[mathcore] Work around libstdc++ <experimental/simd> AVX-512 bug

The `libstdc++`'s `_VecBltnBtmsk` (AVX-512 mask) ABI fails to compile
with non-GCC front ends (Clang, Intel icpx) due to a static_assert
requiring `long long` and `long` to be the same type, reproducible at
least with GCC 15 and 16.

Detect this in CMake and, when the AVX-512 path is broken but the
256-bit AVX path compiles, pin ROOT's SIMD alias in `Math/Types.h` to
`simd_abi::__avx` (which corresponds to AVX-256). ABI stays consistent
across all translation units and interpreted contexts, no `-mno-avx512f`
is needed, and the rest of ROOT keeps native AVX-512 codegen.

Closes #22370.

(cherry picked from commit 83dd2e4)

Author: guitargeek

cmake/modules/RootConfiguration.cmake

@@ -380,6 +380,11 @@ if(ROOT_HAVE_EXPERIMENTAL_SIMD)
 else()
   set(hasstdexperimentalsimd undef)
 endif()
+if(ROOT_EXPERIMENTAL_SIMD_PIN_AVX_ABI)
+  set(experimentalsimdpinavxabi define)
+else()
+  set(experimentalsimdpinavxabi undef)
+endif()
 if(dataframe)
   set(hasdataframe define)
 else()

cmake/modules/SearchInstalledSoftware.cmake

@@ -1792,6 +1792,52 @@ else()
   " ROOT_HAVE_EXPERIMENTAL_SIMD)
 endif()
 
+# On platforms with AVX-512, the libstdc++ implementation of
+# <experimental/simd> (from GCC up to at least 16) fails to compile with
+# non-GCC front ends (Clang, Intel icpx) because of a static_assert in the
+# _VecBltnBtmsk (AVX-512 mask) ABI that requires `long long` and `long` to be
+# the same type. The bug fires only for the AVX-512 mask ABI path, so we work
+# around it by pinning ROOT's simd alias (Math/Types.h) to the 256-bit AVX2
+# ABI variant instead of the platform-native ABI. That keeps the ABI of
+# Float_v/Double_v/... consistent across all TUs (no `-mno-avx512f` needed)
+# and lets the rest of ROOT keep its native AVX-512 codegen.
+set(ROOT_EXPERIMENTAL_SIMD_PIN_AVX_ABI FALSE CACHE INTERNAL
+    "Pin <experimental/simd> alias to the 256-bit ABI to dodge libstdc++ AVX-512 bug")
+if(ROOT_HAVE_EXPERIMENTAL_SIMD)
+  set(_simd_realistic_test "
+      #include <experimental/simd>
+      int main() {
+          std::experimental::native_simd<double> a(1.0), b(2.0);
+          where(a > b, a) = b;
+          return 0;
+      }
+  ")
+  check_cxx_source_compiles("${_simd_realistic_test}"
+                            ROOT_EXPERIMENTAL_SIMD_FULL_USAGE_OK)
+  if(NOT ROOT_EXPERIMENTAL_SIMD_FULL_USAGE_OK)
+    set(_simd_pinned_abi_test "
+        #include <experimental/simd>
+        namespace stx = std::experimental;
+        int main() {
+            stx::simd<double, stx::simd_abi::__avx> a(1.0), b(2.0);
+            where(a > b, a) = b;
+            return 0;
+        }
+    ")
+    check_cxx_source_compiles("${_simd_pinned_abi_test}"
+                              ROOT_EXPERIMENTAL_SIMD_AVX_ABI_OK)
+    if(ROOT_EXPERIMENTAL_SIMD_AVX_ABI_OK)
+      set(ROOT_EXPERIMENTAL_SIMD_PIN_AVX_ABI TRUE CACHE INTERNAL "" FORCE)
+      message(STATUS "Working around libstdc++ <experimental/simd> AVX-512 bug "
+                     "by pinning Math/Types.h to the 256-bit AVX ABI")
+    else()
+      message(STATUS "Disabling experimental/simd-based features: libstdc++ "
+                     "header fails to compile in this configuration")
+      set(ROOT_HAVE_EXPERIMENTAL_SIMD FALSE CACHE INTERNAL "" FORCE)
+    endif()
+  endif()
+endif()
+
 #------------------------------------------------------------------------------------
 # Check if the pyspark package is installed on the system.
 # Needed to run tests of the distributed RDataFrame module that use pyspark.

config/RConfigure.in

@@ -38,6 +38,7 @@
 #@hascocoa@ R__HAS_COCOA    /**/
 #@hasvdt@ R__HAS_VDT    /**/
 #@hasstdexperimentalsimd@ R__HAS_STD_EXPERIMENTAL_SIMD    /**/
+#@experimentalsimdpinavxabi@ R__EXPERIMENTAL_SIMD_PIN_AVX_ABI    /**/
 #@usecxxmodules@ R__USE_CXXMODULES   /**/
 #@uselibc++@ R__USE_LIBCXX    /**/
 #@has_found_attribute_always_inline@ R__HAS_ATTRIBUTE_ALWAYS_INLINE /**/

math/mathcore/inc/Math/Types.h

@@ -13,8 +13,21 @@ namespace ROOT {
 
 namespace Internal {
 
+#if defined(R__EXPERIMENTAL_SIMD_PIN_AVX_ABI) && defined(__AVX512F__)
+// libstdc++'s <experimental/simd> _VecBltnBtmsk (AVX-512 mask) ABI fails to
+// compile with non-GCC front ends (Clang, Intel icpx) due to a static_assert
+// requiring `long long` and `long` to be the same type. When AVX-512 is
+// enabled in this TU we'd otherwise hit that path, so pin to the 256-bit AVX
+// ABI instead. The fallback is guarded by __AVX512F__ so that environments
+// without AVX-512 in scope (notably rootcling/cling, which parses headers
+// without -mavx*) still see the regular `native` ABI and pick the
+// always-supported scalar fallback.
+template <typename T>
+using SIMDTag = std::experimental::simd_abi::__avx;
+#else
 template <typename T>
 using SIMDTag = std::experimental::simd_abi::native<T>;
+#endif
 
 } // namespace Internal