Experimental support for RVV runtime dispatch in clang19, refs llvm/l…

…lvm-project#56592

PiperOrigin-RevId: 660355333

hwy/detect_targets.h

@@ -605,12 +605,11 @@
 #endif  // HWY_HAVE_AUXV
 
 #ifndef HWY_HAVE_RUNTIME_DISPATCH_RVV  // allow override
-// The riscv_vector.h in (at least) Clang 16-18 requires compiler flags, see
+// The riscv_vector.h in Clang 16-18 requires compiler flags, see
 // https://github.com/llvm/llvm-project/issues/56592. GCC 13.3 also has an
 // #error check, whereas 14.1 fails with "argument type 'vuint16m8_t' requires
 // the V ISA extension": https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115325.
-// Hence disable runtime dispatch for now.
-#if HWY_ARCH_RISCV && 0
+#if HWY_ARCH_RISCV && HWY_COMPILER_CLANG >= 1900
 #define HWY_HAVE_RUNTIME_DISPATCH_RVV 1
 #else
 #define HWY_HAVE_RUNTIME_DISPATCH_RVV 0

hwy/ops/set_macros-inl.h

@@ -629,8 +629,12 @@
 
 #define HWY_NAMESPACE N_RVV
 
+#if HWY_COMPILER_CLANG >= 1900
+// https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#181-zvl-minimum-vector-length-standard-extensions
+#define HWY_TARGET_STR "Zvl128b,Zve64d"
+#else
 // HWY_TARGET_STR remains undefined so HWY_ATTR is a no-op.
-// (rv64gcv is not a valid target)
+#endif
 
 //-----------------------------------------------------------------------------
 // EMU128