[Bug target/67366] Poor assembly generation for unaligned memory accesses on ARM v6 & v7 cpus

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67366

--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 27 Aug 2015, rearnsha at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67366
> 
> --- Comment #2 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
> (In reply to Richard Biener from comment #1)
> > I think this boils down to the fact that memcpy expansion is done too late
> > and
> > that (with more recent GCC) the "inlining" done on the GIMPLE level is
> > restricted
> > to !SLOW_UNALIGNED_ACCESS but arm defines STRICT_ALIGNMENT to 1
> > unconditionally.
> > 
> 
> Yep, we have to define STRICT_ALIGNMENT to 1 because not all load instructions
> work with misaligned addresses (ldm, for example).  The only way to handle
> misaligned copies is through the movmisalign API.

Are the movmisalign handled ones reasonably efficient?  That is, more
efficient than memcpy/memmove?  Then we should experiment with

Index: gcc/gimple-fold.c
===================================================================
--- gcc/gimple-fold.c   (revision 227252)
+++ gcc/gimple-fold.c   (working copy)
@@ -708,7 +708,9 @@ gimple_fold_builtin_memory_op (gimple_st
                  /* If the destination pointer is not aligned we must be 
able
                     to emit an unaligned store.  */
                  && (dest_align >= GET_MODE_ALIGNMENT (TYPE_MODE (type))
-                     || !SLOW_UNALIGNED_ACCESS (TYPE_MODE (type), 
dest_align)))
+                     || !SLOW_UNALIGNED_ACCESS (TYPE_MODE (type), 
dest_align)
+                     || (optab_handler (movmisalign_optab, TYPE_MODE 
(type))
+                         != CODE_FOR_nothing)))
                {
                  tree srctype = type;
                  tree desttype = type;
@@ -720,7 +722,10 @@ gimple_fold_builtin_memory_op (gimple_st
                    srcmem = tem;
                  else if (src_align < GET_MODE_ALIGNMENT (TYPE_MODE 
(type))
                           && SLOW_UNALIGNED_ACCESS (TYPE_MODE (type),
-                                                    src_align))
+                                                    src_align)
+                          && (optab_handler (movmisalign_optab,
+                                             TYPE_MODE (type))
+                              == CODE_FOR_nothing))
                    srcmem = NULL_TREE;
                  if (srcmem)
                    {