[m-rev.] for review: fix mantis bug 56

Zoltan Somogyi zs at csse.unimelb.edu.au
Mon May 26 12:26:32 AEST 2008
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Fix a bug that prevented the compiler from being able to bootstrap in debugging
grades. This was Mantis bug #56. (The bug was not affected by the presence or
absence of trailing, or the issue of debug vs decldebug.)

We have previously found that the problem could be eliminated by compiling
polymorphism.m with --no-common-struct. I isolated the problem to the code
we used to generate at the start of the else case in the only if-then-else
in fixup_pred_polymorphism when --common-struct is enabled:

	MR_r1 = MR_sv(20);
                /* Placing PredInfo1 */
        MR_r1 = MR_sv(28);
                /* Placing TypeInfo_34 (depth 1) */
        MR_sv(28) = MR_sv(29);
                /* Placing TypeInfo_37 */

The bug was of course the second assignment to r1. It was caused not by
anything done by common.m, but was a bug (an over-eager optimization)
in the code generator that just happened to be tickled *extremely* rarely.

compiler/var_locn.m:
	The bug was caused by a sometimes-false assumption. The assumption
	was that when we need to free up an lval because we are about to
	place a value in it, we can move it into the register preferred by
	follow_vars for the variable being moved, provided that register isn't
	locked.

	This is true when we are placing the arguments of a call, because the
	preferred location for any value being moved is either a stack slot
	(if the value is not an argument), or a register we haven't set yet
	(if the value is a following argument). It cannot be a register we have
	already set, because in that case there would be no need to move the
	value out of a later register.

	However, the assumption is false when placing resume vars between
	the stack and original resume maps, as in fixup_pred_polymorphism.
	In that case, we do not impose any particular order on the sequence
	of place_var operations required. The bug was that when we needed
	to free up a location (stackvar 28), we moved the value in it into r1,
	even though r1 already contained a value we needed.

	The fix is to make sure that we never pick a register currently in use.
	Since for calls, the in-use check should not fail, this should not
	lead to significantly worse code being generated.

Zoltan.

cvs diff: Diffing .
cvs diff: Diffing analysis
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cvs diff: Diffing boehm_gc/Mac_files
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cvs diff: Diffing boehm_gc/libatomic_ops-1.2/src/atomic_ops/sysdeps/sunc
cvs diff: Diffing boehm_gc/libatomic_ops-1.2/tests
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cvs diff: Diffing boehm_gc/windows-untested
cvs diff: Diffing boehm_gc/windows-untested/vc60
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cvs diff: Diffing boehm_gc/windows-untested/vc71
cvs diff: Diffing browser
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Index: compiler/var_locn.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/var_locn.m,v
retrieving revision 1.61
diff -u -b -r1.61 var_locn.m
--- compiler/var_locn.m	28 Feb 2008 00:08:33 -0000	1.61
+++ compiler/var_locn.m	25 May 2008 12:24:22 -0000
@@ -859,7 +859,7 @@
         Code, !VLI) :-
     check_var_is_unknown(!.VLI, Var),
 
-    select_preferred_reg_or_stack_check(!.VLI, Var, Lval),
+    select_preferred_reg_or_stack(!.VLI, Var, Lval),
     get_var_name(!.VLI, Var, VarName),
     list.length(Vector, Size),
     (
@@ -1316,7 +1316,7 @@
         Lval = SelectLval,
         Code = empty
     ;
-        select_preferred_reg_or_stack_check(!.VLI, Var, Lval),
+        select_preferred_reg_or_stack(!.VLI, Var, Lval),
         var_locn_place_var(ModuleInfo, Var, Lval, Code, !VLI)
     ).
 
@@ -1527,8 +1527,7 @@
             EffAffectedVars = [MovedVar]
         ),
 
-        CheckInUse = no,
-        select_preferred_reg_or_stack(!.VLI, MovedVar, Pref, CheckInUse),
+        select_preferred_reg_or_stack(!.VLI, MovedVar, Pref),
         \+ Pref = Lval,
         \+ list.member(Pref, ForbiddenLvals),
         ( \+ var_locn_lval_in_use(!.VLI, Pref) ->
@@ -1878,26 +1877,20 @@
 
 %----------------------------------------------------------------------------%
 
-:- pred select_preferred_reg_avoid(var_locn_info::in, prog_var::in,
-    list(lval)::in, lval::out) is det.
-
-select_preferred_reg_avoid(VLI, Var, Avoid, Lval) :-
-    select_preferred_reg(VLI, Var, yes, Avoid, Lval).
-
-:- pred select_preferred_reg(var_locn_info::in, prog_var::in,
-    lval::out) is det.
+:- pred select_preferred_reg(var_locn_info::in, prog_var::in, lval::out)
+    is det.
 
 select_preferred_reg(VLI, Var, Lval) :-
-    select_preferred_reg(VLI, Var, yes, [], Lval).
+    select_preferred_reg_avoid(VLI, Var, [], Lval).
 
