PR ada/20548

	* common.opt (-fstack-check): Do not declare the variable here.
	(-fstack-check=): New option variant.
	* doc/invoke.texi (Code Gen Options): Document it.
	* expr.h (STACK_OLD_CHECK_PROTECT): New macro.
	(STACK_CHECK_PROTECT): Bump to 3 pages if DWARF-2 EH is used.
	(STACK_CHECK_STATIC_BUILTIN): New macro.
	* doc/tm.texi (Stack Checking): Document STACK_CHECK_STATIC_BUILTIN.
	* opts.c: Include expr.h.
	(common_handle_option) <OPT_fold_stack_check_>: New case.
	<OPT_fstack_check>: Likewise.
	* calls.c (initialize_argument_information): Use TYPE_SIZE_UNIT
	consistently in the test for variable-sized types.  Adjust for
	new behaviour of flag_stack_check.
	* explow.c: Include except.h.
	(allocate_dynamic_stack_space): Do not take into account
	STACK_CHECK_MAX_FRAME_SIZE for static builtin stack checking.
	* function.c (gimplify_parameters): Use DECL_SIZE_UNIT in the test
	for variable-sized parameters.  Treat all parameters whose size is
	greater than STACK_CHECK_MAX_VAR_SIZE as variable-sized if generic
	stack checking is enabled.
	* gimplify.c (gimplify_decl_expr): Treat non-static objects whose
	size is greater than STACK_CHECK_MAX_VAR_SIZE as variable-sized
	if generic stack checking is enabled.
	(expand_function_end): Adjust for new behaviour of flag_stack_check.
	* reload1.c (reload): Likewise.
	* stmt.c (expand_decl): Assert that all automatic variables have
	fixed size at this point and remove dead code.
	* flags.h (stack_check_type): New enumeration type.
	(flag_stack_check): Declare.
	* toplev.c (flag_stack_check): New global variable.
	* Makefile.in (opts.o): Add dependency on EXPR_H.
	(explow.o): Add dependency on except.h.
ada/
	* gcc-interface/decl.c (gnat_to_gnu_entity): Use DECL_SIZE_UNIT in the
	setjmp test consistently.  Adjust for new behaviour of flag_stack_check.
	* gcc-interface/utils2.c (build_call_alloc_dealloc): Remove redundant
	test of flag_stack_check.  Adjust for new behaviour of flag_stack_check.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@139159 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 28 insertions, 2 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 56a0bdadc55..40c3df16594 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -15207,8 +15207,34 @@ a single-threaded environment since stack overflow is automatically
 detected on nearly all systems if there is only one stack.
 
 Note that this switch does not actually cause checking to be done; the
-operating system must do that.  The switch causes generation of code
-to ensure that the operating system sees the stack being extended.
+operating system or the language runtime must do that.  The switch causes
+generation of code to ensure that they see the stack being extended.
+
+You can additionally specify a string parameter: @code{no} means no
+checking, @code{generic} means force the use of old-style checking,
+@code{specific} means use the best checking method and is equivalent
+to bare @option{-fstack-check}.
+
+Old-style checking is a generic mechanism that requires no specific
+target support in the compiler but comes with the following drawbacks:
+
+@enumerate
+@item
+Modified allocation strategy for large objects: they will always be
+allocated dynamically if their size exceeds a fixed threshold.
+
+@item
+Fixed limit on the size of the static frame of functions: when it is
+topped by a particular function, stack checking is not reliable and
+a warning is issued by the compiler.
+
+@item
+Inefficiency: because of both the modified allocation strategy and the
+generic implementation, the performances of the code are hampered.
+@end enumerate
+
+Note that old-style stack checking is also the fallback method for
+@code{specific} if no target support has been added in the compiler.
 
 @item -fstack-limit-register=@var{reg}
 @itemx -fstack-limit-symbol=@var{sym}