[m-rev.] for review: explicitly free dead cells

Peter Wang novalazy at gmail.com
Thu Aug 21 15:43:35 AEST 2008


On icfp2000 it's about 3% faster than the ctgc-enabled ray tracer in
hlc.gc.  Don't know about the compiler yet.

Zoltan, do you think it will be hard to solve the problem noted in
unify_gen.m?


Branches: main

Add the ability, when structure reuse is enabled, to free dead heap cells
which can't otherwise be reused.  This currently only works with the high-level
C backend.

Fix a bug where we might try to reuse static data in MLDS grades.  As with the
the LLDS grades, check at run-time that the cell to reuse is within the heap
range.

compiler/options.m:
	Add `--structure-reuse-free-cells' option (disabled by default).

compiler/structure_reuse.direct.choose_reuse.m:
	Fix a bug which caused the code to mark deconstructions with `can_cgc'
	to never be run.  Run the code if `--structure-reuse-free-cells'
	is enabled.

compiler/ml_unify_gen.m:
	Make the code which implements cell reuse generate code like this:

	    MR_assign_if_in_heap(Lval, Rval);
	    if (Lval) {
		/* assign fields */
	    } else {
		Lval = MR_new_object(...);
	    }

	where `MR_assign_if_in_heap' assigns NULL to Lval if Rval doesn't look
	like a dynamically allocated address.

	Strip tag bits off the argument to `delete_object' statements.

compiler/mlds.m:
	Add an MLDS statement `assign_if_in_heap'.

	Change `delete_object' to take an rval argument instead of an lval,
	for stripping tag bits.

compiler/mlds_to_c.m:
	Generate output for `assign_if_in_heap' and `delete_object'.

compiler/ml_elim_nested.m:
compiler/ml_optimize.m:
compiler/mlds_to_gcc.m:
compiler/mlds_to_il.m:
compiler/mlds_to_java.m:
	Conform to MLDS statement changes.

compiler/unify_gen.m:
	Add commented out code to generate `free_heap' instructions in LLDS
	grades.

runtime/mercury_heap.h:
	Make `MR_free_heap' macro test that its argument is the heap range
	before calling `GC_FREE', so we don't try to free static data.

	Add `MR_assign_if_in_heap'.

diff --git a/compiler/ml_elim_nested.m b/compiler/ml_elim_nested.m
index af37263..b916b1e 100644
--- a/compiler/ml_elim_nested.m
+++ b/compiler/ml_elim_nested.m
@@ -1762,9 +1762,14 @@ fixup_atomic_stmt(Atomic0, Atomic, !Info) :-
         fixup_rval(Rval0, Rval, !Info),
         Atomic = assign(Lval, Rval)
     ;
-        Atomic0 = delete_object(Lval0),
+        Atomic0 = assign_if_in_heap(Lval0, Rval0),
         fixup_lval(Lval0, Lval, !Info),
-        Atomic = delete_object(Lval)
+        fixup_rval(Rval0, Rval, !Info),
+        Atomic = assign_if_in_heap(Lval, Rval)
+    ;
+        Atomic0 = delete_object(Rval0),
+        fixup_rval(Rval0, Rval, !Info),
+        Atomic = delete_object(Rval)
     ;
         Atomic0 = new_object(Target0, MaybeTag, HasSecTag, Type, MaybeSize,
             MaybeCtorName, Args0, ArgTypes, MayUseAtomic),
diff --git a/compiler/ml_optimize.m b/compiler/ml_optimize.m
index b454dbf..0e8342a 100644
--- a/compiler/ml_optimize.m
+++ b/compiler/ml_optimize.m
@@ -1166,9 +1166,14 @@ eliminate_var_in_atomic_stmt(Stmt0, Stmt, !VarElimInfo) :-
         eliminate_var_in_rval(Rval0, Rval, !VarElimInfo),
         Stmt = assign(Lval, Rval)
     ;
-        Stmt0 = delete_object(Lval0),
+        Stmt0 = assign_if_in_heap(Lval0, Rval0),
         eliminate_var_in_lval(Lval0, Lval, !VarElimInfo),
-        Stmt = delete_object(Lval)
+        eliminate_var_in_rval(Rval0, Rval, !VarElimInfo),
+        Stmt = assign_if_in_heap(Lval, Rval)
+    ;
+        Stmt0 = delete_object(Rval0),
+        eliminate_var_in_rval(Rval0, Rval, !VarElimInfo),
+        Stmt = delete_object(Rval)
     ;
         Stmt0 = new_object(Target0, MaybeTag, HasSecTag, Type,
             MaybeSize, MaybeCtorName, Args0, ArgTypes, MayUseAtomic),
diff --git a/compiler/ml_unify_gen.m b/compiler/ml_unify_gen.m
index 44552a9..5dc901f 100644
--- a/compiler/ml_unify_gen.m
+++ b/compiler/ml_unify_gen.m
@@ -210,7 +210,9 @@ ml_gen_unification(Unification, CodeModel, Context, Decls, Statements,
         % this is safe.
         CanCGC = can_cgc,
         ml_gen_var(!.Info, Var, VarLval),
-        Stmt = ml_stmt_atomic(delete_object(VarLval)),
+        % XXX avoid strip_tag when we know what tag it will have
+        Delete = delete_object(unop(std_unop(strip_tag), lval(VarLval))),
+        Stmt = ml_stmt_atomic(Delete),
         CGC_Statements = [statement(Stmt, mlds_make_context(Context)) ]
     ;
         CanCGC = cannot_cgc,
@@ -810,7 +812,10 @@ ml_gen_new_object(MaybeConsId, Tag, HasSecTag, MaybeCtorName, Var,
 
         ml_gen_type(!.Info, Type, MLDS_DestType),
         CastVar2Rval = unop(cast(MLDS_DestType), Var2Rval),
-        AssignStatement = ml_gen_assign(Var1Lval, CastVar2Rval, Context),
+        MLDS_Context = mlds_make_context(Context),
+        AssignStatement = statement(
+            ml_stmt_atomic(assign_if_in_heap(Var1Lval, CastVar2Rval)),
+            MLDS_Context),
 
         % For each field in the construction unification we need to generate
         % an rval.  ExtraRvals need to be inserted at the start of the object.
@@ -823,8 +828,23 @@ ml_gen_new_object(MaybeConsId, Tag, HasSecTag, MaybeCtorName, Var,
             Context, FieldStatements, TakeAddrInfos, !Info),
         ml_gen_field_take_address_assigns(TakeAddrInfos, VarLval, MLDS_Type,
             MaybeTag, Context, !.Info, TakeAddrStatements),
-        Statements = [AssignStatement | ExtraRvalStatements] ++
-            FieldStatements ++ TakeAddrStatements,
+        IfBody = statement(ml_stmt_block([],
+            ExtraRvalStatements ++ FieldStatements ++ TakeAddrStatements),
+            MLDS_Context),
+
+        % If the reassignment isn't possible because the target is statically
+        % allocated then fall back to dynamic allocation.
+        ml_gen_new_object(MaybeConsId, Tag, HasSecTag, MaybeCtorName, Var,
+            ExtraRvals, ExtraTypes, ArgVars, ArgModes, TakeAddr,
+            construct_dynamically, Context, DynamicDecls, DynamicStmt, !Info),
+        IfElse = statement(ml_stmt_block(DynamicDecls, DynamicStmt),
+            MLDS_Context),
+
+        IfStatement = statement(
+            ml_stmt_if_then_else(lval(Var1Lval), IfBody, yes(IfElse)),
+            mlds_make_context(Context)),
+
+        Statements = [AssignStatement, IfStatement],
         Decls = []
     ;
         HowToConstruct = construct_in_region(_RegVar),
diff --git a/compiler/mlds.m b/compiler/mlds.m
index 0047d26..c6f86d2 100644
--- a/compiler/mlds.m
+++ b/compiler/mlds.m
@@ -1238,9 +1238,15 @@
             % Assign the value specified by rval to the location
             % specified by lval.
 
+    ;       assign_if_in_heap(mlds_lval, mlds_rval)
+            % assign_if_in_heap(Location, Value):
+            % Assign the address specified by rval to the location specified
+            % by lval if the address is in the bounds of the garbage collected
+            % heap.  Otherwise assign a null pointer to lval.
+
     % Heap management.
 
-    ;       delete_object(mlds_lval)
+    ;       delete_object(mlds_rval)
             % Compile time garbage collect (ie explicitly
             % deallocate) the memory used by the lval.
 
diff --git a/compiler/mlds_to_c.m b/compiler/mlds_to_c.m
index a7caf57..8964ceb 100644
--- a/compiler/mlds_to_c.m
+++ b/compiler/mlds_to_c.m
@@ -3134,8 +3134,19 @@ mlds_output_atomic_stmt(Indent, _FuncInfo, assign(Lval, Rval), _, !IO) :-
     mlds_output_rval(Rval, !IO),
     io.write_string(";\n", !IO).
 
-mlds_output_atomic_stmt(_Indent, _FuncInfo, delete_object(_Lval), _, !IO) :-
-    sorry(this_file, "delete_object not implemented").
+mlds_output_atomic_stmt(Indent, _FuncInfo, assign_if_in_heap(Lval, Rval), _, !IO) :-
+    mlds_indent(Indent, !IO),
+    io.write_string("MR_assign_if_in_heap(", !IO),
+    mlds_output_lval(Lval, !IO),
+    io.write_string(", ", !IO),
+    mlds_output_rval(Rval, !IO),
+    io.write_string(");\n", !IO).
+
+mlds_output_atomic_stmt(Indent, _FuncInfo, delete_object(Rval), _, !IO) :-
+    mlds_indent(Indent, !IO),
+    io.write_string("MR_free_heap(", !IO),
+    mlds_output_rval(Rval, !IO),
+    io.write_string(");\n", !IO).
 
 mlds_output_atomic_stmt(Indent, FuncInfo, NewObject, Context, !IO) :-
     NewObject = new_object(Target, MaybeTag, _HasSecTag, Type, MaybeSize,
diff --git a/compiler/mlds_to_gcc.m b/compiler/mlds_to_gcc.m
index 8692a63..3bc1580 100644
--- a/compiler/mlds_to_gcc.m
+++ b/compiler/mlds_to_gcc.m
@@ -3094,6 +3094,9 @@ gen_atomic_stmt(DefnInfo, assign(Lval, Rval), _) -->
 	build_rval(Rval, DefnInfo, GCC_Rval),
 	gcc__gen_assign(GCC_Lval, GCC_Rval).
 
+gen_atomic_stmt(_, assign_if_in_heap(_, _), _) -->
+	{ sorry(this_file, "gen_atomic_stmt: assign_if_in_heap") }.
+
 	%
 	% heap management
 	%
diff --git a/compiler/mlds_to_il.m b/compiler/mlds_to_il.m
index dcec99a..1725ed9 100644
--- a/compiler/mlds_to_il.m
+++ b/compiler/mlds_to_il.m
@@ -531,7 +531,8 @@ rename_cond(match_range(RvalA, RvalB))
 
 rename_atomic(comment(S)) = comment(S).
 rename_atomic(assign(L, R)) = assign(rename_lval(L), rename_rval(R)).
-rename_atomic(delete_object(O)) = delete_object(rename_lval(O)).
+rename_atomic(assign_if_in_heap(L, R)) = assign(rename_lval(L), rename_rval(R)).
+rename_atomic(delete_object(O)) = delete_object(rename_rval(O)).
 rename_atomic(new_object(L, Tag, HasSecTag, Type, MaybeSize, Ctxt, Args,
         Types, MayUseAtomic))
     = new_object(rename_lval(L), Tag, HasSecTag, Type, MaybeSize,
@@ -2034,10 +2035,14 @@ atomic_statement_to_il(assign(Lval, Rval), Instrs, !Info) :-
         LoadRvalInstrs,
         StoreLvalInstrs
     ]).
+
+atomic_statement_to_il(assign_if_in_heap(_, _), _, !Info) :-
+    sorry(this_file, "assign_if_in_heap").
+
 atomic_statement_to_il(comment(Comment), Instrs, !Info) :-
     Instrs = node([comment(Comment)]).
 
-atomic_statement_to_il(delete_object(Target), Instrs, !Info) :-
+atomic_statement_to_il(delete_object(_Target), Instrs, !Info) :-
     % XXX We assume the code generator knows what it is doing and is only
     % going to delete real objects (e.g. reference types). It would perhaps
     % be prudent to check the type of delete_object (if it had one) to
@@ -2046,8 +2051,13 @@ atomic_statement_to_il(delete_object(Target), Instrs, !Info) :-
     % We implement delete_object by storing null in the lval, which hopefully
     % gives the garbage collector a good solid hint that this storage is
     % no longer required.
-    get_load_store_lval_instrs(Target, LoadInstrs, StoreInstrs, !Info),
-    Instrs = tree_list([LoadInstrs, instr_node(ldnull), StoreInstrs]).
+    %
+    % XXX commented out because `delete_object' was changed to take an rval
+    % instead of an lval
+    %
+    % get_load_store_lval_instrs(Target, LoadInstrs, StoreInstrs, !Info),
+    % Instrs = tree_list([LoadInstrs, instr_node(ldnull), StoreInstrs]).
+    Instrs = empty.
 
 atomic_statement_to_il(new_object(Target0, _MaybeTag, HasSecTag, Type, Size,
         MaybeCtorName, Args0, ArgTypes0, _MayUseAtomic), Instrs, !Info) :-
diff --git a/compiler/mlds_to_java.m b/compiler/mlds_to_java.m
index 2221552..3cb3221 100644
--- a/compiler/mlds_to_java.m
+++ b/compiler/mlds_to_java.m
@@ -2811,6 +2811,9 @@ output_atomic_stmt(Indent, ModuleInfo, FuncInfo, assign(Lval, Rval), _, !IO) :-
     ),
     io.write_string(";\n", !IO).
 
+output_atomic_stmt(_, _, _, assign_if_in_heap(_, _), _, !IO) :-
+    sorry(this_file, "output_atomic_stmt: assign_if_in_heap").
+
     % heap management
     %
 output_atomic_stmt(_Indent, _, _FuncInfo, delete_object(_Lval), _, _, _) :-
diff --git a/compiler/options.m b/compiler/options.m
index faee9af..7feb4fb 100644
--- a/compiler/options.m
+++ b/compiler/options.m
@@ -631,6 +631,7 @@
     ;           structure_reuse_constraint_arg
     ;           structure_reuse_max_conditions
     ;           structure_reuse_repeat
+    ;           structure_reuse_free_cells
 
     % Stuff for the old termination analyser.
     ;       termination
@@ -1349,6 +1350,7 @@ option_defaults_2(special_optimization_option, [
     structure_reuse_constraint_arg      -   int(0),
     structure_reuse_max_conditions      -   int(10),
     structure_reuse_repeat              -   int(0),
+    structure_reuse_free_cells          -   bool(no),
     termination                         -   bool(no),
     termination_single_args             -   int(0),
     termination_norm                    -   string("total"),
@@ -2316,6 +2318,7 @@ long_option("structure-reuse-constraint-arg", structure_reuse_constraint_arg).
 long_option("ctgc-constraint-arg",  structure_reuse_constraint_arg).
 long_option("structure-reuse-max-conditions", structure_reuse_max_conditions).
 long_option("structure-reuse-repeat", structure_reuse_repeat).
+long_option("structure-reuse-free-cells", structure_reuse_free_cells).
 
 % HLDS->LLDS optimizations
 long_option("smart-indexing",       smart_indexing).
@@ -3697,6 +3700,11 @@ options_help_ctgc -->
 %       "--structure-reuse-max-conditions",
 %       "\tSoft limit on the number of reuse conditions to accumulate",
 %       "\tfor a procedure. (default: 10)"
+
+% This option is likely to break many optimisations which haven't been updated.
+%       "--structure-reuse-free-cells",
+%       "\tImmediately free cells which are known to be dead but which",
+%       "\tcannot be reused."
     ]).
 
 :- pred options_help_termination(io::di, io::uo) is det.
diff --git a/compiler/structure_reuse.direct.choose_reuse.m b/compiler/structure_reuse.direct.choose_reuse.m
index 7bd82b7..d081a6a 100644
--- a/compiler/structure_reuse.direct.choose_reuse.m
+++ b/compiler/structure_reuse.direct.choose_reuse.m
@@ -135,8 +135,13 @@ determine_reuse(Strategy, ModuleInfo, ProcInfo, DeadCellTable, !Goal,
     process_goal(BackGroundInfo, DeadCellTable, RemainingDeadCellTable,
         !Goal, reuse_as_init, ReuseAs, !IO),
 
-    % Check for cell caching.
-    check_for_cell_caching(RemainingDeadCellTable, !Goal, !IO).
+    globals.io_lookup_bool_option(structure_reuse_free_cells, FreeCells, !IO),
+    (
+        FreeCells = yes,
+        check_for_cell_caching(RemainingDeadCellTable, !Goal, !IO)
+    ;
+        FreeCells = no
+    ).
 
