find: by default, disable the cost-based optimiser.

The cost-based optimiser re-orders predicates based on their expected
cost.  This re-ordering (as currently implemented) results in
user-visible changes to the order of operations.  An optimiser should
not do that.  For example, "-empty -readable" and "-readable -empty"
don't actually have the same effect since "-empty" fails on an
unreadable directory.  This fixes savannah bug #58427 (unless the user
specifies -O2).

* find/util.c(set_option_defaults): set default optimisation level to
1 instead of 2.
* find/tree.c(build_expression_tree): call do_arm_swaps (i.e. apply
cost-based optimisations) only at optimisation level 2 and above.
* find/find.1(-O): explain this change.
* doc/find.texi(Optimisation Options): explain this change.
* NEWS: mention this change.

NEWS

@@ -3,6 +3,11 @@ GNU findutils NEWS - User visible changes.      -*- outline -*- (allout)
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
 ** Bug Fixes
+  Find now defaults to optimisation level 1 rather than 2 and the
+  cost-based optimiser will only run at level 2 and above.  This
+  should prevent changes of operation order which result in
+  user-visible differences in behaviour.
+
   If the -P option to xargs is not used, xargs will not change the way
   in which the SIGUSR1 and SIGUSR2 signals are handled.  This means that
   they will cause the program to terminate if the signals were not

doc/find.texi

@@ -3526,12 +3526,13 @@ Use of an unrecognised formatting directive with @samp{-fprintf}
 The @samp{-O@var{level}} option sets @code{find}'s optimisation level
 to @var{level}.  The default optimisation level is 1.
 
-At certain optimisation levels, @code{find} reorders tests to speed up
-execution while preserving the overall effect; that is, predicates
-with side effects are not reordered relative to each other.  The
-optimisations performed at each optimisation level are as follows.
+At certain optimisation levels (but not by default), @code{find}
+reorders tests to speed up execution while preserving the overall
+effect; that is, predicates with side effects are not reordered
+relative to each other.  The optimisations performed at each
+optimisation level are as follows.
 
-@table @samp
+@table @asis
 @item 0
 Currently equivalent to optimisation level 1.
 
@@ -3553,7 +3554,6 @@ type @samp{FOO} which is not known (that is, present in
 @file{/etc/mtab}) at the time @code{find} starts, that predicate is
 equivalent to @samp{-false}.
 
-
 @item 3
 At this optimisation level, the full cost-based query optimiser is
 enabled.  The order of tests is modified so that cheap (i.e., fast)
@@ -3565,6 +3565,14 @@ earlier, and for @samp{-a}, predicates which are likely to fail are
 evaluated earlier.
 @end table
 
+The re-ordering of operations performed by the cost-based optimiser
+can result in user-visible behaviour change.  For example, the
+@samp{-readable} and @samp{-empty} predicates are sensitive to
+re-ordering.  If they are run in the order @samp{-empty -readable}, an
+error message will be issued for unreadable directories.  If they are
+run in the order @samp{-readable -empty}, no error message will be
+issued. This is the reason why such operation re-ordering is not
+performed at the default optimisation level.
 
 @node Debug Options
 @subsection Debug Options

find/find.1

@@ -325,6 +325,19 @@ level 1) will not be changed in the 4.3.x release series.  The
 findutils test suite runs all the tests on
 .B find
 at each optimisation level and ensures that the result is the same.
+
+The re-ordering of operations performed by the cost-based optimiser
+can result in user-visible behaviour change.  For example, the
+.B \-readable
+and
+.B \-empty
+predicates are sensitive to re-ordering.  If they are run in the order
+.BR "\-empty \-readable" ,
+an error message will be issued for unreadable directories.  If they
+are run in the order
+.B \-readable \-empty
+no error message will be issued. This is the reason why such operation
+re-ordering is not performed at the default optimisation level.
 .
 .SH EXPRESSION
 The part of the command line after the list of starting points is the

find/tree.c

@@ -1430,7 +1430,10 @@ build_expression_tree (int argc, char *argv[], int end_of_leading_options)
   /* Check that the tree is in normalised order (opt_expr does this) */
   check_normalization (eval_tree, true);
 
-  do_arm_swaps (eval_tree);
+  if (options.optimisation_level > 1)
+    {
+      do_arm_swaps (eval_tree);
+    }
 
   /* Check that the tree is still in normalised order */
   check_normalization (eval_tree, true);

find/util.c

@@ -1035,7 +1035,7 @@ set_option_defaults (struct options *p)
     p->output_block_size = 1024;
 
   p->debug_options = 0uL;
-  p->optimisation_level = 2;
+  p->optimisation_level = 1;
 
   if (getenv ("FIND_BLOCK_SIZE"))
     {