On Wed, Jan 30, 2019 at 3:42 PM Alexander Monakov <amona...@ispras.ru> wrote:
>
> On Mon, 21 Jan 2019, Alexander Monakov wrote:
>
> > Ah, I see now. I agree transparent_union ought to work, but today both GCC
> > and Clang will reject such attempt; I've filed PR 88955 for the GCC issue.
> >
> > So unfortunately such code would still need a cast or an unnamed temporary,
> > which may be relatively obvious to the reader.
>
> In the end it may be preferable to have it anyway, so here's a revised version
> with an extra line in the example for passing arguments via compound literals.
>
> Checked with 'make html', OK to apply?
OK unless somebody objects within a reasonable time frame.
Richard.
> PR c/88698
> * doc/extend.texi (Vector Extensions): Add an example of using vector
> types together with x86 intrinsics.
>
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 95d22ac1e3c..34a76927b12 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -10632,6 +10632,47 @@ v4si g = __builtin_convertvector (f, v4si); /* g is
> @{1,-2,3,7@} */
> v4si h = __builtin_convertvector (c, v4si); /* h is @{1,5,0,10@} */
> @end smallexample
>
> +@cindex vector types, using with x86 intrinsics
> +Sometimes it is desirable to write code using a mix of generic vector
> +operations (for clarity) and machine-specific vector intrinsics (to
> +access vector instructions that are not exposed via generic built-ins).
> +On x86, intrinsic functions for integer vectors typically use the same
> +vector type @code{__m128i} irrespective of how they interpret the vector,
> +making it necessary to cast their arguments and return values from/to
> +other vector types. In C, you can make use of a @code{union} type:
> +@c In C++ such type punning via a union is not allowed by the language
> +@smallexample
> +#include <immintrin.h>
> +
> +typedef unsigned char u8x16 __attribute__ ((vector_size (16)));
> +typedef unsigned int u32x4 __attribute__ ((vector_size (16)));
> +
> +typedef union @{
> + __m128i mm;
> + u8x16 u8;
> + u32x4 u32;
> +@} v128;
> +@end smallexample
> +
> +@noindent
> +for variables that can be used with both built-in operators and x86
> +intrinsics:
> +
> +@smallexample
> +v128 x, y = @{ 0 @};
> +memcpy (&x, ptr, sizeof x);
> +y.u8 += 0x80;
> +x.mm = _mm_adds_epu8 (x.mm, y.mm);
> +x.u32 &= 0xffffff;
> +
> +/* Instead of a variable, a compound literal may be used to pass the
> + return value of an intrinsic call to a function expecting the union: */
> +v128 foo (v128);
> +x = foo ((v128) @{_mm_adds_epu8 (x.mm, y.mm)@});
> +@c This could be done implicitly with __attribute__((transparent_union)),
> +@c but GCC does not accept it for unions of vector types (PR 88955).
> +@end smallexample
> +
> @node Offsetof
> @section Support for @code{offsetof}
> @findex __builtin_offsetof
