[libc++] Add `__truncating_cast` for safely casting float types to integers

This is needed anytime we need to clamp an arbitrary floating point
value to an integer type.

Thanks to Eric Fiselier for the patch.

Differential Revision: https://reviews.llvm.org/D66836

llvm-svn: 370891

1 files changed, 90 insertions, 0 deletions

diff --git a/libcxx/test/libcxx/numerics/clamp_to_integral.pass.cpp b/libcxx/test/libcxx/numerics/clamp_to_integral.pass.cpp
new file mode 100644
index 00000000000..cb3336f52ba
--- /dev/null
+++ b/libcxx/test/libcxx/numerics/clamp_to_integral.pass.cpp
@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// __clamp_to_integral<IntT>(RealT)
+
+// Test the conversion function that truncates floating point types to the
+// closest representable value for the specified integer type, or
+// numeric_limits<IntT>::max()/min() if the value isn't representable.
+
+#include <limits>
+#include <cassert>
+#include <cmath>
+
+template <class IntT>
+void test() {
+  typedef std::numeric_limits<IntT> Lim;
+  const bool MaxIsRepresentable = sizeof(IntT) < 8;
+  const bool IsSigned = std::is_signed<IntT>::value;
+  struct TestCase {
+    double Input;
+    IntT Expect;
+    bool IsRepresentable;
+  } TestCases[] = {
+      {0, 0, true},
+      {1, 1, true},
+      {IsSigned ? static_cast<IntT>(-1) : 0,
+       IsSigned ? static_cast<IntT>(-1) : 0, true},
+      {Lim::lowest(), Lim::lowest(), true},
+      {static_cast<double>(Lim::max()), Lim::max(), MaxIsRepresentable},
+      {static_cast<double>(Lim::max()) + 1, Lim::max(), false},
+      {static_cast<double>(Lim::max()) + 1024, Lim::max(), false},
+      {nextafter(static_cast<double>(Lim::max()), INFINITY), Lim::max(), false},
+  };
+  for (TestCase TC : TestCases) {
+    auto res = std::__clamp_to_integral<IntT>(TC.Input);
+    assert(res == TC.Expect);
+    if (TC.IsRepresentable) {
+      auto other = static_cast<IntT>(std::trunc(TC.Input));
+      assert(res == other);
+    } else
+      assert(res == Lim::min() || res == Lim::max());
+  }
+}
+
+template <class IntT>
+void test_float() {
+  typedef std::numeric_limits<IntT> Lim;
+  const bool MaxIsRepresentable = sizeof(IntT) < 4;
+  ((void)MaxIsRepresentable);
+  const bool IsSigned = std::is_signed<IntT>::value;
+  struct TestCase {
+    float Input;
+    IntT Expect;
+    bool IsRepresentable;
+  } TestCases[] = {
+      {0, 0, true},
+      {1, 1, true},
+      {IsSigned ? static_cast<IntT>(-1) : 0,
+       IsSigned ? static_cast<IntT>(-1) : 0, true},
+      {Lim::lowest(), Lim::lowest(), true},
+      {static_cast<float>(Lim::max()), Lim::max(), MaxIsRepresentable },
+       {nextafter(static_cast<float>(Lim::max()), INFINITY), Lim::max(), false},
+  };
+  for (TestCase TC : TestCases) {
+    auto res = std::__clamp_to_integral<IntT>(TC.Input);
+    assert(res == TC.Expect);
+    if (TC.IsRepresentable) {
+      auto other = static_cast<IntT>(std::trunc(TC.Input));
+      assert(res == other);
+    } else
+      assert(res == Lim::min() || res == Lim::max());
+  }
+}
+
+int main() {
+  test<short>();
+  test<unsigned short>();
+  test<int>();
+  test<unsigned>();
+  test<long long>();
+  test<unsigned long long>();
+  test_float<short>();
+  test_float<int>();
+  test_float<long long>();
+}