     % Select the register into which Var should be put. If the follow_vars map
     % maps Var to a register, then select that register, unless it is already
-    % in use, and CheckInUse = yes.
+    % in use.
     %
-:- pred select_preferred_reg(var_locn_info::in, prog_var::in,
-    bool::in, list(lval)::in, lval::out) is det.
+:- pred select_preferred_reg_avoid(var_locn_info::in, prog_var::in,
+    list(lval)::in, lval::out) is det.
 
-select_preferred_reg(VLI, Var, CheckInUse, Avoid, Lval) :-
+select_preferred_reg_avoid(VLI, Var, Avoid, Lval) :-
     var_locn_get_follow_var_map(VLI, FollowVarMap),
     (
         map.search(FollowVarMap, Var, PrefLocn),
@@ -1908,12 +1901,7 @@
         (
             PrefLocn = abs_reg(N),
             PrefLval = reg(reg_r, N),
-            (
-                CheckInUse = yes,
-                \+ var_locn_lval_in_use(VLI, PrefLval)
-            ;
-                CheckInUse = no
-            ),
+            \+ var_locn_lval_in_use(VLI, PrefLval),
             \+ list.member(PrefLval, Avoid)
         ->
             Lval = PrefLval
@@ -1926,24 +1914,17 @@
 
     % Select the register or stack slot into which Var should be put. If the
     % follow_vars map maps Var to a register, then select that register,
-    % unless it is already in use and CheckInUse = yes. If the follow_vars map
-    % does not contain Var, then Var is not needed in a register in the near
-    % future, and this we select Var's stack slot, unless it is in use and
-    % CheckInUse = yes. If all else fails, we get spare, unused register.
-    % (Note that if the follow_vars pass has not been run, then all follow vars
-    % maps will be empty, which would cause this predicate to try to put far
-    % too many things in stack slots.)
-    %
-:- pred select_preferred_reg_or_stack_check(var_locn_info::in,
-    prog_var::in, lval::out) is det.
-
-select_preferred_reg_or_stack_check(VLI, Var, Lval) :-
-    select_preferred_reg_or_stack(VLI, Var, Lval, yes).
-
-:- pred select_preferred_reg_or_stack(var_locn_info::in,
-    prog_var::in, lval::out, bool::in) is det.
+    % unless it is already in use. If the follow_vars map does not contain Var,
+    % then Var is not needed in a register in the near future, and this we
+    % select Var's stack slot, unless it is in use. If all else fails, we get
+    % a spare, unused register. (Note that if the follow_vars pass has not
+    % been run, then all follow vars maps will be empty, which would cause
+    % this predicate to try to put far too many things in stack slots.)
+    %
+:- pred select_preferred_reg_or_stack(var_locn_info::in, prog_var::in,
+    lval::out) is det.
 
-select_preferred_reg_or_stack(VLI, Var, Lval, CheckInUse) :-
+select_preferred_reg_or_stack(VLI, Var, Lval) :-
     var_locn_get_follow_var_map(VLI, FollowVarMap),
     (
         map.search(FollowVarMap, Var, PrefLocn),
@@ -1954,12 +1935,7 @@
         (
             PrefLocn = abs_reg(N),
             PrefLval = reg(reg_r, N),
-            (
-                CheckInUse = yes,
                 \+ var_locn_lval_in_use(VLI, PrefLval)
-            ;
-                CheckInUse = no
-            )
         ->
             Lval = PrefLval
         ;
@@ -1970,12 +1946,7 @@
             var_locn_get_stack_slots(VLI, StackSlots),
             map.search(StackSlots, Var, StackSlotLocn),
             StackSlot = stack_slot_to_lval(StackSlotLocn),
-            (
-                CheckInUse = yes,
                 \+ var_locn_lval_in_use(VLI, StackSlot)
-            ;
-                CheckInUse = no
-            )
         ->
             Lval = StackSlot
         ;
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