mercury/compiler/tag_switch.m

%-----------------------------------------------------------------------------%
% Copyright (C) 1994-2000 The University of Melbourne.
% This file may only be copied under the terms of the GNU General
% Public License - see the file COPYING in the Mercury distribution.
%-----------------------------------------------------------------------------%

% tag_switch.m - generate switches based on primary and secondary tags.

% Author: zs.

%-----------------------------------------------------------------------------%

:- module tag_switch.

:- interface.

:- import_module prog_data, hlds_goal, hlds_data.
:- import_module switch_util, code_model.
:- import_module llds, code_info.

:- import_module list.

	% Generate intelligent indexing code for tag based switches.

:- pred tag_switch__generate(list(extended_case), prog_var, code_model,
	can_fail, store_map, label, branch_end, branch_end, code_tree,
	code_info, code_info).
:- mode tag_switch__generate(in, in, in, in, in, in, in, out, out, in, out)
	is det.

:- implementation.

:- import_module builtin_ops, hlds_module, hlds_pred, code_gen, trace.
:- import_module options, globals, type_util, prog_data.

:- import_module assoc_list, map, tree, bool, int, string.
:- import_module require, std_util.

%-----------------------------------------------------------------------------%

	% The idea is to generate two-level switches, first on the primary
	% tag and then on the secondary tag. Since more than one function
	% symbol can be eliminated by a failed primary tag test, this reduces
	% the expected the number of comparisons required before finding the
	% code corresponding to the actual value of the switch variable.
	% We also get a speedup compared to non-tag switches by extracting
	% the primary and secondary tags once instead of repeatedly for
	% each functor test.
	%
	% We have four methods we can use for generating the code for the
	% switches on both primary and secondary tags.
	%
	% 1. try-me-else chains have the form
	%
	%		if (tag(var) != tag1) goto L1
	%		code for tag1
	%		goto end
	%	L1:	if (tag(var) != tag2) goto L2
	%		code for tag2
	%		goto end
	%	L2:	...
	%	Ln:	code for last possible tag value (or failure)
	%		goto end
	%
	% 2. try chains have the form
	%
	%		if (tag(var) == tag1) goto L1
	%		if (tag(var) == tag2) goto L2
	%		...
	%		code for last possible tag value (or failure)
	%		goto end
	%	L1:	code for tag1
	%		goto end
	%	L2:	code for tag2
	%		goto end
	%		...
	%
	% 3. jump tables have the form
	%
	%		goto tag(var) of L1, L2, ...
	%	L1:	code for tag1
	%		goto end
	%	L2:	code for tag2
	%		goto end
	%		...
	%
	% 4. binary search switches have the form
	%
	%		if (tag(var)) > 1) goto L23
	%		if (tag(var)) != 0) goto L1
	%		code for tag 0
	%		goto end
	%	L1:	code for tag 1
	%		goto end
	%	L23:	if (tag(var)) != 2) goto L3
	%		code for tag 2
	%		goto end
	%	L3:	code for tag 3
	%		goto end
	%
	% Note that for a det switch with two tag values, try-me-else chains
	% and try chains are equivalent.

	% Which method is best depends on the number of possible tag values,
	% on the costs of taken/untaken branches and table lookups on the given
	% architecture, and on the frequency with which the various
	% alternatives are taken.
	%
	% While the first two are in principle known at compile time,
	% the third is not. Nevertheless, for switches on primary tags
	% we can use the heuristic that the more secondary tags assigned to
	% a primary tag, the more likely that the switch variable will have
	% that primary tag at runtime.
	%
	% Try chains are good for switches with small numbers of alternatives
	% on architectures where untaken branches are cheaper than taken
	% branches.
	%
	% Try-me-else chains are good for switches with very small numbers of
	% alternatives on architectures where taken branches are cheaper than
	% untaken branches (which are rare these days).
	%
	% Jump tables are good for switches with large numbers of alternatives.
	% The cost of jumping through a jump table is relatively high, since
	% it involves a memory access and an indirect branch (which most
	% current architectures do not handle well), but this cost is
	% independent of the number of alternatives.
	%
	% Binary search switches are good for switches where the number of
	% alternatives is large enough for the reduced expected number of
	% branches executed to overcome the extra overhead of the subtraction
	% required for some conditional branches (compared to try chains
	% and try-me-else chains), but not large enough to make the
	% expected cost of the expected number of comparisons exceed the
	% expected cost of a jump table lookup and dispatch.

	% For try-me-else chains, we want tag1 to be the most frequent case,
	% tag 2 the next most frequent case, etc.
	%
	% For det try chains, we want the last tag value to be the most
	% frequent case, since it can be reached without taken jumps.
	% We want tag1 to be the next most frequent, tag2 the next most
	% frequent after that, etc.
	%
	% For semidet try chains, there is no last possible tag value (the
	% code for failure occupies its position), so we want tag1 to be
	% the most frequent case, tag 2 the next most frequent case, etc.
	%
	% For jump tables, the position of the labels in the computed goto
	% must conform to their numerical value. The order of the code
	% fragments does not really matter, although the last has a slight
	% edge in that no goto is needed to reach the code following the
	% switch. If there is no code following the switch (which happens
	% very frequently), then even this advantage is nullified.
	%
	% For binary search switches, we want the case of the most frequently
	% occurring tag to be the first, since this code is reached with no
	% taken branches and ends with an unconditional branch, whereas
	% reaching the code of the other cases requires at least one taken
	% *conditional* branch. In general, at each binary decision we
	% want the more frequently reached cases to be in the half that
	% immediately follows the if statement implementing the decision.

