[m-rev.] for review: LLDS code gen change for AGC

Fergus Henderson fjh at cs.mu.OZ.AU
Fri Nov 7 05:28:42 AEDT 2003
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For review by Zoltan.

Estimated hours taken: 3
Branches: main

Another step towards supporting accurate GC for the LLDS back-end:
modify the LLDS code generator to allocate the proper forwarding pointer
slots for type_infos and typeclass_infos.

compiler/unify_gen.m:
compiler/code_info.m:
compiler/var_locn.m:
	When allocating type_infos or typeclass_infos on the heap,
	if accurate garbage collection is enabled then ensure that we
	reserve a word before the start of the object.  This word is
	used by the collector to store a forwarding pointer.
	For ordinary objects, we store the forwarding pointer in the
	old copy of the object, but we can't clobber the old copies of
	type_infos and typeclass_infos, since they might be used _during_
	garbage collection.

Workspace: /home/jupiter/fjh/ws-jupiter/mercury
Index: compiler/code_info.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/code_info.m,v
retrieving revision 1.281
diff -u -d -r1.281 code_info.m
--- compiler/code_info.m	5 Nov 2003 03:17:35 -0000	1.281
+++ compiler/code_info.m	6 Nov 2003 17:46:55 -0000
@@ -3020,7 +3020,9 @@
 :- pred code_info__assign_expr_to_var(prog_var::in, rval::in, code_tree::out,
 	code_info::in, code_info::out) is det.
 
-:- pred code_info__assign_cell_to_var(prog_var::in, tag::in,
+	% code_info__assign_cell_to_var(Var, ReserveWordAtStart, Ptag, Vector,
+	% 	Size, TypeMsg, Code, !CI)
+:- pred code_info__assign_cell_to_var(prog_var::in, bool::in, tag::in,
 	list(maybe(rval))::in, maybe(term_size_value)::in, string::in,
 	code_tree::out, code_info::in, code_info::out) is det.
 
@@ -3157,11 +3159,13 @@
 	),
 	code_info__set_var_locn_info(VarLocnInfo, !CI).
 
-code_info__assign_cell_to_var(Var, Ptag, Vector, Size, TypeMsg, Code, !CI) :-
+code_info__assign_cell_to_var(Var, ReserveWordAtStart, Ptag, Vector, Size,
+		TypeMsg, Code, !CI) :-
 	code_info__get_var_locn_info(!.CI, VarLocnInfo0),
 	code_info__get_static_cell_info(!.CI, StaticCellInfo0),
-	var_locn__assign_cell_to_var(Var, Ptag, Vector, Size, TypeMsg, Code,
-		StaticCellInfo0, StaticCellInfo, VarLocnInfo0, VarLocnInfo),
+	var_locn__assign_cell_to_var(Var, ReserveWordAtStart, Ptag, Vector,
+		Size, TypeMsg, Code, StaticCellInfo0, StaticCellInfo,
+		VarLocnInfo0, VarLocnInfo),
 	code_info__set_static_cell_info(StaticCellInfo, !CI),
 	code_info__set_var_locn_info(VarLocnInfo, !CI).
 
