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Re: Different conflicts between byacc and bison
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From: |
Akim Demaille |
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Subject: |
Re: Different conflicts between byacc and bison |
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Date: |
Sat, 6 Nov 2021 17:04:57 +0100 |
Hi Domingo,
> Le 3 nov. 2021 à 19:27, Domingo Alvarez Duarte <mingodad@gmail.com> a écrit :
>
> Hello !
>
> I'm trying to build the CFRONT parser
Wow... I was unaware cfront still existed.
> with bison instead of byacc and I've got bison to accept the grammar
> https://github.com/mingodad/cfront-3/blob/master/src/gram.y but the generated
> parser by bison has a small difference in the conflicts found in the grammar
> (see bellow)
That's an interesting finding, thanks for reporting it!
FWIW, I believe we will change the way conflicts are counted in Bison. I don't
know when, and backward compatibility will have to be taken into account, but
it was found that Bison disagrees with Bison. Indeed, when looking for
counterexamples and when merely counting the number of conflicts, we don't get
the same results, and the latter should comply with the former. See
https://github.com/akimd/bison/issues/73#issuecomment-792282225.
> cfront-3/src$ bison -y gram.y
> gram.y: warning: 20 shift/reduce conflicts [-Wconflicts-sr]
> gram.y: warning: 4 reduce/reduce conflicts [-Wconflicts-rr]
>
> cfront-3/srv$ byacc gram.y
> byacc: 21 shift/reduce conflicts, 3 reduce/reduce conflicts.
I have not tried to compare the SR conflicts (the automata are very similar,
but the state numbers are different and I don't feel like tracking the
correspondance right now). But the RR conflicted state where Bison and Byacc
differ is:
In Bison (very verbose mode, including counterexamples)
> State 101
>
> 57 tname: . qualified_tname
> 58 | . qualified_tname lt temp_inst_parms gt
> 59 | . NAME lt temp_inst_parms gt
> 174 scope_qualifiers: . tn_list
> 175 tn_list: . tscope
> 176 | . tn_list tscope
> 177 qualified_tname: . tn_list TNAME
> 178 | . TNAME
> 188 decl: tname LP . MUL ID RP arg_list
> 189 | tname LP . AND ID RP arg_list
> 190 | tname LP . elist RP
> 191 | tname LP . RP fct_attributes
> 192 | tname LP . MEMPTR decl RP arg_list
> 266 elist: . ex_list
> 267 ex_list: . initializer
> 268 | . ex_list CM initializer
> 269 initializer: . e
> 270 | . LC elist RC
> 296 e: . e ASSIGN e
> 297 | . e PLUS e
> 298 | . e MINUS e
> 299 | . e MUL e
> 300 | . e AND e
> 301 | . e OR e
> 302 | . e ER e
> 303 | . e SHIFTOP e
> 304 | . e EQUOP e
> 305 | . e DIVOP e
> 306 | . e RELOP e
> 307 | . e LT e
> 308 | . e GT e
> 309 | . e ANDAND e
> 310 | . e OROR e
> 311 | . e ASOP e
> 312 | . e CM e
> 313 | . e QUEST e COLON e
> 314 | . e REFMUL e
> 315 | . DELETE term
> 316 | . DELETE LB e RB term
> 317 | . MEM DELETE term
> 318 | . MEM DELETE LB e RB term
> 319 | . term
> 320 | . THROW term
> 321 | %empty . [RP, LT, GT, ER, OR, ANDAND, OROR, QUEST, ASSIGN, CM,
> ASOP, RELOP, EQUOP, DIVOP, SHIFTOP, REFMUL]
> 322 term: . NEW cast_type
> 323 | . NEW new_type
> 324 | . MEM NEW cast_type
> 325 | . MEM NEW new_type
> 326 | . term ICOP
> 327 | . cast_type term
> 328 | . MUL term
> 329 | . AND term
> 330 | . MINUS term
> 331 | . PLUS term
