gcc: warn about overly aligned stack variables

[Matthew Dempsky]

GCC supports an "aligned" attribute to specify a minimum alignment for
types/objects.  However, if an object is allocated on the stack and
its alignment exceeds the "preferred stack boundary", then GCC 4.2
silently ignores the alignment.

This bit us 4 years ago when the SCSI stack started allocating mutexes
on the stack: HPPA mutexes need to be 16-byte aligned, but HPPA's
stack is naturally only 8-byte aligned.

Since newer GCC properly support overly aligned stack allocations, it
seems prudent to at least make base GCC emit a warning if its going to
ignore an alignment request.

With the diff below, compiling a source file like this:

        typedef int __attribute__((aligned(512))) aligned_int;

        aligned_int good;

        void foo() {
                aligned_int bad;
        }

now produces a warning like this:

        $ cc -c test.c
        test.c: In function 'foo':
        test.c:6: warning: ignoring alignment for stack allocated 'bad'

I verified this doesn't break an amd64 kernel build, but I haven't had
time to look beyond that.  Sharing in case anyone's interested and/or
wants to test further themselves.

Index: gcc/cfgexpand.c
===================================================================
RCS file: /home/matthew/cvs-mirror/cvs/src/gnu/gcc/gcc/cfgexpand.c,v
retrieving revision 1.4
diff -u -p -r1.4 cfgexpand.c
--- gcc/cfgexpand.c     6 May 2014 23:32:34 -0000       1.4
+++ gcc/cfgexpand.c     20 Jun 2014 22:55:53 -0000
@@ -159,8 +159,10 @@ get_decl_align_unit (tree decl)
 
   align = DECL_ALIGN (decl);
   align = LOCAL_ALIGNMENT (TREE_TYPE (decl), align);
-  if (align > PREFERRED_STACK_BOUNDARY)
+  if (align > PREFERRED_STACK_BOUNDARY) {
+    warning (0, "ignoring alignment for stack allocated %q+D", decl);
     align = PREFERRED_STACK_BOUNDARY;
+  }
   if (cfun->stack_alignment_needed < align)
     cfun->stack_alignment_needed = align;