[Math] Migrate from VecCore and Vc to std::experimental::simd

Use `std::experimental::simd` for the vectorized TMath and vectorized
TFormula backend if available. We assume it's available if ROOT is
compiled with C++20 or later on Linux. In reality, it is avialble for
new-enough Clang and GCC compilers (GCC >= 11), but it is not worth it
at this point to check the compiler version explicitly, to keep things
simple. People who build with C++20 are also expected to use new
compilers.

The fact that ROOT C++17 users lose the vectorized TMath and TFormula
features is an acceptable collateral damage of this migration, as the
features are rarely used and the code/build system simplification is
significant. If users really relied on these niche features, they are
expected to build with C++20.

Windows and Apple silicon users are unaffected by this migration. The
VecCore+Vc combination did not compile on Windows before anyway, and Vc
does not support Apple silicon as Vc didn't support vectorization on ARM
via Neon (it was [in developement](https://github.com/VcDevel/Vc?tab=readme-ov-file#vc-portable-zero-overhead-c-types-for-explicitly-data-parallel-programming) but never finished).

As a result of this migration, the `vc`, `veccore`, `builtin_vc`, and
`builtin_veccore` build options are deprecated.

The migration also fixes this long-standing JIRA issue about failing
TFormula vectorization tests, because Vc symbols could not be resolved:
https://its.cern.ch/jira/browse/ROOT-10614
This was a fundamental limitation of the statically-linked Vc library,
as symbols could not be looked up by the interpreter at runtime. This
limitation is gone with the migration to `std::experimental::simd`.

The unit test that checks if GenVector works with Vc types was changed
to use `std::experimental::simd` types instead.

Author: guitargeek

.ci/copy_headers.sh

@@ -6,7 +6,7 @@ set -ex
 mkdir ../build
 cd ../build
 cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -Dall=On -Dtesting=On -Dx11=Off -Dalien=Off \
-      -Dcuda=Off -Dtmva-gpu=Off -Dveccore=Off ../root
+      -Dcuda=Off -Dtmva-gpu=Off ../root
 # We need to prebuild a minimal set of targets which are responsible for header copy
 # or generation.
 make -j4 move_headers intrinsics_gen clang-tablegen-targets ClangDriverOptions \

.github/workflows/root-ci-config/buildconfig/global.txt

@@ -24,9 +24,7 @@ builtin_openui5=ON
 builtin_pcre=OFF
 builtin_tbb=OFF
 builtin_unuran=OFF
-builtin_vc=OFF
 builtin_vdt=OFF
-builtin_veccore=OFF
 builtin_xrootd=OFF
 builtin_xxhash=OFF
 builtin_zeromq=ON
@@ -97,9 +95,7 @@ tmva=ON
 unfold=ON
 unuran=ON
 uring=OFF
-vc=OFF
 vdt=ON
-veccore=OFF
 vecgeom=OFF
 webgui=ON
 win_broken_tests=OFF

.github/workflows/root-ci-config/buildconfig/mac-beta.txt

@@ -17,7 +17,6 @@ builtin_pcre=ON
 builtin_tbb=ON
 builtin_unuran=ON
 builtin_vdt=ON
-builtin_veccore=ON
 builtin_xrootd=ON
 builtin_xxhash=ON
 builtin_zeromq=ON

.github/workflows/root-ci-config/buildconfig/mac14.txt

@@ -18,9 +18,7 @@ builtin_openssl=ON
 builtin_pcre=ON
 builtin_tbb=ON
 builtin_unuran=ON
-builtin_vc=ON
 builtin_vdt=ON
-builtin_veccore=ON
 builtin_xrootd=ON
 builtin_xxhash=ON
 builtin_zeromq=ON

.github/workflows/root-ci-config/buildconfig/mac15.txt

@@ -16,7 +16,6 @@ builtin_pcre=ON
 builtin_tbb=ON
 builtin_unuran=ON
 builtin_vdt=ON
-builtin_veccore=ON
 builtin_xrootd=ON
 builtin_xxhash=ON
 builtin_zeromq=ON

.github/workflows/root-ci-config/buildconfig/mac26.txt

@@ -17,7 +17,6 @@ builtin_pcre=ON
 builtin_tbb=ON
 builtin_unuran=ON
 builtin_vdt=ON
-builtin_veccore=ON
 builtin_xrootd=ON
 builtin_xxhash=ON
 builtin_zeromq=ON

README/ReleaseNotes/v640/index.md

@@ -71,6 +71,33 @@ This change affects the following classes:  `TFile`, `TMapFile`, `TMemFile`, `TD
 
 ## Math
 
+### Migration from VecCore/Vc to `std::experimental::simd` for Vectorization
+
+We have migrated the vectorized backends of **TMath** and **TFormula** from **VecCore/Vc** to `std::experimental::simd`, where available.
+
+On Linux, `std::experimental::simd` is assumed to be available when ROOT is compiled with C++20 or later, which in practice corresponds to sufficiently recent GCC and Clang compilers. To keep the build system simple and robust, ROOT does not explicitly check compiler versions: users opting into C++20 are expected to use modern toolchains.
+
+**Impact on Linux users**
+
+ROOT builds with C++17 on Linux no longer provide vectorized TMath and TFormula.
+This is an intentional and accepted trade-off of the migration. These vectorized features were rarely used, while maintaining them significantly complicated the codebase and build configuration.
+
+Users who rely on vectorized **TMath** or the vectorized **TFormula** backend are encouraged to build ROOT with C++20.
+Doing so restores vectorization through `std::experimental::simd`, providing a more robust and future-proof implementation.
+
+**Windows and Apple silicon users are unaffected**
+
+VecCore/Vc did not work on Windows previously, and Vc never provided production-ready support for ARM/Neon, so Apple silicon did not benefit from vectorization before this change.
+
+**Build system changes**
+
+As part of this migration, the following build options are deprecated. From ROOT 6.42, setting them will result in configuration errors.
+
+  * `vc`
+  * `veccore`
+  * `builtin_vc`
+  * `builtin_veccore`
+
 ## RooFit
 
