Discussion of these changes can be found on the Mercury developers
mailing list archives from June 2018.
COPYING.LIB:
Add a special linking exception to the LGPL.
*:
Update references to COPYING.LIB.
Clean up some minor errors that have accumulated in copyright
messages.
Estimated hours taken: 20
Branches: main
When creating a new backtrackable reference that uses choicepoint ids
to avoid redundant trailing, set the initial value of the id using
MR_null_choicepoint_id() rather than MR_current_choicepoint_id().
If the current id is used, any updates that are performed before the
next choicepoint is created will not be trailed since the code for
updating the reference checks the stored id against the id that is
current at the time of the update, and only adds a trail entry if they
differ.
Normally this wouldn't be a problem because if execution ever does
backtrack to that initial choicepoint then it means it has backtracked
to a point before the reference was created, hence the reference should
no longer be live and it wouldn't matter that the reference was not
untrailed back to its initial value. In this case the only effect of
this change is that at most one unnecessary entry may be added to the
trail.
However, if a predicate that creates such a reference is tabled, then the
reference will still exist in the table even after backtracking, and will
be produced again if that predicate is later called. At this point the
reference will not necessarily have its correct initial value.
doc/reference_manual.texi:
In the section on avoiding redundant trailing, advise users to
use MR_null_choicepoint_id() if a mutable data structure is
created by a predicate or function that is tabled.
extras/references/reference.m:
Use MR_null_choicepoint_id() in the implementation of
new_reference/2, and document the reason.
extras/references/tests/Mmakefile:
extras/references/tests/glob_test_2.m:
extras/references/tests/glob_test_2.exp:
A test case that fails in grade asm_fast.gc.tr with the previous
version of reference.m, but succeeds now.
Estimated hours taken: 3
Branches: main
extras/references/nb_reference.m:
extras/references/reference.m:
extras/references/tests/glob_test.m:
extras/references/tests/ref_test.m:
Fix places where the code previously assumed that `nb_reference(T)' and
`reference(T)' had the same representation as `c_pointer'.
This is not the case in some grades, e.g. the `--reserve-tag' grades.
Estimated hours taken: 0.5
Branches: main
extras/references/c_reference.h:
extras/references/nb_reference.m:
extras/references/reference.m:
extras/references/scoped_update.m:
Add "MR_" prefixes to a number of identifiers which were missing them.
Estimated hours taken: 12 (this figure a wild guess, because most of this
stuff was done about 6 months ago and left to rot).
This change basically exposes the C types used for implementing the
`reference' and `nb_reference' types in extras/references, so that if the
user wishes to allocate these types somewhere other than on the heap, they
can. This is needed by HAL in order to implement global variables.
extras/references/c_reference.h:
New file, intended to expose the C types used by the `reference'
and `nb_reference' modules, so that the user can allocate them
somewhere other than on the heap if they need to.
extras/references/nb_reference.m:
Added a new predicate `init/2' for initialising a user-allocated
`nb_reference'.
extras/references/reference.m:
Moved the `ME_Reference' type to c_reference.h, and added a new
predicate `init/2' for initialising a user-allocated `reference'.
extras/references/README:
Added entries for the new `c_reference.h' and `tests/glob_test.m'
files, as well as correcting the name of `tests/ref_test.m'.
extras/references/tests/Mmakefile:
Added `glob_test' to the list of programs to build.
extras/references/tests/glob_test.m:
Test case for statically-allocated reference types, implementing
global variables. Adapted from output from the HAL compiler.
extras/references/tests/glob_test.exp:
Expected output from `glob_test'.
Estimated hours taken: 70 (plus whatever pets spent when he wrote the
original version of this)
This change adds a new extras directory, "references". This directory
contains two impure reference type modules and a module that allows scoped
non-backtrackable update, along with examples of using them and tests.
These modules are intended to be useful when HAL is retargetted to Mercury,
for implementing global variables (backtracking and non-backtracking), and
may also be useful for the debugger.
In order to implement these features, a new memory zone "global heap" was
added to the runtime system, for a heap which is not reclaimed on failure,
along with a pair of functions for copying terms to this heap.
runtime/mercury_deep_copy.c:
runtime/mercury_deep_copy.h:
Added two functions, MR_make_permanent() and
MR_make_partially_permanent(), which essentially do a deep copy of a
term to the global heap.
(In conservative GC grades, these functions actually do nothing).
runtime/mercury_engine.c:
runtime/mercury_engine.h:
Added fields global_heap_zone and e_global_hp (for the global heap
and its heap pointer) to the MR_mercury_engine_struct, along with
appropriate initialisation, etc.
Defined MR_heap_zone, MR_solutions_heap_zone, and
MR_global_heap_zone for convenient access to the corresponding field
of the relevant Mercury engine.
runtime/mercury_memory.c:
Added code for handling the size and zone size of the global heap.
runtime/mercury_regorder.h:
runtime/mercury_regs.h:
Defined MR_global_hp (the global heap pointer for general use),
along with corresponding other changes.
runtime/mercury_wrapper.c:
runtime/mercury_wrapper.h:
Added declarations and initialisation of the size and zone_size of
the global_heap.
Added an entry for MR_GLOBAL_HP_RN to print_register_usage_counts()
(plus missing entries for MR_SOL_HP_RN, MR_MIN_HP_REC and
MR_MIN_SOL_HP_REC).
New files:
extras/references/Mmakefile:
Mmakefile for building and testing these modules.
extras/references/README:
Description of contents of this directory.
extras/references/global.m:
A wrapper module for building a library containing the nb_reference,
reference and scoped_update modules.
extras/references/nb_reference.m:
Implements references which are not backtracked on failure.
extras/references/reference.m:
Implements references which *are* backtracked on failure.
extras/references/scoped_update.m:
Allows nested scoping of non-backtracking references.
extras/references/samples/Mmakefile:
extras/references/samples/max_of.m:
extras/references/samples/max_test.exp:
extras/references/samples/max_test.m:
An example of using a non-backtracking reference (to find the
maximum of the solutions generated by a predicate), with tests.
extras/references/tests/Mmakefile:
extras/references/tests/ref_test.exp:
extras/references/tests/ref_test.m:
Some tests of references (backtracking and non-backtracking) and
scoping.
