[Bug c++/98317] New: Vector Extensions aligned(1) not generating unaligned loads/stores

[Bug c++/98317] New: Vector Extensions aligned(1) not generating unaligned loads/stores

[danielhanchen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98317

            Bug ID: 98317
           Summary: Vector Extensions aligned(1) not generating unaligned
                    loads/stores
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: danielhanchen at gmail dot com
  Target Milestone: ---

The ordering of aligned(1) causes GCC to generate movaps / movups.

typedef float   float128_tv1    __attribute__ ((aligned(1), vector_size(16)));
typedef float   float128_tv2    __attribute__ ((vector_size(16), aligned(1)));

float128_tv1 provides MOVAPS
float128_tv2 provides MOVUPS

It seems like the ordering of the arguments changes the assembly.

https://gcc.godbolt.org/z/5qs7e7

It seems like GCC 10.2 and 9.2 all have this issue.
Unless if this was already documentated, this issue can cause massive issues if
memory is unaligned and an aligned load/store is used instead.

[Bug c++/98317] Vector Extensions aligned(1) not generating unaligned loads/stores

[danielhanchen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98317

Daniel Han-Chen <danielhanchen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |danielhanchen at gmail dot com

--- Comment #1 from Daniel Han-Chen <danielhanchen at gmail dot com> ---
https://gcc.godbolt.org/z/sGWevT

I also tried separating the __attribute__s


typedef float   float128_tv1    __attribute__ ((aligned(1), vector_size(16)));
typedef float   float128_tv2    __attribute__ ((vector_size(16), aligned(1)));
typedef float   float128_tv3    __attribute__((aligned(1))) __attribute__
((vector_size(16)));
typedef float   float128_tv4    __attribute__ ((vector_size(16)))
__attribute__((aligned(1)));


aligned as the first argument still fails.

[Bug c++/98317] Vector Extensions aligned(1) not generating unaligned loads/stores

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98317

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I'm not convinced it is a bug.  Because vector_size by definition increases the
alignment, say if you have vector_size on int, the vector type doesn't have
alignof(int) alignment, but alignment of the vector type.
Similarly, if one uses
typedef int T __attribute__((aligned (1)));
typedef T V __attribute__((vector_size (16)));
I'd argue that V should have still 16-byte alignment.
Having the attributes on the same typedef or on the same type is a grey area,
but what we do currently matches what is done with the separate typedefs.
I'd strongly suggest to use separate typedefs, i.e. one that makes a vector
type
typedef float V __attribute__((vector_size (16)));
and then another one that makes it unaligned:
typedef V U __attribute__((aligned (1)));

[Bug c++/98317] Vector Extensions aligned(1) not generating unaligned loads/stores

[danielhanchen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98317

--- Comment #3 from Daniel Han-Chen <danielhanchen at gmail dot com> ---
Oh ok then.

It's cause I was trying to do unaligned loads by following:
https://stackoverflow.com/questions/9318115/loading-data-for-gccs-vector-extensions

In it, it mentioned using typedef char __attribute__ ((vector_size (16),aligned
(1))) unaligned_byte16, which works, though the other way does not.

But I like your solution by declaring the type as aligned(1) separately.

[Bug c++/98317] Vector Extensions aligned(1) not generating unaligned loads/stores

[danielhanchen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98317

Daniel Han-Chen <danielhanchen at gmail dot com> changed:

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

--- Comment #4 from Daniel Han-Chen <danielhanchen at gmail dot com> ---
Jakub mentioned his solution, so all good now.