patch [6/5] check for conflict with -fstrict-volatile-bitfields and -std=

[Sandra Loosemore]

On 06/17/2013 06:02 PM, Sandra Loosemore wrote:

I had another thought:  perhaps -fstrict-volatile-bitfields could remain
the default on targets where it currently is, but it can be overridden
by an appropriate -std= option.  Perhaps also GCC could give an error if
-fstrict-volatile-bitfields is given explicitly with an incompatible
-std= option.

Like this.  This patch is intended to be applied on top of the other 5 
pieces in this series, although in theory it's independent of them.  OK 
to commit, and does this resolve the objection to part 3?

-Sandra



2013-06-19  Sandra Loosemore  <san...@codesourcery.com>

	gcc/c-family/
	* c-opts.c (c_common_post_options): Check for conflict between
	-std= and -fstrict-volatile-bitfields.

	gcc/
	* doc/invoke.texi (Code Gen Options): Document what happens when
	-fstrict-volatile-bitfields conflicts with -std=.
	Index: gcc/c-family/c-opts.c
===================================================================
--- gcc/c-family/c-opts.c	(revision 199963)
+++ gcc/c-family/c-opts.c	(working copy)
@@ -813,6 +813,18 @@ c_common_post_options (const char **pfil
   C_COMMON_OVERRIDE_OPTIONS;
 #endif
 
+  /* C11 and C++11 specify a memory model that is incompatible with
+     -fstrict-volatile-bitfields.  Warn if that option is given explicitly
+     and prevent backends from defaulting to turning it on.  */
+  if (flag_isoc11 || cxx_dialect >= cxx11)
+    {
+      if (flag_strict_volatile_bitfields > 0)
+	warning (0, "-fstrict-volatile-bitfields conflicts with the "
+		    "C11 and C++11 memory model");
+      else
+	flag_strict_volatile_bitfields = 0;
+    }
+
   /* Excess precision other than "fast" requires front-end
      support.  */
   if (c_dialect_cxx ())
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 199963)
+++ gcc/doc/invoke.texi	(working copy)
@@ -20899,4 +20899,12 @@
 AAPCS, @option{-fstrict-volatile-bitfields} is the default.
 
+Note that @option{-fstrict-volatile-bitfields} is incompatible with
+the bit-field access behavior required by the ISO C11 and C++11
+standards.  GCC warns if @option{-fstrict-volatile-bitfields} is given
+explicitly with an incompatible @option{-std=} option.  On targets
+that otherwise default to @option{-fstrict-volatile-bitfields},
+providing an incompatible @option{-std=} option implicitly disables
+@option{-fstrict-volatile-bitfields}.
+
 @item -fsync-libcalls
 @opindex fsync-libcalls