Update -Warray-bounds with -fstrict-flex-arrays.

     A. add the following to clarify the relationship between -Warray-bounds
        and the LEVEL of -fstrict-flex-array:

     By default, the trailing array of a structure will be treated as a
     flexible array member by '-Warray-bounds' or '-Warray-bounds=N' if
     it is declared as either a flexible array member per C99 standard
     onwards ('[]'), a GCC zero-length array extension ('[0]'), or an
     one-element array ('[1]').  As a result, out of bounds subscripts
     or offsets into zero-length arrays or one-element arrays are not
     warned by default.

     You can add the option '-fstrict-flex-arrays' or
     '-fstrict-flex-arrays=LEVEL' to control how this option treat
     trailing array of a structure as a flexible array member.

     when LEVEL<=1, no change to the default behavior.

     when LEVEL=2, additional warnings will be issued for out of bounds
     subscripts or offsets into one-element arrays;

     when LEVEL=3, in addition to LEVEL=2, additional warnings will be
     issued for out of bounds subscripts or offsets into zero-length
     arrays.

     B. change -Warray-bounds=2 to exclude its control on how to treat
        trailing arrays as flexible array members:

     '-Warray-bounds=2'
          This warning level also warns about the intermediate results
          of pointer arithmetic that may yield out of bounds values.
          This warning level may give a larger number of false positives
          and is deactivated by default.

gcc/ChangeLog:

	* attribs.cc (strict_flex_array_level_of): New function.
	* attribs.h (strict_flex_array_level_of): Prototype for new function.
	* doc/invoke.texi: Update -Warray-bounds by specifying the impact from
	-fstrict-flex-arrays. Also update -Warray-bounds=2 by eliminating its
	impact on treating trailing arrays as flexible array members.
	* gimple-array-bounds.cc (get_up_bounds_for_array_ref): New function.
	(check_out_of_bounds_and_warn): New function.
	(array_bounds_checker::check_array_ref): Update with call to the above
	new functions.
	* tree.cc (array_ref_flexible_size_p): Add one new argument.
	(component_ref_sam_type): New function.
	(component_ref_size): Control with level of strict-flex-array.
	* tree.h (array_ref_flexible_size_p): Update prototype.
	(enum struct special_array_member): Add two new enum values.
	(component_ref_sam_type): New prototype.

gcc/c/ChangeLog:

	* c-decl.cc (is_flexible_array_member_p): Call new function
	strict_flex_array_level_of.

gcc/testsuite/ChangeLog:

	* gcc.dg/Warray-bounds-11.c: Update warnings for -Warray-bounds=2.
	* gcc.dg/Warray-bounds-flex-arrays-1.c: New test.
	* gcc.dg/Warray-bounds-flex-arrays-2.c: New test.
	* gcc.dg/Warray-bounds-flex-arrays-3.c: New test.
	* gcc.dg/Warray-bounds-flex-arrays-4.c: New test.
	* gcc.dg/Warray-bounds-flex-arrays-5.c: New test.
	* gcc.dg/Warray-bounds-flex-arrays-6.c: New test.

1 files changed, 22 insertions, 5 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 277ac35ad16..726392409b6 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -8040,17 +8040,34 @@ is enabled by @option{-Wall}.  It is more effective when @option{-ftree-vrp}
 is active (the default for @option{-O2} and above) but a subset of instances
 are issued even without optimization.
 
+By default, the trailing array of a structure will be treated as a flexible
+array member by @option{-Warray-bounds} or @option{-Warray-bounds=@var{n}}
+if it is declared as either a flexible array member per C99 standard onwards
+(@samp{[]}), a GCC zero-length array extension (@samp{[0]}), or an one-element
+array (@samp{[1]}). As a result, out of bounds subscripts or offsets into
+zero-length arrays or one-element arrays are not warned by default.
+
+You can add the option @option{-fstrict-flex-arrays} or
+@option{-fstrict-flex-arrays=@var{level}} to control how this
+option treat trailing array of a structure as a flexible array member:
+
+when @var{level}<=1, no change to the default behavior.
+
+when @var{level}=2, additional warnings will be issued for out of bounds
+subscripts or offsets into one-element arrays;
+
+when @var{level}=3, in addition to @var{level}=2, additional warnings will be
+issued for out of bounds subscripts or offsets into zero-length arrays.
+
 @table @gcctabopt
 @item -Warray-bounds=1
 This is the default warning level of @option{-Warray-bounds} and is enabled
 by @option{-Wall}; higher levels are not, and must be explicitly requested.
 
 @item -Warray-bounds=2
-This warning level also warns about out of bounds accesses to trailing
-struct members of one-element array types (@pxref{Zero Length}) and about
-the intermediate results of pointer arithmetic that may yield out of bounds
-values.  This warning level may give a larger number of false positives and
-is deactivated by default.
+This warning level also warns about the intermediate results of pointer
+arithmetic that may yield out of bounds values. This warning level may
+give a larger number of false positives and is deactivated by default.
 @end table
 
 @item -Warray-compare