re PR middle-end/39701 (Revision 145846 caused many test failures)

2009-04-10  Paolo Bonzini  <bonzini@gnu.org>

	PR tree-optimization/39701
	* doc/invoke.texi (Optimization Options): Document change in
	meaning and initialization of -fdelete-null-pointer-checks.

From-SVN: r145948

gcc/ChangeLog

+2009-04-10  Paolo Bonzini  <bonzini@gnu.org>
+
+	PR tree-optimization/39701
+	* doc/invoke.texi (Optimization Options): Document change in
+	meaning and initialization of -fdelete-null-pointer-checks.
+
 2009-04-10  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR middle-end/39701

gcc/doc/invoke.texi

@@ -5888,17 +5888,22 @@ Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
 
 @item -fdelete-null-pointer-checks
 @opindex fdelete-null-pointer-checks
-Use global dataflow analysis to identify and eliminate useless checks
-for null pointers.  The compiler assumes that dereferencing a null
-pointer would have halted the program.  If a pointer is checked after
-it has already been dereferenced, it cannot be null.
-
-In some environments, this assumption is not true, and programs can
-safely dereference null pointers.  Use
-@option{-fno-delete-null-pointer-checks} to disable this optimization
+Assume that programs cannot safely dereference null pointers, and that
+no code or data element resides there.  This enables simple constant
+folding optimizations at all optimization levels.  In addition, other
+optimization passes in GCC use this flag to control global dataflow
+analyses that eliminate useless checks for null pointers; these assume
+that if a pointer is checked after it has already been dereferenced,
+it cannot be null.
+
+Note however that in some environments this assumption is not true.
+Use @option{-fno-delete-null-pointer-checks} to disable this optimization
 for programs which depend on that behavior.
 
-Enabled by default.
+Some targets, especially embedded ones, disable this option at all levels.
+Otherwise it is enabled at all levels: @option{-O0}, @option{-O1},
+@option{-O2}, @option{-O3}, @option{-Os}.  Passes that use the information
+are enabled independently at different optimization levels.
 
 @item -fexpensive-optimizations
 @opindex fexpensive-optimizations