[libc-commits] [libc] [libc][stdfix] Add sqrt for fixed point types. (PR #83042)

[via libc-commits]

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@@ -0,0 +1,139 @@
+//===-- Calculate square root of fixed point numbers. -----*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_FIXEDPOINT_SQRT_H
+#define LLVM_LIBC_SRC___SUPPORT_FIXEDPOINT_SQRT_H
+
+#include "include/llvm-libc-macros/stdfix-macros.h"
+#include "src/__support/CPP/bit.h"
+#include "src/__support/CPP/type_traits.h"
+#include "src/__support/macros/attributes.h"   // LIBC_INLINE
+#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
+
+#include "fx_rep.h"
+
+#ifdef LIBC_COMPILER_HAS_FIXED_POINT
+
+namespace LIBC_NAMESPACE::fixed_point {
+
+namespace internal {
+
+template <typename T> struct SqrtConfig;
+
+template <> struct SqrtConfig<unsigned short fract> {
+  using Type = unsigned short fract;
+  static constexpr int EXTRA_STEPS = 0;
+};
+
+template <> struct SqrtConfig<unsigned fract> {
+  using Type = unsigned fract;
+  static constexpr int EXTRA_STEPS = 1;
+};
+
+template <> struct SqrtConfig<unsigned long fract> {
+  using Type = unsigned long fract;
+  static constexpr int EXTRA_STEPS = 2;
+};
+
+template <>
+struct SqrtConfig<unsigned short accum> : SqrtConfig<unsigned fract> {};
+
+template <>
+struct SqrtConfig<unsigned accum> : SqrtConfig<unsigned long fract> {};
+
+// TODO: unsigned long accum type is 64-bit, and will need 64-bit fract type.
+// Probably we will use DyadicFloat<64> for intermediate computations instead.
+
+// Linear approximation for the initial values, with errors bounded by:
+//   max(1.5 * 2^-11, eps)
+// Generated with Sollya:
+// > for i from 4 to 15 do {
+//     P = fpminimax(sqrt(x), 1, [|8, 8|], [i * 2^-4, (i + 1)*2^-4],
+//                   fixed, absolute);
+//     print("{", coeff(P, 1), "uhr,", coeff(P, 0), "uhr},");
+//   };
+// Clang is having issues with this array:
+// static constexpr unsigned short fract SQRT_FIRST_APPROX[12][2] = {
+//     {0x1.e8p-1uhr, 0x1.0cp-2uhr}, {0x1.bap-1uhr, 0x1.28p-2uhr},
+//     {0x1.94p-1uhr, 0x1.44p-2uhr}, {0x1.74p-1uhr, 0x1.6p-2uhr},
+//     {0x1.6p-1uhr, 0x1.74p-2uhr},  {0x1.4ep-1uhr, 0x1.88p-2uhr},
+//     {0x1.3ep-1uhr, 0x1.9cp-2uhr}, {0x1.32p-1uhr, 0x1.acp-2uhr},
+//     {0x1.22p-1uhr, 0x1.c4p-2uhr}, {0x1.18p-1uhr, 0x1.d4p-2uhr},
+//     {0x1.08p-1uhr, 0x1.fp-2uhr},  {0x1.04p-1uhr, 0x1.f8p-2uhr},
+// };
+// We are using their storage type instead.
+// TODO(https://github.com/llvm/llvm-project/issues/83050): Use fixed point
+// array when the bug is fixed.
+static constexpr uint8_t SQRT_FIRST_APPROX[12][2] = {
+    {244, 67},  {221, 74},  {202, 81},  {186, 88},  {176, 93},  {167, 98},
+    {159, 103}, {153, 107}, {145, 113}, {140, 117}, {132, 124}, {130, 126},
+};
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lntue wrote:

Given that the current predefined detection macro `__FRACT_FBIT__` that we use just added to very recently, and basic compiler support for fixed point is still on-going, I think it is ok for us to require clang trunk to build fixed point functions for now.  Later on, we might set the requirement to build fixed point functions to be clang-19 once it's released.

https://github.com/llvm/llvm-project/pull/83042