:- type switch_method	--->	try_me_else_chain
			;	try_chain
			;	jump_table
			;	binary_search.

tag_switch__generate(Cases, Var, CodeModel, CanFail, StoreMap, EndLabel,
		MaybeEnd0, MaybeEnd, Code)
		-->
	% group the cases based on primary tag value
	% and find out how many constructors share each primary tag value

	code_info__get_module_info(ModuleInfo),
	code_info__get_proc_info(ProcInfo),
	{ proc_info_vartypes(ProcInfo, VarTypes) },
	{ map__lookup(VarTypes, Var, Type) },
	{ switch_util__get_ptag_counts(Type, ModuleInfo,
		MaxPrimary, PtagCountMap) },
	{ map__to_assoc_list(PtagCountMap, PtagCountList) },
	{ map__init(PtagCaseMap0) },
	{ switch_util__group_cases_by_ptag(Cases, PtagCaseMap0, PtagCaseMap) },

	{ map__count(PtagCaseMap, PtagsUsed) },
	code_info__get_globals(Globals),
	{ globals__lookup_int_option(Globals, dense_switch_size,
		DenseSwitchSize) },
	{ globals__lookup_int_option(Globals, try_switch_size,
		TrySwitchSize) },
	{ globals__lookup_int_option(Globals, binary_switch_size,
		BinarySwitchSize) },
	( { PtagsUsed >= DenseSwitchSize } ->
		{ PrimaryMethod = jump_table }
	; { PtagsUsed >= BinarySwitchSize } ->
		{ PrimaryMethod = binary_search }
	; { PtagsUsed >= TrySwitchSize } ->
		{ PrimaryMethod = try_chain }
	;
		{ PrimaryMethod = try_me_else_chain }
	),

	% We get a register for holding the tag. The tag is needed only
	% by the switch, and no other code gets control between producing
	% the tag value and all uses of it, so we can release the register
	% for use by the code of the various cases.

	% We forgo using the register if the primary tag is needed only once,
	% or if the "register" we get is likely to be slower than
	% recomputing the tag from scratch.

	code_info__produce_variable_in_reg(Var, VarCode, VarLval),
	{ VarRval = lval(VarLval) },
	code_info__acquire_reg(r, PtagReg),
	code_info__release_reg(PtagReg),
	{
		PrimaryMethod \= jump_table,
		PtagsUsed >= 2,
		globals__lookup_int_option(Globals, num_real_r_regs,
			NumRealRegs),
		(
			NumRealRegs = 0
		;
			( PtagReg = reg(r, PtagRegNo) ->
				PtagRegNo =< NumRealRegs
			;
				error("improper reg in tag switch")
			)
		)
	->
		PtagCode = node([
			assign(PtagReg, unop(tag, VarRval))
				- "compute tag to switch on"
		]),
		PtagRval = lval(PtagReg)
	;
		PtagCode = empty,
		PtagRval = unop(tag, VarRval)
	},

	% We generate FailCode and EndCode here because the last case within
	% a primary tag may not be the last case overall.

	code_info__get_next_label(FailLabel),
	{ FailLabelCode = node([
		label(FailLabel) -
			"switch has failed"
	]) },
	(
		{ CanFail = cannot_fail },
		{ FailCode = node([
			goto(do_not_reached) - "oh-oh, det switch failed"
		]) }
	;
		{ CanFail = can_fail },
		code_info__generate_failure(FailCode)
	),
	{ LabelledFailCode = tree(FailLabelCode, FailCode) },

	{ EndCode = node([label(EndLabel) - "end of tag switch"]) },

	(
		{ PrimaryMethod = binary_search },
		{ switch_util__order_ptags_by_value(0, MaxPrimary, PtagCaseMap,
			PtagCaseList) },
		tag_switch__generate_primary_binary_search(PtagCaseList,
			0, MaxPrimary, PtagRval, VarRval, CodeModel, CanFail,
			StoreMap, EndLabel, FailLabel, PtagCountMap,
			no, MaybeEnd, CasesCode)
	;
		{ PrimaryMethod = jump_table },
		{ switch_util__order_ptags_by_value(0, MaxPrimary, PtagCaseMap,
			PtagCaseList) },
		tag_switch__generate_primary_jump_table(PtagCaseList,
			0, MaxPrimary, VarRval, CodeModel, StoreMap,
			EndLabel, FailLabel, PtagCountMap, MaybeEnd0, MaybeEnd,
			Labels, TableCode),
		{ SwitchCode = node([
			computed_goto(PtagRval, Labels) -
				"switch on primary tag"
		]) },
		{ CasesCode = tree(SwitchCode, TableCode) }
	;
		{ PrimaryMethod = try_chain },
		{ switch_util__order_ptags_by_count(PtagCountList, PtagCaseMap,
			PtagCaseList0) },
		{
			CanFail = cannot_fail,
			PtagCaseList0 = [MostFreqCase | OtherCases]
		->
			list__append(OtherCases, [MostFreqCase], PtagCaseList)
		;
			PtagCaseList = PtagCaseList0
		},
		tag_switch__generate_primary_try_chain(PtagCaseList,
			PtagRval, VarRval, CodeModel, CanFail, StoreMap,
			EndLabel, FailLabel, PtagCountMap, empty, empty,
			MaybeEnd0, MaybeEnd, CasesCode)
	;
		{ PrimaryMethod = try_me_else_chain },
		{ switch_util__order_ptags_by_count(PtagCountList, PtagCaseMap,
			PtagCaseList) },
		tag_switch__generate_primary_try_me_else_chain(PtagCaseList,
			PtagRval, VarRval, CodeModel, CanFail, StoreMap,
			EndLabel, FailLabel, PtagCountMap, MaybeEnd0, MaybeEnd,
			CasesCode)
	),

	{ Code =
		tree(VarCode,
		tree(PtagCode,
		tree(CasesCode,
		tree(LabelledFailCode,
		     EndCode))))
	}.