> 332 | . NOT term
> 333 | . COMPL term
> 334 | . ICOP term
> 335 | . SIZEOF term
> 336 | . SIZEOF cast_type
> 337 | . term LB e RB
> 338 | . term REF prim
> 339 | . term REF MEMQ prim
> 340 | . term REF MEMQ TNAME
> 341 | . term REF dtorspec
> 342 | . term REF scope_qualifiers prim
> 343 | . term REF scope_qualifiers TNAME
> 344 | . term DOT prim
> 345 | . term DOT MEMQ prim
> 346 | . term DOT MEMQ TNAME
> 347 | . term DOT dtorspec
> 348 | . term DOT scope_qualifiers prim
> 349 | . term DOT scope_qualifiers TNAME
> 350 | . prim
> 351 | . scope_qualifiers prim
> 352 | . tn_list COMPL tag
> 353 | . tn_list COMPL TYPE
> 354 | . term_elist
> 355 | . term_lp e RP
> 356 | . ZERO
> 357 | . ICON
> 358 | . FCON
> 359 | . STRING
> 360 | . CCON
> 361 | . THIS
> 370 term_elist: . TYPE LP elist RP
> 371 | . tname LP elist RP
> 372 | . NEW term_lp elist RP cast_type
> 373 | . NEW term_lp elist RP new_type
> 374 | . MEM NEW term_lp elist RP cast_type
> 375 | . MEM NEW term_lp elist RP new_type
> 376 | . term LP elist RP
> 377 ptname: . PTNAME lt temp_inst_parms gt
> 378 tscope: . TSCOPE
> 379 | . MEM
> 380 | . ptname TSCOPE
> 381 prim: . ID
> 382 | . OPERATOR oper
> 383 | . OPERATOR c_type
> 384 cast_type: . term_lp type cast_decl RP
> 385 term_lp: . LP
> 395 arg_lp: LP . [ENUM, RP, CM, NAME, TYPE, TNAME, ELLIPSIS, AGGR, MEM,
> TSCOPE, DECL_MARKER, LINKAGE, PTNAME]
>
> DELETE shift, and go to state 154
> NEW shift, and go to state 155
> OPERATOR shift, and go to state 156
> SIZEOF shift, and go to state 157
> THIS shift, and go to state 158
> LP shift, and go to state 159
> RP shift, and go to state 195
> NOT shift, and go to state 160
> COMPL shift, and go to state 161
> MUL shift, and go to state 196
> AND shift, and go to state 197
> PLUS shift, and go to state 164
> MINUS shift, and go to state 165
> LC shift, and go to state 198
> ID shift, and go to state 166
> STRING shift, and go to state 167
> ICON shift, and go to state 168
> FCON shift, and go to state 169
> CCON shift, and go to state 170
> NAME shift, and go to state 12
> ZERO shift, and go to state 171
> ICOP shift, and go to state 172
> THROW shift, and go to state 174
> MEMPTR shift, and go to state 200
> PTNAME shift, and go to state 22
>
> ENUM reduce using rule 395 (arg_lp)
> RP [reduce using rule 321 (e)]
> RP [reduce using rule 395 (arg_lp)]
> CM reduce using rule 321 (e)
> CM [reduce using rule 395 (arg_lp)]
> NAME [reduce using rule 395 (arg_lp)]
> TYPE reduce using rule 395 (arg_lp)
> TNAME reduce using rule 395 (arg_lp)
> ELLIPSIS reduce using rule 395 (arg_lp)
> AGGR reduce using rule 395 (arg_lp)
> MEM reduce using rule 395 (arg_lp)
> TSCOPE reduce using rule 395 (arg_lp)
> DECL_MARKER reduce using rule 395 (arg_lp)
> LINKAGE reduce using rule 395 (arg_lp)
> PTNAME [reduce using rule 395 (arg_lp)]
> $default reduce using rule 321 (e)
>
> tname go to state 201
> scope_qualifiers go to state 182
> tn_list go to state 202
> qualified_tname go to state 39
> elist go to state 203
> ex_list go to state 204
> initializer go to state 205
> e go to state 206
> term go to state 185
> term_elist go to state 186
> ptname go to state 53
> tscope go to state 42
> prim go to state 187
> cast_type go to state 188
> term_lp go to state 189
>
> Conflict between rule 321 and token MUL resolved as shift (NO_EXPR < MUL).
