[Bug target/52419] Wrong expansion of misaligned vector store

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52419

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-02-29 13:46:01 UTC ---
We are not prepared to handle bitsize != GET_MODE_BITSIZE in expand_assignment
for the movmisalign case.  The following fixes it

Index: gcc/expr.c
===================================================================
--- gcc/expr.c  (revision 184656)
+++ gcc/expr.c  (working copy)
@@ -4687,7 +4687,13 @@ expand_assignment (tree to, tree from, b
              != CODE_FOR_nothing))
        {
          misalignp = true;
-         to_rtx = gen_reg_rtx (mode);
+         /* If we only partly store to the misaligned memory region
+            perform a read-modify-write cycle.  Otherwise use a scratch
+            register for the 'read' part.  */
+         if (bitsize != GET_MODE_BITSIZE (mode))
+           to_rtx = expand_normal (tem);
+         else
+           to_rtx = gen_reg_rtx (mode);
        }
       else
        {

of course we then generate

foo:
.LFB0:
        .cfi_startproc
        movdqu  (%rdi), %xmm0
        movhps  .LC0(%rip), %xmm0
        movdqu  %xmm0, (%rdi)
        ret

which is rather sub-optimal.  Not sure who would be supposed to fix that.
OTOH we could as well simply avoid going the movmisalign paths when
the word is not accessed in full.  But that's harder because we'd somehow
have to circumvent the expand_normal case to fall into the movmisalign
trap.