%-----------------------------------------------------------------------------%

	% Generate a switch on a primary tag value using a try-me-else chain.

:- pred tag_switch__generate_primary_try_me_else_chain(ptag_case_list,
	rval, rval, code_model, can_fail, store_map, label, label,
	ptag_count_map, branch_end, branch_end,
	code_tree, code_info, code_info).
:- mode tag_switch__generate_primary_try_me_else_chain(in, in, in, in, in, in,
	in, in, in, in, out, out, in, out) is det.

tag_switch__generate_primary_try_me_else_chain([], _, _, _, _, _, _, _, _, _,
		_, _) -->
	{ error("generate_primary_try_me_else_chain: empty switch") }.
tag_switch__generate_primary_try_me_else_chain([PtagGroup | PtagGroups],
		TagRval, VarRval, CodeModel, CanFail, StoreMap,
		EndLabel, FailLabel, PtagCountMap, MaybeEnd0, MaybeEnd, Code)
		-->
	{ PtagGroup = Primary - (StagLoc - StagGoalMap) },
	{ map__lookup(PtagCountMap, Primary, CountInfo) },
	{ CountInfo = StagLoc1 - MaxSecondary },
	{ StagLoc = StagLoc1 ->
		true
	;
		error("secondary tag locations differ in generate_primary_try_me_else_chain")
	},
	(
		{ PtagGroups = [_|_] ; CanFail = can_fail }
	->
		code_info__remember_position(BranchStart),
		code_info__get_next_label(ElseLabel),
		{ TestRval = binop(ne, TagRval,
			unop(mktag, const(int_const(Primary)))) },
		{ TestCode = node([
			if_val(TestRval, label(ElseLabel)) -
				"test primary tag only"
		]) },
		tag_switch__generate_primary_tag_code(StagGoalMap,
			Primary, MaxSecondary, StagLoc, VarRval, CodeModel,
			StoreMap, EndLabel, FailLabel, MaybeEnd0, MaybeEnd1,
			TagCode),
		{ ElseCode = node([
			label(ElseLabel) -
				"handle next primary tag"
		]) },
		{ ThisTagCode =
			tree(TestCode,
			tree(TagCode,
			     ElseCode))
		},
		( { PtagGroups = [_|_] } ->
			code_info__reset_to_position(BranchStart),
			tag_switch__generate_primary_try_me_else_chain(
				PtagGroups, TagRval, VarRval, CodeModel,
				CanFail, StoreMap, EndLabel, FailLabel,
				PtagCountMap, MaybeEnd1, MaybeEnd,
				OtherTagsCode),
			{ Code = tree(ThisTagCode, OtherTagsCode) }
		;
			% FailLabel ought to be the next label anyway,
			% so this goto will be optimized away (unless the
			% layout of the failcode in the caller changes).
			{ FailCode = node([
				goto(label(FailLabel)) -
					"primary tag with no code to handle it"
			]) },
			{ MaybeEnd = MaybeEnd1 },
			{ Code = tree(ThisTagCode, FailCode) }
		)
	;
		tag_switch__generate_primary_tag_code(StagGoalMap,
			Primary, MaxSecondary, StagLoc, VarRval, CodeModel,
			StoreMap, EndLabel, FailLabel, MaybeEnd0, MaybeEnd,
			Code)
	).

%-----------------------------------------------------------------------------%

	% Generate a switch on a primary tag value using a try chain.

:- pred tag_switch__generate_primary_try_chain(ptag_case_list,
	rval, rval, code_model, can_fail, store_map, label, label,
	ptag_count_map, code_tree, code_tree, branch_end, branch_end,
	code_tree, code_info, code_info).
:- mode tag_switch__generate_primary_try_chain(in, in, in, in, in, in,
	in, in, in, in, in, in, out, out, in, out) is det.

