[Bug c++/64655] New: Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[Bug c++/64655] New: Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

            Bug ID: 64655
           Summary: Vectorizer is always using load aligned instructions
                    with objects with the "aligned" attribute
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: adrien at guinet dot me

I think there is an issue with the way gcc uses the __attribute__ information
when generating SIMD load instructions during auto vectorization.

For instance, using a structure like this one:

struct A
{
        uint32_t i[N] __attribute__((aligned(32)));
        uint32_t j[N] __attribute__((aligned(32)));
        uint32_t r[N] __attribute__((aligned(32)));
};

with a loop like this one:

struct A *a = (struct A*) malloc(sizeof(struct A));
for (size_t i = 0; i < N; i++) {
   a->r[i] = a->i[i] + a->j[i];
}

then the vectorizer will use "load aligned" instructions. The issue is that,
even if i, j and r are aligned *inside* the A structure, nothing tells gcc that
the "a" pointer is actually correctly aligned.

You can reproduce this using the attached "test_align.c" test case, and
compiling it like this (using AVX2 for instance which needs 32-bytes
alignment):

$ gcc -O3 -march=core-avx2 test_align.c -std=c99 -o test_align

Running it on a computer supporting AVX2 will segfault. If you don't have AVX2
on your machine, you can use the excellent Intel SDE
(https://software.intel.com/en-us/articles/intel-software-development-emulator)
which will use PIN to "emulate" AVX2. Running the sample under SDE will even
tell us this:

$ sde64 -- ./test_align 
SDE ERROR:  TID: 0 executed instruction with an unaligned memory reference to
address 0x602ab0 INSTR: 0x000400550: IFORM: VMOVDQA_YMMqq_MEMqq :: vmovdqa
ymm0, ymmword ptr [rax+0x1aa0]
        IMAGE:    /tmp/test_align
        FUNCTION: main

Indeed, if we take a look at the assembly produced, we see:

[.. init code with rand() calls .., then, without any guards checking aligned
pointers]
lea     rdx, [r13+1A80h]
mov     rax, r13
loc_400550:
  vmovdqa ymm0, ymmword ptr [rax+1AA0h]  <- aligned load
  vpaddd  ymm0, ymm0, ymmword ptr [rax]  <- aligned load
  add     rax, 20h
  vmovdqa ymmword ptr [rax+3520h], ymm0 <- aligned store
  cmp     rax, rdx
  jnz     short loc_400550

If we remove the aligned attributes, ending with this structure:

struct A
{
    uint32_t i[N];
    uint32_t j[N];
    uint32_t r[N];
}

then gcc generates guard to check for unaligned pointers, and everything runs
fine!

Note that clang uses unaligned loads even with the aligned attributes (and thus
the binary does not segfault). The disassembly from the binary produced with
clang 3.5 is this one:

mov     rax, 0FFFFFFFFFFFFF960h
loc_400670:                             
  vmovdqu ymm0, ymmword ptr [r14+rax*4+3510h]
  vpaddd  ymm0, ymm0, ymmword ptr [r14+rax*4+1A80h]
  vmovdqu ymmword ptr [r14+rax*4+4FA0h], ymm0
  add     rax, 8
  jnz     short loc_400670

Bug seen in GCC 4.8.3 and GCC 4.9.2.

Thanks for any thoughts about this!

Regards,

Adrien.

[Bug c++/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

--- Comment #1 from Adrien Guinet <adrien at guinet dot me> ---
Created attachment 34476
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34476&action=edit
test case reproducing the issue

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
That is a user error.  malloc doesn't guarantee bigger alignment than alignment
of basic C types.  If you need to allocate heap memory with bigger alignments,
you need to use different allocation functions, like posix_memalign, memalign,
aligned_alloc, valloc, pvalloc or mmap.

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

--- Comment #3 from Adrien Guinet <adrien at guinet dot me> ---
I don't think this is a user error. Some attributes might be declared aligned
inside a structure, without any specifications that every instanced objects
must be themselves aligned on a specific boundary. This is espacially true with
C++ !

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

Adrien Guinet <adrien at guinet dot me> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #4 from Adrien Guinet <adrien at guinet dot me> ---
IMHO, bug isn't resolved!

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

--- Comment #5 from Adrien Guinet <adrien at guinet dot me> ---
Moreover, the test case runs fine without any automatic vectorisation and
crashes when it is applied, so from my point of view there is something wrong
with this optimisation!

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
You are wrong on this.  You've told the compiler that those arrays will be
aligned to 32 bytes, but they are not, so running the code is undefined
behavior.  malloc doesn't have a way to pass alignment requirements to it, so
there is no way it could know about the alignment requirements of struct A.

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

Adrien Guinet <adrien at guinet dot me> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #7 from Adrien Guinet <adrien at guinet dot me> ---
>From my understanding of
https://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html, the "aligned"
attribute inside a structure aligns the object inside this structure, and
nothing more, as the example says:

"You can also specify the alignment of structure fields. For example, to create
a double-word aligned int pair, you could write:

          struct foo { int x[2] __attribute__ ((aligned (8))); };


This is an alternative to creating a union with a double member that forces the
union to be double-word aligned. "

If I create a union, there is no alignment information. Anyway, I'd be happy to
read the official definition if it is specified somewhere!

Thanks!

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Adrien Guinet from comment #7)
> From my understanding of
> https://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html, the "aligned"
> attribute inside a structure aligns the object inside this structure, and
> nothing more, as the example says:
> 
> "You can also specify the alignment of structure fields. For example, to
> create a double-word aligned int pair, you could write:
> 
>           struct foo { int x[2] __attribute__ ((aligned (8))); };
>           
> 
> This is an alternative to creating a union with a double member that forces
> the union to be double-word aligned. "
> 
> If I create a union, there is no alignment information. Anyway, I'd be happy
> to read the official definition if it is specified somewhere!

Your understanding is wrong.  If certain field requires some alignment, then
obviously the whole structure requires that alignment too, otherwise the field
would not be aligned as requested (unless the struct is __attribute__((packed))
of course).  Just try to print __alignof__ (struct A), you'll see it is 32.
Anyway, please stop reopening, GCC bugzilla is for reporting compiler bugs, not
a place to learn C and/or the GCC extensions.  Please try gcc-help mailing
list.

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
.

[Bug c/64655] Vectorizer is always using load aligned instructions with objects with the "aligned" attribute

[adrien at guinet dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64655

--- Comment #10 from Adrien Guinet <adrien at guinet dot me> ---
Well, okay, that implication didn't look that obvious. Sorry for the constant
bug reopening, it was not my intent. And thanks for the clarifications!