | end_if)     ;

this reduced parse time of large programs 2/3, not reduce linear time, deep prediction on if_statement , statement_list still observed. closer antlr 4 runtime seems show semantic predicates not used @ during prediction.

does know of trick make "optional end-if" unambiguous, or make predict fast?

actually there ambiguity statements before "else", before end-if.

note: paring sample 30 levels of nested else-if uses 3 seconds on machine. , i'm facing large programs have many occurrence of such deep nesting , uses more 10 minutes parse. 1 of them goes stack overflow when predicting.

another note: i'm using antlr 4.2.


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