tag_switch__generate_primary_try_chain([], _, _, _, _, _, _, _, _, _, _, _,
		_, _) -->
	 { error("empty list in generate_primary_try_chain") }.
tag_switch__generate_primary_try_chain([PtagGroup | PtagGroups],
		TagRval, VarRval, CodeModel, CanFail, StoreMap, EndLabel,
		FailLabel, PtagCountMap, PrevTests0, PrevCases0,
		MaybeEnd0, MaybeEnd, Code) -->
	{ PtagGroup = Primary - (StagLoc - StagGoalMap) },
	{ map__lookup(PtagCountMap, Primary, CountInfo) },
	{ CountInfo = StagLoc1 - MaxSecondary },
	{ StagLoc = StagLoc1 ->
		true
	;
		error("secondary tag locations differ in generate_primary_try_chain")
	},
	(
		{ PtagGroups = [_|_] ; CanFail = can_fail }
	->
		code_info__remember_position(BranchStart),
		code_info__get_next_label(ThisPtagLabel),
		{ TestRval = binop(eq, TagRval,
			unop(mktag, const(int_const(Primary)))) },
		{ TestCode = node([
			if_val(TestRval, label(ThisPtagLabel)) -
				"test primary tag only"
		]) },
		{ LabelCode = node([
			label(ThisPtagLabel) -
				"this primary tag"
		]) },
		tag_switch__generate_primary_tag_code(StagGoalMap,
			Primary, MaxSecondary, StagLoc, VarRval, CodeModel,
			StoreMap, EndLabel, FailLabel, MaybeEnd0, MaybeEnd1,
			TagCode),
		{ PrevTests = tree(PrevTests0, TestCode) },
		{ PrevCases = tree(tree(LabelCode, TagCode), PrevCases0) },
		( { PtagGroups = [_|_] } ->
			code_info__reset_to_position(BranchStart),
			tag_switch__generate_primary_try_chain(PtagGroups,
				TagRval, VarRval, CodeModel, CanFail, StoreMap,
				EndLabel, FailLabel, PtagCountMap,
				PrevTests, PrevCases, MaybeEnd1, MaybeEnd,
				Code)
		;
			{ FailCode = node([
				goto(label(FailLabel)) -
					"primary tag with no code to handle it"
			]) },
			{ MaybeEnd = MaybeEnd1 },
			{ Code = tree(PrevTests, tree(FailCode, PrevCases)) }
		)
	;
		{ Comment = node([
			comment("fallthrough to last tag value") - ""
		]) },
		tag_switch__generate_primary_tag_code(StagGoalMap,
			Primary, MaxSecondary, StagLoc, VarRval,
			CodeModel, StoreMap, EndLabel, FailLabel,
			MaybeEnd0, MaybeEnd, TagCode),
		{ Code =
			tree(PrevTests0,
			tree(Comment,
			tree(TagCode,
			     PrevCases0)))
		}
	).

%-----------------------------------------------------------------------------%

	% Generate the cases for a primary tag using a dense jump table
	% that has an entry for all possible primary tag values.

:- pred tag_switch__generate_primary_jump_table(ptag_case_list, int, int,
	rval, code_model, store_map, label, label, ptag_count_map,
	branch_end, branch_end, list(label), code_tree, code_info, code_info).
:- mode tag_switch__generate_primary_jump_table(in, in, in, in,
	in, in, in, in, in, in, out, out, out, in, out) is det.

tag_switch__generate_primary_jump_table(PtagGroups, CurPrimary, MaxPrimary,
		VarRval, CodeModel, StoreMap, EndLabel, FailLabel, PtagCountMap,
		MaybeEnd0, MaybeEnd, Labels, Code) -->
	( { CurPrimary > MaxPrimary } ->
		{ PtagGroups = [] ->
			true
		;
			error("caselist not empty when reaching limiting primary tag")
		},
		{ MaybeEnd = MaybeEnd0 },
		{ Labels = [] },
		{ Code = empty }
	;
		{ NextPrimary is CurPrimary + 1 },
		( { PtagGroups = [CurPrimary - PrimaryInfo | PtagGroups1] } ->
			{ PrimaryInfo = StagLoc - StagGoalMap },
			{ map__lookup(PtagCountMap, CurPrimary, CountInfo) },
			{ CountInfo = StagLoc1 - MaxSecondary },
			{ StagLoc = StagLoc1 ->
				true
			;
				error("secondary tag locations differ in generate_primary_jump_table")
			},
			code_info__get_next_label(NewLabel),
			{ LabelCode = node([
				label(NewLabel) -
					"start of a case in primary tag switch"
			]) },
			( { PtagGroups1 = [] } ->
				tag_switch__generate_primary_tag_code(
					StagGoalMap, CurPrimary, MaxSecondary,
					StagLoc, VarRval, CodeModel,
					StoreMap, EndLabel, FailLabel,
					MaybeEnd0, MaybeEnd1, ThisTagCode)
			;
				code_info__remember_position(BranchStart),
				tag_switch__generate_primary_tag_code(
					StagGoalMap, CurPrimary, MaxSecondary,
					StagLoc, VarRval, CodeModel,
					StoreMap, EndLabel, FailLabel,
					MaybeEnd0, MaybeEnd1, ThisTagCode),
				code_info__reset_to_position(BranchStart)
			),
			tag_switch__generate_primary_jump_table(PtagGroups1,
				NextPrimary, MaxPrimary, VarRval, CodeModel,
				StoreMap, EndLabel, FailLabel, PtagCountMap,
				MaybeEnd1, MaybeEnd, OtherLabels, OtherCode),
			{ Labels = [NewLabel | OtherLabels] },
			{ Code =
				tree(LabelCode,
				tree(ThisTagCode,
				     OtherCode))
			}
		;
			tag_switch__generate_primary_jump_table(PtagGroups,
				NextPrimary, MaxPrimary, VarRval, CodeModel,
				StoreMap, EndLabel, FailLabel, PtagCountMap,
				MaybeEnd0, MaybeEnd, OtherLabels, Code),
			{ Labels = [FailLabel | OtherLabels] }
		)
	).

%-----------------------------------------------------------------------------%

	% Generate the cases for a primary tag using a binary search.
	% This invocation looks after primary tag values in the range
	% MinPtag to MaxPtag (including both boundary values).