> Conflict between rule 321 and token AND resolved as shift (NO_EXPR < AND).
> Conflict between rule 321 and token PLUS resolved as shift (NO_EXPR <
> PLUS).
> Conflict between rule 321 and token MINUS resolved as shift (NO_EXPR <
> MINUS).
> Conflict between rule 395 and token TYPE resolved as reduce (TYPE < LP).
> Conflict between rule 395 and token TNAME resolved as reduce (TNAME < LP).
> Conflict between rule 395 and token MEM resolved as reduce (%left MEM).
> Conflict between rule 395 and token TSCOPE resolved as reduce (TSCOPE <
> LP).
>
> shift/reduce conflict on token RP:
> 321 e: %empty .
> 191 decl: tname LP . RP fct_attributes
> Example: tname LP . RP
> Shift derivation
> decl
> `-> 191: tname LP . RP fct_attributes
> `-> 191: %empty
> Example: tname LP . RP
> Reduce derivation
> decl
> `-> 190: tname LP elist RP
> `-> 266: ex_list
> `-> 267: initializer
> `-> 269: e
> `-> 321: %empty .
>
> reduce/reduce conflict on tokens RP, CM:
> 321 e: %empty .
> 395 arg_lp: LP .
> Example: tname LP . RP
> First reduce derivation
> decl
> `-> 190: tname LP elist RP
> `-> 266: ex_list
> `-> 267: initializer
> `-> 269: e
> `-> 321: %empty .
> Example: tname LP . RP
> Second reduce derivation
> decl
> `-> 186: tname arg_list
> `-> 396: arg_lp arg_type_list ellipsis_opt
> RP fct_attributes
> `-> 395: LP . `-> 396: %empty `-> 396: %empty
> `-> 396: %empty
>
> shift/reduce conflict on token RP:
> 395 arg_lp: LP .
> 191 decl: tname LP . RP fct_attributes
> Example: tname LP . RP
> Shift derivation
> decl
> `-> 191: tname LP . RP fct_attributes
> `-> 191: %empty
> Example: tname LP . RP
> Reduce derivation
> decl
> `-> 186: tname arg_list
> `-> 396: arg_lp arg_type_list ellipsis_opt
> RP fct_attributes
> `-> 395: LP . `-> 396: %empty `-> 396: %empty
> `-> 396: %empty
>
> shift/reduce conflict on token NAME:
> 395 arg_lp: LP .
> 59 tname: . NAME lt temp_inst_parms gt
> First example: tname LP . NAME lt temp_inst_parms gt LP elist RP RP
> ASSIGN initializer SM EOFTOK
> Shift derivation
> $accept
> `-> 0: ext_def
>
> EOFTOK
> `-> 1: external_def
> `-> 21: fct_dcl
> `-> 23: decl
>
> ASSIGN initializer SM
> `-> 190: tname LP elist
>
> RP
> `-> 266: ex_list
> `-> 267:
> initializer
> `->
> 269: e
>
> `-> 319: term
>
> `-> 354: term_elist
>
> `-> 371: tname LP elist
> RP
>
> `-> 59: . NAME lt temp_inst_parms gt
> Second example: tname LP . NAME lt temp_inst_parms gt arg_decl
> ellipsis_opt RP fct_attributes arg_dcl_list check_inline @5 base_init block
> EOFTOK
> Reduce derivation
> $accept
> `-> 0: ext_def
>
>
> EOFTOK
> `-> 1: external_def
> `-> 20: fct_def
> `-> 30: decl
>
> arg_dcl_list check_inline @5 base_init
> block
> `-> 186: tname arg_list
> `-> 396: arg_lp
> arg_type_list
> ellipsis_opt RP fct_attributes
> `-> 395: LP .
> `-> 398: at
>
> `-> 399: arg_type
>
> `-> 392: type
> arg_decl
>
> `-> 67: tp
>
> `-> 62: tname
>
> `-> 59: NAME lt temp_inst_parms gt
>
> shift/reduce conflict on token PTNAME:
> 395 arg_lp: LP .
