[libc-commits] [libc] [libc][stdfix] Implement fixed point bitsfx functions in llvm libc (PR #128413)

[via libc-commits]

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@@ -0,0 +1,56 @@
+//===-- Utility class to test bitsfx functions ------------------*- 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 "test/UnitTest/Test.h"
+
+#include "src/__support/fixed_point/fx_rep.h"
+
+template <typename T, typename XType>
+class BitsFxTest : public LIBC_NAMESPACE::testing::Test {
+
+  using FXRep = LIBC_NAMESPACE::fixed_point::FXRep<T>;
+  static constexpr T zero = FXRep::ZERO();
+  static constexpr T max = FXRep::MAX();
+  static constexpr T min = FXRep::MIN();
+  static constexpr T one_half = FXRep::ONE_HALF();
+  static constexpr T one_fourth = FXRep::ONE_FOURTH();
+  static constexpr T eps = FXRep::EPS();
+
+  // (0.42)_10 =
+  // (0.0110101110000101000111101011100001010001111010111000010100011110)_2 =
+  // (0.0x6b851eb851eb851e)_16
+  static constexpr unsigned long long zero_point_forty_two =
+      0x6b851eb851eb851eULL;
+
+public:
+  typedef XType (*BitsFxFunc)(T);
+
+  void testSpecialNumbers(BitsFxFunc func) {
+    EXPECT_EQ(static_cast<XType>(0), func(zero));
+    EXPECT_EQ(static_cast<XType>(1ULL << (FXRep::FRACTION_LEN - 1)),
+              func(one_half));
+    EXPECT_EQ(static_cast<XType>(1ULL << (FXRep::FRACTION_LEN - 2)),
+              func(one_fourth));
+    EXPECT_EQ(static_cast<XType>(1), func(eps));
+
+    // (0.6875)_10 = (0.1011)_2
+    EXPECT_EQ(static_cast<XType>(11ULL << (FXRep::FRACTION_LEN - 4)),
+              func(0.6875));
+
+    EXPECT_EQ(
+        static_cast<XType>(zero_point_forty_two >> (64 - FXRep::FRACTION_LEN)),
+        func(0.42));
+  }
----------------
PiJoules wrote:

Note that if the value cannot precisely fit in the fixed point type, rounding up or down is implementation-defined. I think this check assumes we always round down. Perhaps for numbers that don't precisely fit in a fixed point type, we should check the numbers both above and below the true value.

For example, for 0.42 as a `short fract` which has 7 fractional bits for this implementation, we check the result of `bitshr` is 53 or 54.

https://github.com/llvm/llvm-project/pull/128413