:- pred tag_switch__generate_primary_binary_search(ptag_case_list, int, int,
	rval, rval, code_model, can_fail, store_map, label, label,
	ptag_count_map, branch_end, branch_end, code_tree,
	code_info, code_info).
:- mode tag_switch__generate_primary_binary_search(in, in, in,
	in, in, in, in, in, in, in, in, in, out, out, in, out) is det.

tag_switch__generate_primary_binary_search(PtagGroups, MinPtag, MaxPtag,
		PtagRval, VarRval, CodeModel, CanFail, StoreMap,
		EndLabel, FailLabel, PtagCountMap,
		MaybeEnd0, MaybeEnd, Code) -->
	( { MinPtag = MaxPtag } ->
		{ CurPrimary = MinPtag },
		( { PtagGroups = [] } ->
			% there is no code for this tag
			(
				{ CanFail = can_fail },
				{ string__int_to_string(CurPrimary, PtagStr) },
				{ string__append("no code for ptag ", PtagStr,
					Comment) },
				{ Code = node([
					goto(label(FailLabel)) -
						Comment
				]) }
			;
				{ CanFail = cannot_fail },
				{ Code = empty }
			),
			{ MaybeEnd = MaybeEnd0 }
		; { PtagGroups = [CurPrimary - PrimaryInfo] } ->
			{ PrimaryInfo = StagLoc - StagGoalMap },
			{ map__lookup(PtagCountMap, CurPrimary, CountInfo) },
			{ CountInfo = StagLoc1 - MaxSecondary },
			{ StagLoc = StagLoc1 ->
				true
			;
				error("secondary tag locations differ in generate_primary_jump_table")
			},
			tag_switch__generate_primary_tag_code(
				StagGoalMap, CurPrimary, MaxSecondary, StagLoc,
				VarRval, CodeModel, StoreMap, EndLabel,
				FailLabel, MaybeEnd0, MaybeEnd, Code)
		;
			{ error("caselist not singleton or empty when binary search ends") }
		)
	;
		{ LowRangeEnd is (MinPtag + MaxPtag) // 2 },
		{ HighRangeStart is LowRangeEnd + 1 },
		{ InLowGroup = lambda([PtagGroup::in] is semidet, (
			PtagGroup = Ptag - _,
			Ptag =< LowRangeEnd
		)) },
		{ list__filter(InLowGroup, PtagGroups, LowGroups, HighGroups) },
		code_info__get_next_label(NewLabel),
		{ string__int_to_string(MinPtag, LowStartStr) },
		{ string__int_to_string(LowRangeEnd, LowEndStr) },
		{ string__int_to_string(HighRangeStart, HighStartStr) },
		{ string__int_to_string(MaxPtag, HighEndStr) },
		{ string__append_list(["fallthrough for ptags ",
			LowStartStr, " to ", LowEndStr], IfComment) },
		{ string__append_list(["code for ptags ", HighStartStr,
			" to ", HighEndStr], LabelComment) },
		{ LowRangeEndConst = const(int_const(LowRangeEnd)) },
		{ TestRval = binop(>, PtagRval, LowRangeEndConst) },
		{ IfCode = node([
			if_val(TestRval, label(NewLabel)) -
				IfComment
		]) },
		{ LabelCode = node([
			label(NewLabel) -
				LabelComment
		]) },

		code_info__remember_position(BranchStart),
		tag_switch__generate_primary_binary_search(LowGroups,
			MinPtag, LowRangeEnd, PtagRval, VarRval, CodeModel,
			CanFail, StoreMap, EndLabel, FailLabel, PtagCountMap,
			MaybeEnd0, MaybeEnd1, LowRangeCode),
		code_info__reset_to_position(BranchStart),
		tag_switch__generate_primary_binary_search(HighGroups,
			HighRangeStart, MaxPtag, PtagRval, VarRval, CodeModel,
			CanFail, StoreMap, EndLabel, FailLabel, PtagCountMap,
			MaybeEnd1, MaybeEnd, HighRangeCode),

		{ Code =
			tree(IfCode,
			tree(LowRangeCode,
			tree(LabelCode,
			     HighRangeCode)))
		}
	).

%-----------------------------------------------------------------------------%

	% Generate the code corresponding to a primary tag.
	% If this primary tag has secondary tags, decide whether we should
	% use a jump table to implement the secondary switch.

:- pred tag_switch__generate_primary_tag_code(stag_goal_map, tag_bits, int,
	stag_loc, rval, code_model, store_map, label, label,
	branch_end, branch_end, code_tree, code_info, code_info).
:- mode tag_switch__generate_primary_tag_code(in, in, in, in, in, in, in,
	in, in, in, out, out, in, out) is det.

tag_switch__generate_primary_tag_code(GoalMap, Primary, MaxSecondary, StagLoc,
		Rval, CodeModel, StoreMap, EndLabel, FailLabel,
		MaybeEnd0, MaybeEnd, Code) -->
	{ map__to_assoc_list(GoalMap, GoalList) },
	(
		{ StagLoc = none }
	->
		% There is no secondary tag, so there is no switch on it
		( { GoalList = [-1 - Goal] } ->
			trace__maybe_generate_internal_event_code(Goal,
				TraceCode),
			code_gen__generate_goal(CodeModel, Goal, GoalCode),
			code_info__generate_branch_end(StoreMap,
				MaybeEnd0, MaybeEnd, SaveCode),
			{ GotoCode = node([
				goto(label(EndLabel)) -
					"skip to end of primary tag switch"
			]) },
			{ Code =
				tree(TraceCode,
				tree(GoalCode,
				tree(SaveCode,
				     GotoCode)))
			}
		; { GoalList = [] } ->
			{ error("no goal for non-shared tag") }
		;
			{ error("more than one goal for non-shared tag") }
		)
	;
		% There is a secondary tag, so figure out how to switch on it
		code_info__get_globals(Globals),
		{ globals__lookup_int_option(Globals, dense_switch_size,
			DenseSwitchSize) },
		{ globals__lookup_int_option(Globals, binary_switch_size,
			BinarySwitchSize) },
		{ globals__lookup_int_option(Globals, try_switch_size,
			TrySwitchSize) },
		{ MaxSecondary >= DenseSwitchSize ->
			SecondaryMethod = jump_table
		; MaxSecondary >= BinarySwitchSize ->
			SecondaryMethod = binary_search
		; MaxSecondary >= TrySwitchSize ->
			SecondaryMethod = try_chain
		;
			SecondaryMethod = try_me_else_chain
		},