Index: compiler/unify_gen.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/unify_gen.m,v
retrieving revision 1.135
diff -u -d -r1.135 unify_gen.m
--- compiler/unify_gen.m	31 Oct 2003 03:27:30 -0000	1.135
+++ compiler/unify_gen.m	6 Nov 2003 18:24:57 -0000
@@ -375,10 +375,7 @@
 	unify_gen__var_types(!.CI, Args, ArgTypes),
 	unify_gen__generate_cons_args(Args, ArgTypes, Modes, ModuleInfo,
 		Rvals),
-	VarType = code_info__variable_type(!.CI, Var),
-	unify_gen__var_type_msg(VarType, VarTypeMsg),
-	code_info__assign_cell_to_var(Var, Ptag, Rvals, Size, VarTypeMsg, Code,
-		!CI).
+	unify_gen__construct_cell(Var, Ptag, Rvals, Size, Code, !CI).
 unify_gen__generate_construction_2(shared_remote_tag(Ptag, Sectag),
 		Var, Args, Modes, Size, _, Code, !CI) :-
 	code_info__get_module_info(!.CI, ModuleInfo),
@@ -387,10 +384,7 @@
 		Rvals0),
 		% the first field holds the secondary tag
 	Rvals = [yes(const(int_const(Sectag))) | Rvals0],
-	VarType = code_info__variable_type(!.CI, Var),
-	unify_gen__var_type_msg(VarType, VarTypeMsg),
-	code_info__assign_cell_to_var(Var, Ptag, Rvals, Size, VarTypeMsg, Code,
-		!CI).
+	unify_gen__construct_cell(Var, Ptag, Rvals, Size, Code, !CI).
 unify_gen__generate_construction_2(shared_local_tag(Bits1, Num1),
 		Var, _Args, _Modes, _, _, empty, !CI) :-
 	code_info__assign_const_to_var(Var,
@@ -672,8 +666,8 @@
 			yes(const(int_const(NumArgs)))
 			| PredArgs
 		],
-		code_info__assign_cell_to_var(Var, 0, Vector, no, "closure",
-			Code, !CI)
+		code_info__assign_cell_to_var(Var, no, 0, Vector, no,
+			"closure", Code, !CI)
 	).
 
 :- pred unify_gen__generate_extra_closure_args(list(prog_var)::in, lval::in,
@@ -746,6 +740,34 @@
 	),
 	unify_gen__generate_cons_args_2(Vars, Types, UniModes, ModuleInfo,
 		RVals).
+
+:- pred unify_gen__construct_cell(prog_var::in, tag::in, list(maybe(rval))::in,
+	maybe(term_size_value)::in, code_tree::out,
+	code_info::in, code_info::out) is det.
+
+unify_gen__construct_cell(Var, Ptag, Rvals, Size, Code, !CI) :-
+	VarType = code_info__variable_type(!.CI, Var),
+	unify_gen__var_type_msg(VarType, VarTypeMsg),
+	% If we're doing accurate GC, then
+	% for types which hold RTTI that will be traversed
+	% by the collector at GC-time, we need to allocate
+	% an extra word at the start, to hold the forwarding
+	% pointer.  Normally we would just overwrite the
+	% first word of the object in the "from" space,
+	% but this can't be done for objects which will be
+	% referenced during the garbage collection process.
+	(
+		code_info__get_globals(!.CI, Globals),
+		globals__get_gc_method(Globals, GCMethod),
+		GCMethod = accurate,
+		is_introduced_type_info_type(VarType)
+	->
+		ReserveWordAtStart = yes
+	;
+		ReserveWordAtStart = no
+	),
+	code_info__assign_cell_to_var(Var, ReserveWordAtStart, Ptag, Rvals,
+		Size, VarTypeMsg, Code, !CI).
 
 %---------------------------------------------------------------------------%
 
Index: compiler/var_locn.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/var_locn.m,v
retrieving revision 1.12
diff -u -d -r1.12 var_locn.m
--- compiler/var_locn.m	27 Oct 2003 05:42:37 -0000	1.12
+++ compiler/var_locn.m	6 Nov 2003 18:25:37 -0000
@@ -151,14 +151,18 @@
 :- pred var_locn__assign_expr_to_var(prog_var::in, rval::in, code_tree::out,
 	var_locn_info::in, var_locn_info::out) is det.
 
-%	var_locn__assign_cell_to_var(Var, Ptag, Vector, TypeMsg, Code,
-%			StaticCellInfo0, StaticCellInfo,
+%	var_locn__assign_cell_to_var(Var, ReserveWordAtStart, Ptag, Vector,
+%			TypeMsg, Code, StaticCellInfo0, StaticCellInfo,
 %			VarLocnInfo0, VarLocnInfo)
 %		Generates code to assign to Var a pointer, tagged by Ptag, to
 %		the cell whose contents are given by the other arguments,
 %		and updates the state of VarLocnInfo0 accordingly.
+%		If ReserveWordAtStart is yes, and the cell is allocated on
+%		the heap (rather than statically), then reserve an extra
+%		word immediately before the allocated object, for the
+%		garbage collector to use to hold a forwarding pointer.
 
