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From: "siarhei.siamashka at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/55634] New: ARM: gcc vector extensions: storing vector to unaligned memory location does not use VST1.8 NEON instruction
Date: Sun, 09 Dec 2012 22:16:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55634

             Bug #: 55634
           Summary: ARM: gcc vector extensions: storing vector to
                    unaligned memory location does not use VST1.8 NEON
                    instruction
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: siarhei.siamashka@gmail.com


The following test program tries to use GCC vector extensions to add two
vectors together and store the result to unaligned memory location in a
"portable" way with memcpy:

/***********************************************/

#include <string.h>

typedef unsigned int T __attribute__ ((vector_size (16)));

void foo (void *result, T *a, T *b)
{
  T tmp = *a + *b;
  memcpy (result, &tmp, sizeof(tmp));
}

/***********************************************/

Compiling with gcc 4.7.2:

$ arm-none-linux-gnueabi-gcc -O2 -mcpu=cortex-a8 -mfpu=neon -c test.c
$ objdump -d test.o

00000000 <foo>:
   0:    e52d4004     push    {r4}        ; (str r4, [sp, #-4]!)
   4:    ecd12b04     vldmia    r1, {d18-d19}
   8:    e24dd014     sub    sp, sp, #20
   c:    ecd20b04     vldmia    r2, {d16-d17}
  10:    e28dc010     add    ip, sp, #16
  14:    f26208e0     vadd.i32    q8, q9, q8
  18:    ed6c0b04     vstmdb    ip!, {d16-d17}
  1c:    e1a0c00d     mov    ip, sp
  20:    e1a04000     mov    r4, r0
  24:    e8bc000f     ldm    ip!, {r0, r1, r2, r3}
  28:    e5840000     str    r0, [r4]
  2c:    e5841004     str    r1, [r4, #4]
  30:    e5842008     str    r2, [r4, #8]
  34:    e584300c     str    r3, [r4, #12]
  38:    e28dd014     add    sp, sp, #20
  3c:    e8bd0010     pop    {r4}
  40:    e12fff1e     bx    lr

The same test program results in the following code if compiled for x86-64:

0000000000000000 <foo>:
   0:    66 0f 6f 06              movdqa (%rsi),%xmm0
   4:    66 0f fe 02              paddd  (%rdx),%xmm0
   8:    f3 0f 7f 07              movdqu %xmm0,(%rdi)
   c:    c3                       retq   

So x86-64 target is able to use MOVDQU instruction. Hence ARM target should be
able to use VST1.8 as well.