 ## RDataFrame

cmake/modules/RootBuildOptions.cmake

@@ -107,9 +107,7 @@ ROOT_BUILD_OPTION(builtin_pcre OFF "Build bundled copy of PCRE")
 ROOT_BUILD_OPTION(builtin_png OFF "Build bundled copy of libpng")
 ROOT_BUILD_OPTION(builtin_tbb OFF "Build TBB internally (requires network)")
 ROOT_BUILD_OPTION(builtin_unuran OFF "Build bundled copy of unuran")
-ROOT_BUILD_OPTION(builtin_vc OFF "Build Vc internally (requires network)")
 ROOT_BUILD_OPTION(builtin_vdt OFF "Build VDT internally (requires network)")
-ROOT_BUILD_OPTION(builtin_veccore OFF "Build VecCore internally (requires network)")
 ROOT_BUILD_OPTION(builtin_xrootd OFF "Build XRootD internally (requires network)")
 ROOT_BUILD_OPTION(builtin_xxhash OFF "Build bundled copy of xxHash")
 ROOT_BUILD_OPTION(builtin_zeromq OFF "Build ZeroMQ internally (requires network)")
@@ -180,9 +178,7 @@ ROOT_BUILD_OPTION(unfold OFF "Enable the unfold package [GPL]")
 ROOT_BUILD_OPTION(unuran OFF "Enable support for UNURAN (package for generating non-uniform random numbers) [GPL]")
 ROOT_BUILD_OPTION(uring OFF "Enable support for io_uring (requires liburing and Linux kernel >= 5.1)")
 ROOT_BUILD_OPTION(use_gsl_cblas ON "Use the CBLAS library from GSL instead of finding a more optimized BLAS library automatically with FindBLAS (the GSL CBLAS is less performant but more portable)")
-ROOT_BUILD_OPTION(vc OFF "Enable support for Vc (SIMD Vector Classes for C++)")
 ROOT_BUILD_OPTION(vdt ON "Enable support for VDT (fast and vectorisable mathematical functions)")
-ROOT_BUILD_OPTION(veccore OFF "Enable support for VecCore SIMD abstraction library")
 ROOT_BUILD_OPTION(vecgeom OFF "Enable support for VecGeom vectorized geometry library")
 ROOT_BUILD_OPTION(webgui ON "Build Web-based UI components of ROOT")
 ROOT_BUILD_OPTION(win_broken_tests OFF "Enable broken tests on Windows")
@@ -253,9 +249,7 @@ if(all)
  set(tmva-pymva_defvalue ON)
  set(tmva-rmva_defvalue ON)
  set(unuran_defvalue ON)
- set(vc_defvalue ON)
  set(vdt_defvalue ON)
- set(veccore_defvalue ON)
  set(vecgeom_defvalue ON)
  set(x11_defvalue ON)
  set(xml_defvalue ON)
@@ -295,9 +289,7 @@ if(builtin_all)
   set(builtin_png_defvalue ON)
   set(builtin_tbb_defvalue ON)
 #  set(builtin_unuran_defvalue ON) (GPL)
-  set(builtin_vc_defvalue ON)
   set(builtin_vdt_defvalue ON)
-  set(builtin_veccore_defvalue ON)
   set(builtin_xrootd_defvalue ON)
   set(builtin_xxhash_defvalue ON)
   set(builtin_zeromq_defvalue ON)
@@ -415,6 +407,14 @@ if(DEFINED rpath)
       "") # empty line at the end to make the deprecation message more visible
 endif()
 
+foreach(opt vc builtin_vc veccore builtin_veccore)
+  if(${opt})
+    message(DEPRECATION ">>> Option '${opt}' is deprecated and ignored."
+        "ROOT now uses std::experimental::simd for the vectorized TFormula and TMath classes when available (on Linux when compiling with C++20 or higher)."
+        "Using this option will result in configuration errors in ROOT 6.42.")
+  endif()
+endforeach()
+
 foreach(opt minuit2_mpi)
   if(${opt})
       message(WARNING "The option '${opt}' can only be used to minimise thread-safe functions in Minuit2. It cannot be used for Histogram/Graph fitting and for RooFit. If you want to use Minuit2 with MPI support, it is better to build Minuit2 as a standalone library.")

cmake/modules/RootConfiguration.cmake

@@ -365,20 +365,15 @@ if(cocoa)
 else()
   set(hascocoa undef)
 endif()
-if(vc)
-  set(hasvc define)
-else()
-  set(hasvc undef)
-endif()
 if(vdt)
   set(hasvdt define)
 else()
   set(hasvdt undef)
 endif()
-if(veccore)
-  set(hasveccore define)
+if(ROOT_HAVE_EXPERIMENTAL_SIMD)
+  set(hasstdexperimentalsimd define)
 else()
-  set(hasveccore undef)
+  set(hasstdexperimentalsimd undef)
 endif()
 if(dataframe)
   set(hasdataframe define)