Add extendhfxf2 into compiler rt (#113897)

Retry of pr #109090 and #111099.

Co-authored-by: Alexander Richardson <alexrichardson@google.com>

compiler-rt/lib/builtins/CMakeLists.txt

@@ -296,6 +296,7 @@ endif ()
 # long double is not 80 bits on Android or MSVC.
 set(x86_80_BIT_SOURCES
   divxc3.c
+  extendhfxf2.c
   extendxftf2.c
   fixxfdi.c
   fixxfti.c

compiler-rt/lib/builtins/extendhfxf2.c

@@ -0,0 +1,16 @@
+//===-- lib/extendhfxf2.c - half -> long double conversion --------*- C -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#include "int_lib.h"
+#define SRC_HALF
+#define DST_DOUBLE
+#include "fp_extend_impl.inc"
+
+// Long double are expected to be as precise as double.
+COMPILER_RT_ABI xf_float __extendhfxf2(src_t a) {
+  return (xf_float)__extendXfYf2__(a);
+}

compiler-rt/test/builtins/Unit/extendhfxf2_test.c

@@ -0,0 +1,73 @@
+// RUN: %clang_builtins %s %librt -o %t && %run %t
+// REQUIRES: librt_has_extendhfxf2
+
+#include <limits.h>
+#include <math.h> // for isnan, isinf
+#include <stdio.h>
+
+#include "int_lib.h"
+
+#if HAS_80_BIT_LONG_DOUBLE && defined(COMPILER_RT_HAS_FLOAT16)
+
+long double __extendhfxf2(_Float16 f);
+
+int test_extendhfxf2(_Float16 a, long double expected) {
+  long double x = __extendhfxf2(a);
+  __uint16_t *b = (void *)&a;
+  int ret = !((isnan(x) && isnan(expected)) || x == expected);
+  if (ret) {
+    printf("error in test__extendhfxf2(%#.4x) = %.20Lf, "
+           "expected %.20Lf\n",
+           *b, x, expected);
+  }
+  return ret;
+}
+
+char assumption_1[sizeof(_Float16) * CHAR_BIT == 16] = {0};
+
+int main() {
+  // Small positive value
+  if (test_extendhfxf2(0.09997558593750000000f, 0.09997558593750000000L))
+    return 1;
+
+  // Small negative value
+  if (test_extendhfxf2(-0.09997558593750000000f, -0.09997558593750000000L))
+    return 1;
+
+  // Zero
+  if (test_extendhfxf2(0.0f, 0.0L))
+    return 1;
+
+  // Smallest positive non-zero value
+  if (test_extendhfxf2(0x1p-16f, 0x1p-16L))
+    return 1;
+
+  // Smallest negative non-zero value
+  if (test_extendhfxf2(-0x1p-16f, -0x1p-16L))
+    return 1;
+
+  // Positive infinity
+  if (test_extendhfxf2(__builtin_huge_valf16(), __builtin_huge_valf64x()))
+    return 1;
+
+  // Negative infinity
+  if (test_extendhfxf2(-__builtin_huge_valf16(),
+                       (long double)-__builtin_huge_valf64x()))
+    return 1;
+
+  // NaN
+  if (test_extendhfxf2(__builtin_nanf16(""),
+                       (long double)__builtin_nanf64x("")))
+    return 1;
+
+  return 0;
+}
+
+#else
+
+int main() {
+  printf("skipped\n");
+  return 0;
+}
+
+#endif