 %-----------------------------------------------------------------------------%
 
@@ -471,42 +476,47 @@ process_goal(Background, !DeadCellTable, !Goal, !ReuseAs, !IO):-
         % Select the deconstructions-constructions with highest value.
         %
         Match = highest_match_degree_ratio(MatchTable),
-
-        % Maybe dump all the matches recorded in the table, highlight the
-        % match with the highest value.
-        %
-        maybe_write_string(VeryVerbose, "% Reuse results: \n", !IO),
-        maybe_dump_match_table(VeryVerbose, MatchTable, Match, !IO),
-
-        OldGoal = !.Goal,
-        OldReuseAs = !.ReuseAs,
-
-        % Realise the reuses by explicitly annotating the procedure goal.
-        annotate_reuses_in_goal(Background, Match, !Goal),
-
-        % Remove the deconstructions from the available map of dead cells.
-        remove_deconstructions_from_dead_cell_table(Match, !DeadCellTable),
-
-        % Add the conditions involved in the reuses to the existing
-        % conditions.
-        ModuleInfo = Background ^ back_module_info,
-        ProcInfo   = Background ^ back_proc_info,
-        reuse_as_least_upper_bound(ModuleInfo, ProcInfo,
-            match_get_condition(Background, Match), !ReuseAs),
-
-        % If there would be too many reuse conditions on this procedure
-        % by taking the reuse opportunity, just drop it.
-        globals.io_lookup_int_option(structure_reuse_max_conditions,
-            MaxConditions, !IO),
-        ( reuse_as_count_conditions(!.ReuseAs) > MaxConditions ->
-            !:Goal = OldGoal,
-            !:ReuseAs = OldReuseAs
+        (
+            Match ^ con_specs = []
         ;
-            true
-        ),
+            Match ^ con_specs = [_ | _],
+
+            % Maybe dump all the matches recorded in the table, highlight the
+            % match with the highest value.
+            %
+            maybe_write_string(VeryVerbose, "% Reuse results: \n", !IO),
+            maybe_dump_match_table(VeryVerbose, MatchTable, Match, !IO),
+
+            OldGoal = !.Goal,
+            OldReuseAs = !.ReuseAs,
+
+            % Realise the reuses by explicitly annotating the procedure goal.
+            annotate_reuses_in_goal(Background, Match, !Goal),
+
+            % Remove the deconstructions from the available map of dead cells.
+            remove_deconstructions_from_dead_cell_table(Match, !DeadCellTable),
+
+            % Add the conditions involved in the reuses to the existing
+            % conditions.
+            ModuleInfo = Background ^ back_module_info,
+            ProcInfo   = Background ^ back_proc_info,
+            reuse_as_least_upper_bound(ModuleInfo, ProcInfo,
+                match_get_condition(Background, Match), !ReuseAs),
+
+            % If there would be too many reuse conditions on this procedure
+            % by taking the reuse opportunity, just drop it.
+            globals.io_lookup_int_option(structure_reuse_max_conditions,
+                MaxConditions, !IO),
+            ( reuse_as_count_conditions(!.ReuseAs) > MaxConditions ->
+                !:Goal = OldGoal,
+                !:ReuseAs = OldReuseAs
+            ;
+                true
+            ),
 
-        % Process the goal for further reuse-matches.
-        process_goal(Background, !DeadCellTable, !Goal, !ReuseAs, !IO)
+            % Process the goal for further reuse-matches.
+            process_goal(Background, !DeadCellTable, !Goal, !ReuseAs, !IO)
+        )
     ).
 
