Bug 39626 - Correctly implement details of Fortran 2008 BLOCK construct
Summary: Correctly implement details of Fortran 2008 BLOCK construct
Alias: None
Product: gcc
Classification: Unclassified
Component: fortran (show other bugs)
Version: unknown
: P3 enhancement
Target Milestone: ---
Assignee: Daniel Kraft
: 43019 (view as bug list)
Depends on:
Blocks: 39627
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Reported: 2009-04-03 14:48 UTC by Daniel Kraft
Modified: 2012-09-21 18:48 UTC (History)
1 user (show)

See Also:
Known to work:
Known to fail:
Last reconfirmed: 2009-08-30 10:00:24


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Description Daniel Kraft 2009-04-03 14:48:45 UTC
The upcoming Fortran 2008 standard introduces the BLOCK construct which allows to declare local variables with a limited scope inside a procedure.

This is not yet implemented by gfortran.  Some ideas and discussion here: http://gcc.gnu.org/ml/fortran/2009-04/msg00003.html
Comment 1 Steven Bosscher 2009-04-03 16:43:38 UTC
This feature requires a substantial re-work of symbol handling in gfortran (make it block based).
Comment 2 Daniel Kraft 2009-08-30 10:00:24 UTC
I'm not sure it needs really that much changes...  After all, the semantics can probably be simulated completely via replacing the BLOCK-constructs with contained procedures that are called where the BLOCK originally was?

That's of course not a nice implementation and I do not think we should actually do this, but I believe that there needs not be that much change -- I'll work on this and try to find out a good way.
Comment 3 Tobias Burnus 2009-09-10 19:10:38 UTC
See also http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/ee95b68bf0658433#
Especially about the fine points raised by Richard Maine
Comment 4 Daniel Kraft 2009-09-28 14:49:11 UTC
For a "basically workig" (i.e. without some of the finer details and ugly corner cases, but handling all I would "reasonably" expect as user) patch, see http://gcc.gnu.org/ml/fortran/2009-09/msg00255.html.
Comment 5 Daniel Kraft 2009-09-29 07:43:04 UTC
Subject: Bug 39626

Author: domob
Date: Tue Sep 29 07:42:42 2009
New Revision: 152266

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=152266
2009-09-29  Daniel Kraft  <d@domob.eu>

	PR fortran/39626
	* gfortran.h (enum gfc_statement): Add ST_BLOCK and ST_END_BLOCK.
	(struct gfc_namespace): Convert flags to bit-fields and add flag
	`construct_entities' for use with BLOCK constructs.
	(enum gfc_exec_code): Add EXEC_BLOCK.
	(struct gfc_code): Add namespace field to union for EXEC_BLOCK.
	* match.h (gfc_match_block): New prototype.
	* parse.h (enum gfc_compile_state): Add COMP_BLOCK.
	* trans.h (gfc_process_block_locals): New prototype.
	(gfc_trans_deferred_vars): Made public, new prototype.
	* trans-stmt.h (gfc_trans_block_construct): New prototype.
	* decl.c (gfc_match_end): Handle END BLOCK correctly.
	(gfc_match_intent): Error if inside of BLOCK.
	(gfc_match_optional), (gfc_match_value): Ditto.
	* match.c (gfc_match_block): New routine.
	* parse.c (decode_statement): Handle BLOCK statement.
	(case_exec_markers): Add ST_BLOCK.
	(case_end): Add ST_END_BLOCK.
	(gfc_ascii_statement): Handle ST_BLOCK and ST_END_BLOCK.
	(parse_spec): Check for statements not allowed inside of BLOCK.
	(parse_block_construct): New routine.
	(parse_executable): Parse BLOCKs.
	(parse_progunit): Disallow CONTAINS in BLOCK constructs.
	* resolve.c (is_illegal_recursion): Find real container procedure and
	don't get confused by BLOCK constructs.
	(resolve_block_construct): New routine.
	(gfc_resolve_blocks), (resolve_code): Handle EXEC_BLOCK.
	* st.c (gfc_free_statement): Handle EXEC_BLOCK statements.
	* trans-decl.c (saved_local_decls): New static variable.
	(add_decl_as_local): New routine.
	(gfc_finish_var_decl): Add variable as local if inside BLOCK.
	(gfc_trans_deferred_vars): Make public.
	(gfc_process_block_locals): New routine.
	* trans-stmt.c (gfc_trans_block_construct): New routine.
	* trans.c (gfc_trans_code): Handle EXEC_BLOCK statements.

2009-09-29  Daniel Kraft  <d@domob.eu>

	PR fortran/39626
	* gfortran.dg/block_1.f08: New test.
	* gfortran.dg/block_2.f08: New test.
	* gfortran.dg/block_3.f90: New test.
	* gfortran.dg/block_4.f08: New test.
	* gfortran.dg/block_5.f08: New test.
	* gfortran.dg/block_6.f08: New test.
	* gfortran.dg/block_7.f08: New test.
	* gfortran.dg/block_8.f08: New test.


Comment 6 Daniel Kraft 2009-09-29 07:48:56 UTC
Committed my patch linked above.  This implements "basic" BLOCK support (what I as a user would reasonably expect it to behave) but misses a lot of finer details; I'll keep the PR open for those.

See my mailing list message for more information, but what I've got in mind for still missing stuff:

* handle VOLATILE and ASYNCHRONOUS as the draft standard mentions
* implement the clause requiring SAVE to not reference a common-name
* do more stuff with regards to 'construct entities' rather than "ordinary variables", for instance the IMPLICIT handling Richard Maine mentioned in his c.l.f post which is also the XFAIL'ed test-case block_7.f08
Comment 7 Daniel Kraft 2009-09-29 07:49:32 UTC
Also, Tobias mentioned (in the linked thread):

The patch looks good to me. It sometimes makes the diagnostics less
useful, e.g. for

recursive function func(i)
  integer :: i, func
  func = 1
entry func2(i)
    func2 = func(1)
  end block
end function func

But the same "Unclassifiable statement" one gets if one not only removes
"(end )block" but also "recursive".
Comment 8 Daniel Kraft 2010-02-10 18:26:02 UTC
*** Bug 43019 has been marked as a duplicate of this bug. ***
Comment 9 Tobias Burnus 2010-02-17 21:52:44 UTC
See also http://j3-fortran.org/doc/meeting/191/10-126.txt
("Scoping unit fixes for BLOCK construct")