[m-rev.] diff: make --should-pretest-equality the default

[Zoltan Somogyi]

This change is based on the recently posted benchmark results, and has
effectively already been reviewed by Julien.

Zoltan.

compiler/options.m:
doc/user_guide.texi:
	Make --should-pretest-equality the default.

runtime/mercury_unify_compare_body.h:
	Do not do the pretest in builtin.unify or builtin.compare, since
	it is now done in the compiler-generated predicates they invoke.

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Index: compiler/options.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/options.m,v
retrieving revision 1.554
diff -u -r1.554 options.m
--- compiler/options.m	13 Apr 2007 04:56:40 -0000	1.554
+++ compiler/options.m	14 Apr 2007 06:10:58 -0000
@@ -1189,7 +1189,7 @@
                                         % -1 asks handle_options.m to give
                                         % the value, which may be grade
                                         % dependent.
-    should_pretest_equality             -   bool(no),
+    should_pretest_equality             -   bool(yes),
     fact_table_max_array_size           -   int(1024),
     fact_table_hash_percent_full        -   int(90),
     gcc_local_labels                    -   bool(no),
@@ -3967,9 +3967,9 @@
 %       "\tfaster but also bigger compare predicates.",
 
         % This is a developer only option.
-%       "--should-pretest-equality",
-%       "\tIf specified, add a test for the two values being equal as words",
-%       "\tto the starts of potentially expensive unify and compare",
+%       "--no-should-pretest-equality",
+%       "\tIf specified, do not add a test for the two values being equal",
+%       "\tas words to the starts of potentially expensive unify and compare",
 %       "\tpredicates."
 
         "--fact-table-max-array-size <n>",
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Index: doc/user_guide.texi
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/doc/user_guide.texi,v
retrieving revision 1.517
diff -u -r1.517 user_guide.texi
--- doc/user_guide.texi	13 Apr 2007 04:56:43 -0000	1.517
+++ doc/user_guide.texi	14 Apr 2007 06:11:48 -0000
@@ -7600,9 +7600,10 @@
 % but also bigger compare predicates.
 
 % @sp 1
-% @item --should-pretest-equality
+% @item --no-should-pretest-equality
+% @findex --no-should-pretest-equality
 % @findex --should-pretest-equality
-% If specified, add a test for the two values being equal as words
+% If specified, do not add a test for the two values being equal as words
 % to the starts of potentially expensive unify and compare predicates.
 
 @sp 1
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Index: runtime/mercury_unify_compare_body.h
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/runtime/mercury_unify_compare_body.h,v
retrieving revision 1.46
diff -u -r1.46 mercury_unify_compare_body.h
--- runtime/mercury_unify_compare_body.h	13 Apr 2007 04:56:45 -0000	1.46
+++ runtime/mercury_unify_compare_body.h	14 Apr 2007 06:18:31 -0000
@@ -449,7 +449,8 @@
             ** the execution of the call port code and then of either the exit
             ** or the fail port code.
             **
-            ** That is a problem. First, at the moment there is no fast way
+            ** That would be a problem if you wanted to pretest x == y here.
+            ** First, at the moment there is no simple or fast way
             ** to get from the type_ctor_info (which we have) to the proc
             ** layout structures of the type's unify or compare predicates
             ** (which port codes need). Second, even if we put the addresses
@@ -460,18 +461,19 @@
             ** without making calls (since those calls are being
             ** short-circuited here).
             **
-            ** Our solution is two fold. First, in deep-profiling grades,
-            ** we don't check x == y here; instead, we make sure that the
-            ** compiler-generated unify and compare predicates start off
-            ** with that check. Second, since this solution increases both
-            ** code size and execution time, in non-deep-profiling grades
-            ** we do the x == y check here, and not in the compiler-generated
-            ** unify or compare predicates.
-            **
+            ** Our solution is to check x == y not here, but at the starts of
+            ** compiler-generated unify and compare predicates. This delays
+            ** the check until after the extraction of any typeinfos
+            ** representing the arguments of the main type constructor,
+            ** but it also ensures that the check is performed not just on the
+            ** top function symbols of the terms being unified or compared,
+            ** but on all other function symbols too. Measurements show that
+            ** the overhead of these extra tests is less than the amount of
+            ** work that these extra pretests avoid.
             */
 
 #ifndef MR_DEEP_PROFILING
-  #ifdef MR_CHECK_DU_EQ
+  #if 0
     #ifdef  select_compare_code
             if (x == y) {
                 raw_return_answer(MR_COMPARE_EQUAL);
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