diff options
| author | Alexander Monakov <amonakov@ispras.ru> | 2019-02-04 15:00:41 +0000 |
|---|---|---|
| committer | Alexander Monakov <amonakov@ispras.ru> | 2019-02-04 15:00:41 +0000 |
| commit | 598ec38ce7b580fd7784fcd0ad74e5677151ff4a (patch) | |
| tree | b878f291d10e9abdee64994807428da42c06e377 | |
| parent | 046c92ad178236dddf45f2f2d4b42e075c28c80d (diff) | |
doc: showcase a "union of vectors" pattern (PR 88698)
PR c/88698
* doc/extend.texi (Vector Extensions): Add an example of using vector
types together with x86 intrinsics.
git-svn-id: https://gcc.gnu.org/svn/gcc/trunk@268522 138bc75d-0d04-0410-961f-82ee72b054a4
| -rw-r--r-- | gcc/ChangeLog | 6 | ||||
| -rw-r--r-- | gcc/doc/extend.texi | 41 |
2 files changed, 47 insertions, 0 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 3c52283d93c..21d80ad5f9d 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,9 @@ +2019-02-04 Alexander Monakov <amonakov@ispras.ru> + + PR c/88698 + * doc/extend.texi (Vector Extensions): Add an example of using vector + types together with x86 intrinsics. + 2019-02-04 Alan Modra <amodra@gmail.com> * config/rs6000/rs6000.c (rs6000_indirect_call_template_1): Increase diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index b51f4273ade..815788c40ab 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -11054,6 +11054,47 @@ 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 |