		{ StagLoc = remote ->
			OrigStagRval = lval(field(yes(Primary), Rval,
				const(int_const(0)))),
			Comment = "compute remote sec tag to switch on"
		;
			OrigStagRval = unop(unmkbody, Rval),
			Comment = "compute local sec tag to switch on"
		},

		code_info__acquire_reg(r, StagReg),
		code_info__release_reg(StagReg),
		{
			SecondaryMethod \= jump_table,
			MaxSecondary >= 2,
			globals__lookup_int_option(Globals, num_real_r_regs,
				NumRealRegs),
			(
				NumRealRegs = 0
			;
				( StagReg = reg(r, StagRegNo) ->
					StagRegNo =< NumRealRegs
				;
					error("improper reg in tag switch")
				)
			)
		->
			StagCode = node([
				assign(StagReg, OrigStagRval) -
					Comment
			]),
			StagRval = lval(StagReg)
		;
			StagCode = empty,
			StagRval = OrigStagRval
		},


		(
			{ list__length(GoalList, GoalCount) },
			{ FullGoalCount is MaxSecondary + 1 },
			{ FullGoalCount = GoalCount }
		->
			{ CanFail = cannot_fail }
		;
			{ CanFail = can_fail }
		),

		(
			{ SecondaryMethod = jump_table },
			tag_switch__generate_secondary_jump_table(GoalList,
				0, MaxSecondary, CodeModel, StoreMap,
				EndLabel, FailLabel, MaybeEnd0, MaybeEnd,
				Labels, CasesCode),
			{ SwitchCode = node([
				computed_goto(StagRval, Labels) -
					"switch on secondary tag"
			]) },
			{ Code = tree(SwitchCode, CasesCode) }
		;
			{ SecondaryMethod = binary_search },
			tag_switch__generate_secondary_binary_search(GoalList,
				0, MaxSecondary, StagRval, CodeModel, CanFail,
				StoreMap, EndLabel, FailLabel,
				MaybeEnd0, MaybeEnd, Code)
		;
			{ SecondaryMethod = try_chain },
			tag_switch__generate_secondary_try_chain(GoalList,
				StagRval, CodeModel, CanFail, StoreMap,
				EndLabel, FailLabel, empty, empty,
				MaybeEnd0, MaybeEnd, Codes),
			{ Code = tree(StagCode, Codes) }
		;
			{ SecondaryMethod = try_me_else_chain },
			tag_switch__generate_secondary_try_me_else_chain(
				GoalList, StagRval, CodeModel, CanFail,
				StoreMap, EndLabel, FailLabel,
				MaybeEnd0, MaybeEnd, Codes),
			{ Code = tree(StagCode, Codes) }
		)
	).

%-----------------------------------------------------------------------------%

	% Generate a switch on a secondary tag value using a try-me-else chain.

:- pred tag_switch__generate_secondary_try_me_else_chain(stag_goal_list, rval,
	code_model, can_fail, store_map, label, label,
	branch_end, branch_end, code_tree, code_info, code_info).
:- mode tag_switch__generate_secondary_try_me_else_chain(in, in, in, in, in,
	in, in, in, out, out, in, out) is det.

tag_switch__generate_secondary_try_me_else_chain([], _, _, _, _, _, _, _, _, _)
		-->
	{ error("generate_secondary_try_me_else_chain: empty switch") }.
tag_switch__generate_secondary_try_me_else_chain([Case0 | Cases0], StagRval,
		CodeModel, CanFail, StoreMap, EndLabel, FailLabel,
		MaybeEnd0, MaybeEnd, Code) -->
	{ Case0 = Secondary - Goal },
	( { Cases0 = [_|_] ; CanFail = can_fail } ->
		code_info__remember_position(BranchStart),
		code_info__get_next_label(ElseLabel),
		{ TestCode = node([
			if_val(binop(ne, StagRval,
					const(int_const(Secondary))),
				label(ElseLabel))
				- "test remote sec tag only"
		]) },
		trace__maybe_generate_internal_event_code(Goal, TraceCode),
		code_gen__generate_goal(CodeModel, Goal, GoalCode),
		code_info__generate_branch_end(StoreMap, MaybeEnd0, MaybeEnd1,
			SaveCode),
		{ GotoLabelCode = node([
			goto(label(EndLabel)) -
				"skip to end of secondary tag switch",
			label(ElseLabel) -
				"handle next secondary tag"
		]) },
		{ ThisCode =
			tree(TestCode,
			tree(TraceCode,
			tree(GoalCode,
			tree(SaveCode,
			     GotoLabelCode))))
		},
		( { Cases0 = [_|_] } ->
			code_info__reset_to_position(BranchStart),
			tag_switch__generate_secondary_try_me_else_chain(Cases0,
				StagRval, CodeModel, CanFail, StoreMap,
				EndLabel, FailLabel, MaybeEnd1, MaybeEnd,
				OtherCode),
			{ Code = tree(ThisCode, OtherCode) }
		;
			{ FailCode = node([
				goto(label(FailLabel)) -
					"secondary tag does not match"
			]) },
			{ MaybeEnd = MaybeEnd1 },
			{ Code = tree(ThisCode, FailCode) }
		)
	;
		trace__maybe_generate_internal_event_code(Goal, TraceCode),
		code_gen__generate_goal(CodeModel, Goal, GoalCode),
		code_info__generate_branch_end(StoreMap, MaybeEnd0, MaybeEnd,
			SaveCode),
		{ GotoCode = node([
			goto(label(EndLabel)) -
				"skip to end of secondary tag switch"
		]) },
		{ Code =
			tree(TraceCode,
			tree(GoalCode,
			tree(SaveCode,
			     GotoCode)))
		}
	).