-:- pred var_locn__assign_cell_to_var(prog_var::in, tag::in,
+:- pred var_locn__assign_cell_to_var(prog_var::in, bool::in, tag::in,
 	list(maybe(rval))::in, maybe(term_size_value)::in, string::in,
 	code_tree::out, static_cell_info::in, static_cell_info::out,
 	var_locn_info::in, var_locn_info::out) is det.
@@ -330,6 +334,7 @@
 :- import_module libs__tree.
 :- import_module ll_backend__code_util.
 :- import_module ll_backend__exprn_aux.
+:- import_module hlds__error_util.
 
 :- import_module int, string, bag, require, getopt, varset, term.
 
@@ -787,8 +792,8 @@
 
 %----------------------------------------------------------------------------%
 
-var_locn__assign_cell_to_var(Var, Ptag, MaybeRvals0, SizeInfo, TypeMsg, Code,
-		!StaticCellInfo, !VLI) :-
+var_locn__assign_cell_to_var(Var, ReserveWordAtStart, Ptag, MaybeRvals0,
+		SizeInfo, TypeMsg, Code, !StaticCellInfo, !VLI) :-
 	(
 		SizeInfo = yes(SizeSource),
 		(
@@ -817,23 +822,35 @@
 		var_locn__assign_const_to_var(Var, CellPtrRval, !VLI),
 		Code = empty
 	;
-		var_locn__assign_dynamic_cell_to_var(Var, Ptag, MaybeRvals,
-			MaybeOffset, TypeMsg, Code, !VLI)
+		var_locn__assign_dynamic_cell_to_var(Var, ReserveWordAtStart,
+			Ptag, MaybeRvals, MaybeOffset, TypeMsg, Code, !VLI)
 	).
 
-:- pred var_locn__assign_dynamic_cell_to_var(prog_var::in, tag::in,
+:- pred var_locn__assign_dynamic_cell_to_var(prog_var::in, bool::in, tag::in,
 	list(maybe(rval))::in, maybe(int)::in, string::in, code_tree::out,
 	var_locn_info::in, var_locn_info::out) is det.
 
-var_locn__assign_dynamic_cell_to_var(Var, Ptag, Vector, MaybeOffset,
-		TypeMsg, Code, !VLI) :-
+var_locn__assign_dynamic_cell_to_var(Var, ReserveWordAtStart, Ptag, Vector,
+		MaybeOffset, TypeMsg, Code, !VLI) :-
 	var_locn__check_var_is_unknown(!.VLI, Var),
 
 	var_locn__select_preferred_reg_or_stack_check(!.VLI, Var, Lval),
 	var_locn__get_var_name(!.VLI, Var, VarName),
 	list__length(Vector, Size),
+	( ReserveWordAtStart = yes ->
+		( MaybeOffset = yes(_) ->
+			% Accurate GC and term profiling both want to own
+			% the word before this object
+			sorry(this_file,
+			  "accurate GC combined with term size profiling")
+		;
+			TotalOffset = yes(1)
+		)
+	;
+		TotalOffset = MaybeOffset
+	),
 	CellCode = node([
-		incr_hp(Lval, yes(Ptag), MaybeOffset, const(int_const(Size)),
+		incr_hp(Lval, yes(Ptag), TotalOffset, const(int_const(Size)),
 			TypeMsg)
 			- string__append("Allocating heap for ", VarName)
 	]),
@@ -2302,5 +2319,12 @@
 var_locn__set_acquired(A, VI, VI ^ acquired := A).
 var_locn__set_locked(L, VI, VI ^ locked := L).
 var_locn__set_exceptions(E, VI, VI ^ exceptions := E).
+
+%----------------------------------------------------------------------------%
+
+:- func this_file = string.
+this_file = "var_locn.m".
+
+:- end_module ll_backend__var_locn.
 
 %----------------------------------------------------------------------------%

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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