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A rewrite of the state variable transformation from the ground up.
The initial aim was to avoid situations (encountered in the g12 project)
in which the old state variable transformation generated code that
did not satisfy the mode checker, due to unnecessary unifications.
The new system tries hard to minimize the number of unifications added to the
program. It does this by relying extensively on the idea that in a branched
structure such as an disjunction, if two branches both update the same state
variable, and the variables representing the last state of the state variable
in the two branches are (say) X and Y, and we pick X to represent the current
state after the disjunction, then we don't have to put the assignment X := Y
into the second branch; instead, we can RENAME Y to X in that branch.
To avoid renaming a goal several times (for itself, for its parent, for its
grandparent etc), we delay all renamings until the end, when we do it all
in one traversal.
The old state var system was opaque and hard to understand, partly because
its basic operations did different things in different contexts. The new system
is a much more direct expression of the intuitive meaning of state variables;
it keeps track of their state much as the programmer writing the original code
would. It should therefore be significantly easier to understand and to modify
in the future.
The new system can also detect more kinds of errors in the use of state
variables. For example it can discover that some branches of a disjunction or
if-then-else set the initial value of a state variable and some do not.
This is ok if the non-setting-branch cannot succeed; if it can, then it is
a bug. We therefore generate messages about such branches, but print them
only if mode analysis finds a bug in the procedure, since in that case,
the lack of initialization may be the cause of the bug.
doc/reference_manual.texi:
Replaced an old example that didn't know what it was talking about,
and thoroughly confused the issue of what is legal use of state
variables and what is not.
compiler/state_var.m:
Rewrite this module along the lines mentioned above.
compiler/options.m:
Add two new options. One, warn-state-var-shadowing, controls whether
we generate warnings for one state var shadowing another (which
G12 has lots of). The other, --allow-defn-for-builtins, is for
developers only; it is needed to bootstrap changes that add new
builtins. I needed this for a form of the state variable transformation
that used calls to a new builtin predicate to copy the values of state
variables in branches that did not modify them, even though other
branches did. I ultimately used unifications to do this copying,
for reasons documented in state_var.m.
compiler/add_clause.m:
compiler/add_pragma.m:
Respect the new --allow-defn-for-builtins option.
(Previously, we changed the code that now looks up the value of the
option.)
doc/user_guide.texi:
Document the --warn-state-var-shadowing option.
Fix some old documentation about dump options.
compiler/simplify.m:
Fix an old oversight: list the predicates in table_builtin.m that may
have calls introduced to them by table_gen.m.
compiler/superhomogeneous.m:
compiler/field_access.m:
compiler/add_clause.m:
compiler/goal_expr_to_goal.m:
Together with state_var.m, these modules contain the transformation
from the parse tree to the HLDS. Since the change to state_var.m
involves significant changes in its interface (such as separating out
the persistent and location-dependent aspects of the information needed
by the state variable transformation), and needing callbacks at
different points than the old transformation, these modules had to
change extensively as well to conform.
goal_expr_to_goal.m is a new module carved out of add_clause.m.
It deserves a module of its own because its code has a significantly
different purpose than add_clause.m. The two separate modules each
have much better cohesion than the old conjoined module did.
In superhomogeneous.m, replace two predicates that did the same thing
with one predicate.
compiler/make_hlds.m:
compiler/notes/compiler_design.html.m:
Mention the new module.
compiler/hlds_goal.m:
Add a mechanism to do the kind of incremental renaming that the state
variable transformation needs.
Add some utility predicates needed by the new code in other modules.
compiler/hlds_clause.m:
compiler/hlds_pred.m:
Add an extra piece of information to clauses and proc_infos:
a list of informational messages generated by the state variable
transformation about some branches of branched goals not giving initial
values to some state variables, while other branches do.
The state variable transformation fills in this field in clauses
where relevant.
compiler/clause_to_proc.m:
Copy this list of messages from clauses to proc_infos.
compiler/modes.m:
When generating an error message for a procedure, include this list
of messages from the state var transformation in the output.
compiler/handle_options.m:
Add a dump alias for debugging the state var transformation.
compiler/hlds_out_goal.m:
Add a predicate that is useful in trace messages when debugging
the compiler.
compiler/hlds_out_pred.m:
Print goal path and goal id information in clauses as well as
proc_infos, since the state var transformation now uses goal ids.
compiler/prog_item.m:
In lists of quantified vars in scope headers, separate out the vars
introduced as !S from those introduced as !.S and !:S. This makes it
easier for the state var transformation to handle them.
Document that we expect lists of quantified variables and state
variables to contain no duplicates. The state var transformation
is slightly simpler if we impose this requirement, and quantifying
a variable twice in the same scope does not make sense, and is
therefore almost certainly an error.
compiler/prog_io_util.m:
Generate error messages when a variable or state variable IS
listed twice in the same quantification list.
Factor out some code used to generate error messages.
compiler/typecheck.m:
Conform to the changes above.
Break a very large predicate into two smaller pieces.
compiler/add_class.m:
compiler/add_pragma.m:
compiler/add_pred.m:
compiler/assertion.m:
compiler/dead_proc_elim.m:
compiler/dependency_graph.m:
compiler/goal_path.m:
compiler/goal_util.m:
compiler/headvar_names.m:
compiler/hhf.m:
compiler/hlds_out_module.m:
compiler/inlining.m:
compiler/intermod.m:
compiler/mercury_to_mercury.m:
compiler/module_imports.m:
compiler/module_qual.m:
compiler/post_typecheck.m:
compiler/prog_io_goal.m:
compiler/prog_util.m:
compiler/purity.m:
compiler/unify_proc.m:
compiler/unused_imports.m:
Conform to the changes above.
compiler/mode_constraints.m:
compiler/modules.m:
compiler/structure_reuse.analysis.m:
Avoid the warnings we now generate about one state variable shadowing
another.
browser/declarative_user.m:
compiler/hlds_out_util.m:
compiler/ordering_mode_constraints.m:
compiler/table_gen.m:
deep_profiler/read_profile.m:
Improve programming style.
library/require.m:
Add expect_not, a negated version of expect.
library/varset.m:
Return lists of new variables in order, not reverse order.
mdbcomp/mdbcomp.goal_path.m:
compiler/prog_mode.m:
Add a utility predicate.
tests/debugger/tailrec1.exp:
tests/invalid/any_passed_as_ground.err_exp:
tests/invalid/bad_sv_unify_msg.err_exp:
tests/invalid/state_vars_test1.err_exp:
tests/invalid/state_vars_test4.err_exp:
tests/invalid/try_bad_params.err_exp:
tests/invalid/try_detism.err_exp:
tests/invalid/purity/impure_pred_t1_fixed.err_exp:
tests/invalid/purity/impure_pred_t2.err_exp:
Update the expected outputs of these test cases to account for
incidental changes in variable numbers and goal paths after this
change.
tests/general/state_vars_tests.{m,exp}:
Remove the code that expected the state var transformation to do
something that was actually AGAINST the reference manual: treating
the step from the condition to the then part of an if-then-else
expression (not a goal) as a sequence point.
tests/general/state_vars_trace.m:
Add a test case that is not enabled yet, since we don't pass it.
tests/hard_coded/bit_buffer_test.m:
Fix a bug in the test itself: the introduction of a state var twice
in the same scope.
tests/hard_coded/try_syntax_6.m:
Avoid a warning about state var shadowing.
tests/hard_coded/if_then_else_expr_state_var.{m,exp}:
A new test to check the proper handling of state vars in if-then-else
expressions.
tests/hard_coded/Mmakefile:
Enable the new test.
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