[cfe-dev] Use of GCC builtins in xmmintrin.h

[Eric Christopher]

On Mon, Jan 27, 2014 at 6:06 AM, Caparros Cabezas  Victoria
<caparrov at inf.ethz.ch> wrote:
> Hi,
>
> I am working on the LLVM IR of x86 vector intrinsics. In the target-specific header file <xmmintrin.h>
> some intrinsics are defined using GCC builtins, and some other are implemented using the vector support
> provided by LLVM, which I guess is the preferred method whenever it is possible.
>
> What determines when to use LLVM vector instructions and when to use GCC builtins?

When it's easily possible and unambiguous to use the LLVM vector code
then use that, otherwise the builtins.

-eric

> For example, the store low value intrinsic is defined as:
>
> static __inline__ void __attribute__((__always_inline__))
> _mm_store_ss(float *__p, __m128 __a)
> {
>   struct __mm_store_ss_struct {
>     float __u;
>   } __attribute__((__packed__, __may_alias__));
>   ((struct __mm_store_ss_struct*)__p)->__u = __a[0];
> }
>
>
> But the store packed intrinsic is defined as:
>
> static __inline__ void __attribute__((__always_inline__, __nodebug__))
> _mm_storeu_ps(float *__p, __m128 __a)
> {
>   __builtin_ia32_storeups(__p, __a);
> }
>
> Why not?
>
> static __inline__ void __attribute__((__always_inline__, __nodebug__))
> _mm_storeu_ps(float *__p, __m128 __a)
> {
>
>  struct __mm_store_ps_struct {
>     __m128  __u;
>   } __attribute__((__packed__, __may_alias__));
>   ((struct __mm_store_ps_struct*)__p)->__u = __a;
>
> }
>
> Loads are defined this way. That would generate more consisten LLVM IR,
> because at the moment vector loads are translated into native vector
> operations in LLVM IR, but vector stores are translated into calls to
> external intrinsics.
>
> Many thanks in advance,
>
> Victoria
>
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