%-----------------------------------------------------------------------------%

	% Generate a switch on a secondary tag value using a try chain.

:- pred tag_switch__generate_secondary_try_chain(stag_goal_list, rval,
	code_model, can_fail, store_map, label, label, code_tree, code_tree,
	branch_end, branch_end, code_tree, code_info, code_info).
:- mode tag_switch__generate_secondary_try_chain(in, in, in, in, in,
	in, in, in, in, in, out, out, in, out) is det.

tag_switch__generate_secondary_try_chain([], _, _, _, _, _, _, _, _, _, _, _)
		-->
	{ error("generate_secondary_try_chain: empty switch") }.
tag_switch__generate_secondary_try_chain([Case0 | Cases0], StagRval,
		CodeModel, CanFail, StoreMap, EndLabel, FailLabel,
		PrevTests0, PrevCases0, MaybeEnd0, MaybeEnd, Code) -->
	{ Case0 = Secondary - Goal },
	( { Cases0 = [_|_] ; CanFail = can_fail } ->
		code_info__remember_position(BranchStart),
		code_info__get_next_label(ThisStagLabel),
		{ TestCode = node([
			if_val(binop(eq, StagRval,
					const(int_const(Secondary))),
				label(ThisStagLabel))
				- "test remote sec tag only"
		]) },
		{ LabelCode = node([
			label(ThisStagLabel) -
				"handle next secondary tag"
		]) },
		trace__maybe_generate_internal_event_code(Goal, TraceCode),
		code_gen__generate_goal(CodeModel, Goal, GoalCode),
		code_info__generate_branch_end(StoreMap, MaybeEnd0, MaybeEnd1,
			SaveCode),
		{ GotoCode = node([
			goto(label(EndLabel)) -
				"skip to end of secondary tag switch"
		]) },
		{ ThisCode =
			tree(LabelCode,
			tree(TraceCode,
			tree(GoalCode,
			tree(SaveCode,
			     GotoCode))))
		},
		{ PrevTests = tree(PrevTests0, TestCode) },
		{ PrevCases = tree(ThisCode, PrevCases0) },
		( { Cases0 = [_|_] } ->
			code_info__reset_to_position(BranchStart),
			tag_switch__generate_secondary_try_chain(Cases0,
				StagRval, CodeModel, CanFail, StoreMap,
				EndLabel, FailLabel, PrevTests, PrevCases,
				MaybeEnd1, MaybeEnd, Code)
		;
			{ FailCode = node([
				goto(label(FailLabel)) -
					"secondary tag with no code to handle it"
			]) },
			{ MaybeEnd = MaybeEnd1 },
			{ Code = tree(PrevTests, tree(FailCode, PrevCases)) }
		)
	;
		trace__maybe_generate_internal_event_code(Goal, TraceCode),
		code_gen__generate_goal(CodeModel, Goal, GoalCode),
		code_info__generate_branch_end(StoreMap, MaybeEnd0, MaybeEnd,
			SaveCode),
		{ GotoCode = node([
			goto(label(EndLabel)) -
				"skip to end of secondary tag switch"
		]) },
		{ Code =
			tree(PrevTests0,
			tree(TraceCode,
			tree(GoalCode,
			tree(SaveCode,
			tree(GotoCode,
			     PrevCases0)))))
		}
	).

%-----------------------------------------------------------------------------%

	% Generate the cases for a primary tag using a dense jump table
	% that has an entry for all possible secondary tag values.

:- pred tag_switch__generate_secondary_jump_table(stag_goal_list, int, int,
	code_model, store_map, label, label, branch_end, branch_end,
	list(label), code_tree, code_info, code_info).
:- mode tag_switch__generate_secondary_jump_table(in, in, in, in,
	in, in, in, in, out, out, out, in, out) is det.