> 377 ptname: . PTNAME lt temp_inst_parms gt
> First example: tname LP . PTNAME lt temp_inst_parms gt TSCOPE COMPL tag
> RP ASSIGN initializer SM EOFTOK
> Shift derivation
> $accept
> `-> 0: ext_def
>
> EOFTOK
> `-> 1: external_def
> `-> 21: fct_dcl
> `-> 23: decl
>
> ASSIGN initializer SM
> `-> 190: tname LP elist
>
> RP
> `-> 266: ex_list
> `-> 267:
> initializer
> `->
> 269: e
>
> `-> 319: term
>
> `-> 352: tn_list
> COMPL tag
>
> `-> 175: tscope
>
> `-> 380: ptname
> TSCOPE
>
> `-> 377: . PTNAME lt temp_inst_parms
> gt
> Second example: tname LP . PTNAME lt temp_inst_parms gt TSCOPE
> DECL_MARKER arg_decl ellipsis_opt RP fct_attributes arg_dcl_list check_inline
> @5 base_init block EOFTOK
> Reduce derivation
> $accept
> `-> 0: ext_def
>
>
> EOFTOK
> `-> 1: external_def
> `-> 20: fct_def
> `-> 30: decl
>
>
> arg_dcl_list check_inline @5 base_init block
> `-> 186: tname arg_list
> `-> 396: arg_lp
> arg_type_list
> ellipsis_opt RP
> fct_attributes
> `-> 395: LP .
> `-> 398: at
>
> `-> 399: arg_type
>
> `-> 392: type
> arg_decl
>
> `-> 67: tp
>
> `-> 63: tn_list
> DECL_MARKER
>
> `-> 175: tscope
>
> `-> 380: ptname
> TSCOPE
>
> `-> 377: PTNAME lt
> temp_inst_parms gt
The disable reductions are:
> RP [reduce using rule 321 (e)]
> RP [reduce using rule 395 (arg_lp)]
> CM [reduce using rule 395 (arg_lp)]
> NAME [reduce using rule 395 (arg_lp)]
> PTNAME [reduce using rule 395 (arg_lp)]
If we focus on the actions on these lookaheads, we have:
> RP shift, and go to state 195
> RP shift, and go to state 195
> NAME shift, and go to state 12
> PTNAME shift, and go to state 22
> RP [reduce using rule 321 (e)]
> RP [reduce using rule 395 (arg_lp)]
> CM reduce using rule 321 (e)
> CM [reduce using rule 395 (arg_lp)]
> NAME [reduce using rule 395 (arg_lp)]
> PTNAME [reduce using rule 395 (arg_lp)]
So we can see an "easy" RR conflict here:
> CM reduce using rule 321 (e)
> CM [reduce using rule 395 (arg_lp)]
But I agree with Bison that there is a second conflict:
> RP [reduce using rule 321 (e)]
> RP [reduce using rule 395 (arg_lp)]
It turns out that both of them are also involved in a SR conflict with the
shift action on RP, but I fail to see how this would not be a RR conflict.
