Delete the old tree.m module (which did a small subset of what cords now do),

Estimated hours taken: 6
Branches: main

Delete the old tree.m module (which did a small subset of what cords now do),
and switch to using cords instead. This is more standard, as well as very
slightly more efficient, because with cords, e.g. concatenating ten code
fragments of which eight are empty doesn't allocate ten cons cells.
My measurements show a 0.1% reduction in executable size and a 0.3% reduction
in compilation time. Both of those are in the noise; the main reason for the
change is more convenient coding.

compiler/tree.m:
	Remove this module.

compiler/libs.m:
	Remove the inclusion of tree.m.

compiler/notes/compiler_design.html:
	Remove the description of tree.m.

compiler/bytecode.m:
	Switch to using cords to represent code in the bytecode backend.

compiler/llds.m:
	Switch to using cords to represent code in the LLDS backend.

compiler/mlds_to_il.m:
	Switch to using cords to represent IL code being built.

compiler/bytecode_gen.m:
compiler/call_gen.m:
compiler/code_gen.m:
compiler/code_info.m:
compiler/commit_gen.m:
compiler/dense_switch.m:
compiler/disj_gen.m:
compiler/ite_gen.m:
compiler/lookup_switch.m:
compiler/lookup_util.m:
compiler/middle_rec.m:
compiler/par_conj_gen.m:
compiler/pragma_c_gen.m:
compiler/proc_gen.m:
compiler/string_switch.m:
compiler/switch_case.m:
compiler/switch_gen.m:
compiler/tag_switch.m:
compiler/trace_gen.m:
compiler/unify_gen.m:
compiler/var_locn.m:
	Conform to the changes above.

library/cord.m:
	Add a predicate form of map.

compiler/code_gen.m

@@ -34,7 +34,7 @@
 
     % Translate a HLDS goal to LLDS.
     %
-:- pred generate_goal(code_model::in, hlds_goal::in, code_tree::out,
+:- pred generate_goal(code_model::in, hlds_goal::in, llds_code::out,
     code_info::in, code_info::out) is det.
 
 %---------------------------------------------------------------------------%
@@ -47,7 +47,6 @@
 :- import_module hlds.instmap.
 :- import_module libs.compiler_util.
 :- import_module libs.globals.
-:- import_module libs.tree.
 :- import_module ll_backend.call_gen.
 :- import_module ll_backend.commit_gen.
 :- import_module ll_backend.disj_gen.
@@ -60,6 +59,7 @@
 :- import_module parse_tree.prog_data.
 
 :- import_module bool.
+:- import_module cord.
 :- import_module io.
 :- import_module list.
 :- import_module map.
@@ -145,7 +145,7 @@ generate_goal(ContextModel, Goal, Code, !CI) :-
             get_maybe_trace_info(!.CI, yes(_))
         ->
             save_variables_on_stack([CallTableVar], TipSaveCode, !CI),
-            CodeUptoTip = tree(GoalCode, TipSaveCode)
+            CodeUptoTip = GoalCode ++ TipSaveCode
         ;
             CodeUptoTip = GoalCode
         ),
@@ -167,7 +167,7 @@ generate_goal(ContextModel, Goal, Code, !CI) :-
         ( set.member(feature_save_deep_excp_vars, Features) ->
             DeepSaveVars = compute_deep_save_excp_vars(ProcInfo),
             save_variables_on_stack(DeepSaveVars, DeepSaveCode, !CI),
-            Code = tree(CodeUptoTip, DeepSaveCode)
+            Code = CodeUptoTip ++ DeepSaveCode
         ;
             Code = CodeUptoTip
         ),
@@ -187,8 +187,7 @@ generate_goal(ContextModel, Goal, Code, !CI) :-
 
             ( should_trace_code_gen(!.CI) ->
                 io.format("\nGOAL FINISH: %s\n", [s(GoalDesc)], !IO),
-                InstrLists = tree.flatten(Code),
-                list.condense(InstrLists, Instrs),
+                Instrs = cord.list(Code),
                 write_instrs(Instrs, no, yes, !IO)
             ;
                 true
@@ -230,7 +229,7 @@ compute_deep_save_excp_vars(ProcInfo) = DeepSaveVars :-
 %---------------------------------------------------------------------------%
 
 :- pred generate_goal_2(hlds_goal_expr::in, hlds_goal_info::in,
-    code_model::in, set(prog_var)::in, code_tree::out,
+    code_model::in, set(prog_var)::in, llds_code::out,
     code_info::in, code_info::out) is det.
 
 generate_goal_2(GoalExpr, GoalInfo, CodeModel, ForwardLiveVarsBeforeGoal,
@@ -242,7 +241,8 @@ generate_goal_2(GoalExpr, GoalInfo, CodeModel, ForwardLiveVarsBeforeGoal,
         GoalExpr = conj(ConjType, Goals),
         (
             ConjType = plain_conj,
-            generate_goals(Goals, CodeModel, Code, !CI)
+            generate_goals(Goals, CodeModel, Codes, !CI),
+            Code = cord_list_to_cord(Codes)
         ;
             ConjType = parallel_conj,
             par_conj_gen.generate_par_conj(Goals, GoalInfo, CodeModel, Code,
@@ -318,17 +318,16 @@ generate_goal_2(GoalExpr, GoalInfo, CodeModel, ForwardLiveVarsBeforeGoal,
     % to the next.
     %
 :- pred generate_goals(hlds_goals::in, code_model::in,
-    code_tree::out, code_info::in, code_info::out) is det.
+    list(llds_code)::out, code_info::in, code_info::out) is det.
 
-generate_goals([], _, empty, !CI).
-generate_goals([Goal | Goals], CodeModel, Code, !CI) :-
-    generate_goal(CodeModel, Goal, Code1, !CI),
+generate_goals([], _, [], !CI).
+generate_goals([Goal | Goals], CodeModel, [Code | Codes], !CI) :-
+    generate_goal(CodeModel, Goal, Code, !CI),
     get_instmap(!.CI, Instmap),
     ( instmap_is_unreachable(Instmap) ->
-        Code = Code1
+        Codes = []
     ;
-        generate_goals(Goals, CodeModel, Code2, !CI),
-        Code = tree(Code1, Code2)
+        generate_goals(Goals, CodeModel, Codes, !CI)
     ).
 
 %---------------------------------------------------------------------------%