tag_switch__generate_secondary_jump_table(CaseList, CurSecondary, MaxSecondary,
		CodeModel, StoreMap, EndLabel, FailLabel, MaybeEnd0, MaybeEnd,
		Labels, Code) -->
	( { CurSecondary > MaxSecondary } ->
		{ CaseList = [] ->
			true
		;
			error("caselist not empty when reaching limiting secondary tag")
		},
		{ MaybeEnd = MaybeEnd0 },
		{ Labels = [] },
		{ Code = empty }
	;
		{ NextSecondary is CurSecondary + 1 },
		( { CaseList = [CurSecondary - Goal | CaseList1] } ->
			code_info__get_next_label(NewLabel),
			{ LabelCode = node([
				label(NewLabel) -
					"start of case in secondary tag switch"
			]) },
			code_info__remember_position(BranchStart),
			trace__maybe_generate_internal_event_code(Goal,
				TraceCode),
			code_gen__generate_goal(CodeModel, Goal, GoalCode),
			code_info__generate_branch_end(StoreMap,
				MaybeEnd0, MaybeEnd1, SaveCode),
			( { CaseList1 = [] } ->
				[]
			;
				code_info__reset_to_position(BranchStart)
			),
			{ GotoCode = node([
				goto(label(EndLabel)) -
					"branch to end of tag switch"
			]) },
			tag_switch__generate_secondary_jump_table(CaseList1,
				NextSecondary, MaxSecondary, CodeModel,
				StoreMap, EndLabel, FailLabel,
				MaybeEnd1, MaybeEnd, OtherLabels, OtherCode),
			{ Labels = [NewLabel | OtherLabels] },
			{ Code =
				tree(LabelCode,
				tree(TraceCode,
				tree(GoalCode,
				tree(SaveCode,
				tree(GotoCode,
				     OtherCode)))))
			}
		;
			tag_switch__generate_secondary_jump_table(CaseList,
				NextSecondary, MaxSecondary, CodeModel,
				StoreMap, EndLabel, FailLabel,
				MaybeEnd0, MaybeEnd, OtherLabels, Code),
			{ Labels = [FailLabel | OtherLabels] }
		)
	).

%-----------------------------------------------------------------------------%

	% Generate the cases for a secondary tag using a binary search.
	% This invocation looks after secondary tag values in the range
	% MinPtag to MaxPtag (including both boundary values).

:- pred tag_switch__generate_secondary_binary_search(stag_goal_list, int, int,
	rval, code_model, can_fail, store_map, label, label,
	branch_end, branch_end, code_tree, code_info, code_info).
:- mode tag_switch__generate_secondary_binary_search(in, in, in,
	in, in, in, in, in, in, in, out, out, in, out) is det.

tag_switch__generate_secondary_binary_search(StagGoals, MinStag, MaxStag,
		StagRval, CodeModel, CanFail, StoreMap, EndLabel, FailLabel,
		MaybeEnd0, MaybeEnd, Code) -->
	( { MinStag = MaxStag } ->
		{ CurSec = MinStag },
		( { StagGoals = [] } ->
			% there is no code for this tag
			(
				{ CanFail = can_fail },
				{ string__int_to_string(CurSec, StagStr) },
				{ string__append("no code for ptag ", StagStr,
					Comment) },
				{ Code = node([
					goto(label(FailLabel)) -
						Comment
				]) }
			;
				{ CanFail = cannot_fail },
				{ Code = empty }
			),
			{ MaybeEnd = MaybeEnd0 }
		; { StagGoals = [CurSec - Goal] } ->
			trace__maybe_generate_internal_event_code(Goal,
				TraceCode),
			code_gen__generate_goal(CodeModel, Goal, GoalCode),
			code_info__generate_branch_end(StoreMap,
				MaybeEnd0, MaybeEnd, SaveCode),
			{ Code =
				tree(TraceCode,
				tree(GoalCode,
				     SaveCode))
			}
		;
			{ error("goallist not singleton or empty when binary search ends") }
		)
	;
		{ LowRangeEnd is (MinStag + MaxStag) // 2 },
		{ HighRangeStart is LowRangeEnd + 1 },
		{ InLowGroup = lambda([StagGoal::in] is semidet, (
			StagGoal = Stag - _,
			Stag =< LowRangeEnd
		)) },
		{ list__filter(InLowGroup, StagGoals, LowGoals, HighGoals) },
		code_info__get_next_label(NewLabel),
		{ string__int_to_string(MinStag, LowStartStr) },
		{ string__int_to_string(LowRangeEnd, LowEndStr) },
		{ string__int_to_string(HighRangeStart, HighStartStr) },
		{ string__int_to_string(MaxStag, HighEndStr) },
		{ string__append_list(["fallthrough for stags ",
			LowStartStr, " to ", LowEndStr], IfComment) },
		{ string__append_list(["code for stags ", HighStartStr,
			" to ", HighEndStr], LabelComment) },
		{ LowRangeEndConst = const(int_const(LowRangeEnd)) },
		{ TestRval = binop(>, StagRval, LowRangeEndConst) },
		{ IfCode = node([
			if_val(TestRval, label(NewLabel)) -
				IfComment
		]) },
		{ LabelCode = node([
			label(NewLabel) -
				LabelComment
		]) },

		code_info__remember_position(BranchStart),
		tag_switch__generate_secondary_binary_search(LowGoals,
			MinStag, LowRangeEnd, StagRval, CodeModel,
			CanFail, StoreMap, EndLabel, FailLabel,
			MaybeEnd0, MaybeEnd1, LowRangeCode),
		code_info__reset_to_position(BranchStart),
		tag_switch__generate_secondary_binary_search(HighGoals,
			HighRangeStart, MaxStag, StagRval, CodeModel,
			CanFail, StoreMap, EndLabel, FailLabel,
			MaybeEnd1, MaybeEnd, HighRangeCode),

		{ Code =
			tree(IfCode,
			tree(LowRangeCode,
			tree(LabelCode,
			     HighRangeCode)))
		}
	).

%-----------------------------------------------------------------------------%