BTW, counterexample generation "factors" these two conflicts into a single item:
> reduce/reduce conflict on tokens RP, CM:
Byacc's verbose output is:
> 131: shift/reduce conflict (shift 231, reduce 321) on RP
> 131: shift/reduce conflict (shift 231, reduce 395) on RP
> 131: reduce/reduce conflict (reduce 321, reduce 395) on CM
> 131: shift/reduce conflict (shift 12, reduce 395) on NAME
> 131: shift/reduce conflict (shift 22, reduce 395) on PTNAME
> state 131
> decl : tname LP . MUL ID RP arg_list (188)
> decl : tname LP . AND ID RP arg_list (189)
> decl : tname LP . elist RP (190)
> decl : tname LP . RP fct_attributes (191)
> decl : tname LP . MEMPTR decl RP arg_list (192)
> arg_lp : LP . (395)
> e : . (321)
>
> DELETE shift 153
> NEW shift 154
> OPERATOR shift 155
> SIZEOF shift 156
> THIS shift 157
> LP shift 158
> RP shift 231
> NOT shift 159
> COMPL shift 160
> MUL shift 232
> AND shift 233
> PLUS shift 163
> MINUS shift 164
> LC shift 214
> ID shift 165
> STRING shift 166
> ICON shift 167
> FCON shift 168
> CCON shift 169
> NAME shift 12
> ZERO shift 170
> ICOP shift 171
> THROW shift 173
> MEMPTR shift 234
> PTNAME shift 22
> ENUM reduce 395
> LT reduce 321
> GT reduce 321
> ER reduce 321
> OR reduce 321
> ANDAND reduce 321
> OROR reduce 321
> QUEST reduce 321
> ASSIGN reduce 321
> CM reduce 321
> ASOP reduce 321
> RELOP reduce 321
> EQUOP reduce 321
> DIVOP reduce 321
> SHIFTOP reduce 321
> TYPE reduce 395
> TNAME reduce 395
> ELLIPSIS reduce 395
> AGGR reduce 395
> MEM reduce 395
> TSCOPE reduce 395
> DECL_MARKER reduce 395
> REFMUL reduce 321
> LINKAGE reduce 395
>
> initializer goto 223
> ex_list goto 224
> elist goto 235
> e goto 216
> term goto 178
> prim goto 179
> term_elist goto 180
> cast_type goto 181
> tscope goto 37
> tn_list goto 196
> scope_qualifiers goto 184
> qualified_tname goto 40
> tname goto 197
> ptname goto 53
> term_lp goto 188
Since it does not display the disabled actions, I can't tell how it's counting.
But
> 131: reduce/reduce conflict (reduce 321, reduce 395) on CM
should also apply to RP, and that would make two RR conflicts, not just one.
A scaled down grammar showing a similar issue is
> %%
> exp: a 'x' | b 'x' | a 'y' | b 'y' | 'a' 'x' 'y'
> a: 'a'
> b: 'a'
where Bison diagnoses 1 SR and 2 RR but generates counterexamples for 2 SR and
1 RR (on two tokens):
> State 1
>
> 5 exp: 'a' . 'x' 'y'
> 6 a: 'a' . ['x', 'y']
> 7 b: 'a' . ['x', 'y']
>
> 'x' shift, and go to state 5
>
> 'x' [reduce using rule 6 (a)]
> 'x' [reduce using rule 7 (b)]
> 'y' reduce using rule 6 (a)
> 'y' [reduce using rule 7 (b)]
> $default reduce using rule 6 (a)
>
> shift/reduce conflict on token 'x':
> 6 a: 'a' .
> 5 exp: 'a' . 'x' 'y'
> First example: 'a' . 'x' 'y' $end
> Shift derivation
> $accept
> `-> 0: exp $end
> `-> 5: 'a' . 'x' 'y'
> Second example: 'a' . 'x' $end
> Reduce derivation
> $accept
> `-> 0: exp $end
> `-> 1: a 'x'
> `-> 6: 'a' .
>
> reduce/reduce conflict on tokens 'x', 'y':
> 6 a: 'a' .
> 7 b: 'a' .
> Example: 'a' . 'x'
> First reduce derivation
> exp
> `-> 1: a 'x'
> `-> 6: 'a' .
> Example: 'a' . 'x'
> Second reduce derivation
> exp
> `-> 2: b 'x'
> `-> 7: 'a' .
>
> shift/reduce conflict on token 'x':
> 7 b: 'a' .
> 5 exp: 'a' . 'x' 'y'
> First example: 'a' . 'x' 'y' $end
> Shift derivation
> $accept
> `-> 0: exp $end
> `-> 5: 'a' . 'x' 'y'
> Second example: 'a' . 'x' $end
> Reduce derivation
> $accept
> `-> 0: exp $end
> `-> 2: b 'x'
> `-> 7: 'a' .
and byacc generates
> 1: shift/reduce conflict (shift 5, reduce 6) on 'x'
> 1: shift/reduce conflict (shift 5, reduce 7) on 'x'
> 1: reduce/reduce conflict (reduce 6, reduce 7) on 'y'
> state 1
> exp : 'a' . 'x' 'y' (5)
> a : 'a' . (6)
> b : 'a' . (7)
>
> 'x' shift 5
> 'y' reduce 6
It does not count the RR conflict on 'x'.
Cheers!