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Add a generic Eigen backend based on clang vector extensions

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The goal of this MR is to implement a generic SIMD backend (packet ops) for Eigen that uses clang vector extensions instead of platform-dependent intrinsics. Ideally, this should make it possible to build Eigen and achieve reasonable speed on any platform that has a recent clang compiler, without having to write any inline assembly or intrinsics.

Caveats:

* The current implementation is a proof of concept and supports vectorization for float, double, int32_t, and int64_t using fixed-size 512-bit vectors (a somewhat arbitrary choice). I have not done much to tune this for speed yet.
* For now, there is no way to enable this other than setting -DEIGEN_VECTORIZE_GENERIC on the command line.
* This only compiles with newer versions of clang. I have tested that it compiles and all tests pass with clang 19.1.7.

https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors

Closes #2998 and #2997

See merge request libeigen/eigen!2051

Co-authored-by: Rasmus Munk Larsen <rmlarsen@google.com>
Co-authored-by: Antonio Sánchez <cantonios@google.com>
This commit is contained in:
Rasmus Munk Larsen
2025-11-06 21:52:19 +00:00
parent 7c7d84735e
commit ec93a6d098
17 changed files with 1161 additions and 39 deletions
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5
Eigen/src/Core/arch/Default/GenericPacketMathFunctions.h
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@@ -695,7 +695,6 @@ EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS Packet pexp_float(const Pack
template <typename Packet>
EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS Packet pexp_double(const Packet _x) {
Packet x = _x;
const Packet cst_zero = pset1<Packet>(0.0);
const Packet cst_1 = pset1<Packet>(1.0);
const Packet cst_2 = pset1<Packet>(2.0);
@@ -719,7 +718,8 @@ EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS Packet pexp_double(const Pac
// clamp x
Packet zero_mask = pcmp_lt(_x, cst_exp_lo);
x = pmin(x, cst_exp_hi);
Packet x = pmin(_x, cst_exp_hi);
// Express exp(x) as exp(g + n*log(2)).
fx = pmadd(cst_cephes_LOG2EF, x, cst_half);
@@ -1352,7 +1352,6 @@ EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS T ptanh_double(const T& a_x)
const T minus_clamp = pset1<T>(-17.714196154005176);
#endif
const T x = pmax(pmin(a_x, plus_clamp), minus_clamp);
// The following rational approximation was generated by rminimax
// (https://gitlab.inria.fr/sfilip/rminimax) using the following
// command: