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

[Krishna Pandey via libc-commits]

================
@@ -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 =
----------------
krishna2803 wrote:

in an attempt to test for `10.72` (just as an example), i tried:

```cpp
// (10.72)_10 =
// (1010.101110000101000111101011100001010001111010111000010100011110)_2 =
// (a.b851eb851eb851e)_16
static constexpr unsigned long long ten_point_seven_two = 0xab851eb851eb851eULL;

static constexpr T ten_point_seven_two_t = 10.72;

if constexpr (FXRep::INTEGRAL_LEN > 0)
  EXPECT_EQ(
    static_cast<XType>(ten_point_seven_two >> (64 - (FXRep::VALUE_LEN))),
    func(ten_point_seven_two_t)
  );
```

where `FXRep::VALUE_LEN` is just `FXRep::INTEGRAL_LEN + FXRep::FRACTION_LEN`. When I traced the definition for `FXRep::INTEGRAL_LEN`, I found them to be different than the spec for example for `short accum` the spec suggests `s4.7` ([ref, pg11](https://standards.iso.org/ittf/PubliclyAvailableStandards/c051126_ISO_IEC_TR_18037_2008.zip)) while the `FXRep::INTEGRAL_LEN` is `8` for `short fract` defined here:

https://github.com/llvm/llvm-project/blob/dff2ca424c20c672b418ec86ac3a120fad4fb364/libc/include/llvm-libc-macros/stdfix-macros.h#L164-L168

so the above test fails.

---

but when i do:
```cpp
// (10.72)_10 =
// (1010.101110000101000111101011100001010001111010111000010100011110)_2 =
// (a.b851eb851eb851e)_16
static constexpr unsigned long long ten_point_seven_two = 0xab851eb851eb851eULL;

static constexpr T ten_point_seven_two_t = 10.72;

if constexpr (FXRep::INTEGRAL_LEN > 0)
  EXPECT_EQ(
    // just hardcoded 4 here in place of INTEGRAL_LEN
    static_cast<XType>(ten_point_seven_two >> (64 - (FXRep::FRACTION_LEN + 4))),
    func(ten_point_seven_two_t)
  );
```

the test passes! could you please provide some guidance on how to handle this?

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