Re: [PATCH i386 5/8] [AVX-512] Extend vectorizer hooks.

[Uros Bizjak]

On Thu, Jan 2, 2014 at 11:18 PM, Eric Botcazou <ebotca...@adacore.com> wrote:
>> Note that it has unexpected side-effects: previously, in 32-bit mode,
>> 256-bit aggregate objects would have been given 256-bit alignment; now,
>> they will fall back to default alignment, for example 32-bit only.
>
> In case this wasn't clear enough, just compile in 32-bit mode:
>
> int a[8] = { 1, 2, 3, 4, 5, 6, 7, 8};

It looks to me that we don't need to adjust anything with max_align.
Using following patch:

--cut here--
Index: i386.c
===================================================================
--- i386.c      (revision 206311)
+++ i386.c      (working copy)
@@ -26465,6 +26465,7 @@
 int
 ix86_data_alignment (tree type, int align, bool opt)
 {
+#if 0
   int max_align = optimize_size ? BITS_PER_WORD
                                : MIN (512, MAX_OFILE_ALIGNMENT);

@@ -26476,6 +26477,7 @@
          || TREE_INT_CST_HIGH (TYPE_SIZE (type)))
       && align < max_align)
     align = max_align;
+#endif

   /* x86-64 ABI requires arrays greater than 16 bytes to be aligned
      to 16byte boundary.  */
--cut here--

and following testcase:

-- cut here--
float a[8] = { 1, 2, 3, 4, 5, 6, 7, 8};

extern float z[8];

void t (void)
{
  int i;

  for (i = 0; i < 8; i++)
    z[i] = z[i] + a[i];
}
--cut here--

When compiled with -m32 -mavx, we get:

        .align 32
        .type   a, @object
        .size   a, 32
a:

so, the alignment was already raised elsewhere. We get .align 16 for
-msse -m32 when vectorizing.

without -msse (and consequently without vectorizing), we get for -m32:

        .align 4
        .type   a, @object
        .size   a, 32
a:

which corresponds to 32bit ABI rules (we still get .align16 for 64bit ABI).

What bothers me in this testcase is (unrelated) alignment of z[8]
array. Even for 64bit targets, we get:

#(insn:TI 6 5 8 2 (set (reg:V4SF 21 xmm0 [orig:90 vect__4.5 ] [90])
#        (unspec:V4SF [
#                (mem/c:V4SF (reg/f:DI 0 ax [89]) [2 MEM[(float
*)&z]+0 S16 A32])
#            ] UNSPEC_LOADU)) al.c:10 1195 {*sse_loadups}
#     (nil))
        movups  (%rax), %xmm0   # 6     *sse_loadups    [length = 3]

ABI guarantees 16 byte alignment of z[8], but we fail to communicate
this to the compiler.

Uros.