 :- pred remove_deconstructions_from_dead_cell_table(match::in,
@@ -1364,6 +1374,9 @@ maybe_dump_full_table(yes, M, !IO) :-
     % structures, without imposing any reuse constraints are annotated so that
     % these cells can be cached whenever the user specifies that option.
     %
+    % XXX cell caching is not actually implemented, but we use the same
+    % information to free dead cells that have no reuse opportunity
+    %
 :- pred check_for_cell_caching(dead_cell_table::in, hlds_goal::in,
     hlds_goal::out, io::di, io::uo) is det.
 
@@ -1371,9 +1384,11 @@ check_for_cell_caching(DeadCellTable0, !Goal, !IO) :-
     dead_cell_table_remove_conditionals(DeadCellTable0, DeadCellTable),
     globals.io_lookup_bool_option(very_verbose, VeryVerbose, !IO),
     ( dead_cell_table_is_empty(DeadCellTable) ->
-        maybe_write_string(VeryVerbose, "% No cells to be cached.\n", !IO)
+        maybe_write_string(VeryVerbose,
+            "% No cells to be cached/freed.\n", !IO)
     ;
-        maybe_write_string(VeryVerbose, "% Marking cacheable cells.\n", !IO),
+        maybe_write_string(VeryVerbose,
+            "% Marking cacheable/freeable cells.\n", !IO),
         check_for_cell_caching_2(DeadCellTable, !Goal)
     ).
 
diff --git a/compiler/unify_gen.m b/compiler/unify_gen.m
index ec8386e..518dcb1 100644
--- a/compiler/unify_gen.m
+++ b/compiler/unify_gen.m
@@ -135,11 +135,31 @@ generate_unification(CodeModel, Uni, GoalInfo, Code, !CI) :-
             Code = empty
         )
     ;
-        Uni = deconstruct(Var, ConsId, Args, Modes, _CanFail, _CanCGC),
+        Uni = deconstruct(Var, ConsId, Args, Modes, _CanFail, CanCGC),
         ( CodeModel = model_det ->
-            generate_det_deconstruction(Var, ConsId, Args, Modes, Code, !CI)
+            generate_det_deconstruction(Var, ConsId, Args, Modes, Code0, !CI)
         ;
-            generate_semi_deconstruction(Var, ConsId, Args, Modes, Code, !CI)
+            generate_semi_deconstruction(Var, ConsId, Args, Modes, Code0, !CI)
+        ),
+        (
+            CanCGC = can_cgc,
+            % XXX we can't free the cell yet.
+            % Any live variables which are only reachable via Var (or its
+            % aliases?) must be produced beforehand.
+            %
+            % XXX avoid strip_tag when we know what tag it will have
+            % VarName = variable_name(!.CI, Var),
+            % FreeVar = node([
+            %     llds_instr(free_heap(unop(strip_tag, VarRval)),
+            %         "Free " ++ VarName)
+            % ]),
+            ProduceArgs = empty,
+            ProduceVar = empty,
+            FreeVar = empty,
+            Code = tree_list([Code0, ProduceArgs, ProduceVar, FreeVar])
+        ;
+            CanCGC = cannot_cgc,
+            Code = Code0
         )
     ;
         Uni = simple_test(Var1, Var2),
diff --git a/runtime/mercury_heap.h b/runtime/mercury_heap.h
index 5121833..29a0c65 100644
--- a/runtime/mercury_heap.h
+++ b/runtime/mercury_heap.h
@@ -176,7 +176,13 @@
 
   #define   MR_mark_hp(dest)        ((void) 0)
   #define   MR_restore_hp(src)      ((void) 0)
-  #define   MR_free_heap(ptr)       GC_FREE((ptr))
+  #define   MR_free_heap(ptr)                                               \
+            do {                                                            \
+                MR_Word *tmp = (MR_Word *) (ptr);                           \
+                if (MR_in_heap_range(tmp)) {                                \
+                    GC_FREE(tmp);                                           \
+                }                                                           \
+            } while (0)
 
 #else /* not MR_CONSERVATIVE_GC */
 
@@ -448,6 +454,12 @@
                 }                                                           \
             } while (0)
 
+#define     MR_assign_if_in_heap(dest, addr)                                \
+            do {                                                            \
+                MR_Word tmp = (addr);                                       \
+                (dest) = MR_in_heap_range(tmp) ? tmp : (MR_Word) NULL;      \
+            } while (0)
+
 /***************************************************************************/
 
 /*


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