Re: gcc: warn about overly aligned stack variables

[Miod Vallat]

 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.

[...]

 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.

[...]

 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.

I love this. I have verified that kernels for all our gcc4 platforms
still build with this diff.

Miod

 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 -   1.4
 +++ gcc/cfgexpand.c   20 Jun 2014 22:55:53 -
 @@ -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;
  
 

Re: gcc: warn about overly aligned stack variables

[Matthew Dempsky]

On Mon, Jun 23, 2014 at 9:59 AM, Miod Vallat m...@online.fr wrote:
 I love this. I have verified that kernels for all our gcc4 platforms
 still build with this diff.

Awesome!  I also heard back from guenther@ that he completed an i386
base build without warnings too.

Do we want to test this any further (e.g., ports), or should I go
ahead and commit?  If the latter, anyone want to step up to ok it? :)

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 -   1.4
+++ gcc/cfgexpand.c 20 Jun 2014 22:55:53 -
@@ -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;