Add bridging for clang _Float16 type.

src/Float16.h

@@ -5,6 +5,19 @@
 #include <cstdint>
 #include <string>
 
+// clang had _Float16 added as a reserved name in clang 8, but
+// doesn't actually support it on most platforms until clang 15.
+// Ideally there would be a better way to detect if the type
+// is supported, even in a compiler independent fashion, but
+// coming up with one has proven elusive.
+#if !defined(__clang__) || (__clang_major__ >= 15)
+#if defined(__is_identifier)
+#if !__is_identifier(_Float16)
+#define HALIDE_CPP_COMPILER_HAS_FLOAT16
+#endif
+#endif
+#endif
+
 namespace Halide {
 
 /** Class that provides a type that implements half precision
@@ -38,6 +51,13 @@ struct float16_t {
      * positive zero.*/
     float16_t() = default;
 
+#ifdef HALIDE_CPP_COMPILER_HAS_FLOAT16
+    /** Construct a float16_t from compiler's built-in _Float16 type. */
+    explicit float16_t(_Float16 value) {
+        data = *(uint16_t *)&value;
+    }
+#endif
+
     /// @}
 
     // Use explicit to avoid accidently raising the precision
@@ -48,6 +68,13 @@ struct float16_t {
     /** Cast to int */
     explicit operator int() const;
 
+#ifdef HALIDE_CPP_COMPILER_HAS_FLOAT16
+    /** Cast to compiler's built-in _Float16 type. */
+    explicit operator _Float16() const {
+        return *(const _Float16 *)&data;
+    }
+#endif
+
     /** Get a new float16_t that represents a special value */
     // @{
     static float16_t make_zero();

test/correctness/float16_t.cpp

@@ -318,6 +318,17 @@ int main(int argc, char **argv) {
         }
     }
 
+#ifdef HALIDE_CPP_COMPILER_HAS_FLOAT16
+    {
+        float16_t f(1.0f16);
+        _Float16 f2 = (_Float16)f;
+        if (f2 != 1.0f16) {
+            printf("Roundtrip of 16-bit float via _Float16 failed.\n");
+            return -1;
+        }
+    }
+#endif
+
     printf("Success!\n");
     return 0;
 }