[libc-commits] [libc] [libc][math] Implement an integer-only version of double precision sin and cos with 1 ULP errors. (PR #184752)

[via libc-commits]

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+//===-- Trig range reduction and evaluation using integer-only --*- 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_MATH_SINCOS_INTEGER_UTILS_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_SINCOS_INTEGER_UTILS_H
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/big_int.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+#include "src/__support/math_extras.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+namespace integer_only {
+
+struct Frac128 : public UInt<128> {
+  using UInt<128>::UInt;
+
+  constexpr Frac128 operator~() const {
+    Frac128 r;
+    r.val[0] = ~val[0];
+    r.val[1] = ~val[1];
+    return r;
+  }
+
+  constexpr Frac128 operator+(const Frac128 &other) const {
+    UInt<128> r = UInt<128>(*this) + (UInt<128>(other));
+    return Frac128(r.val);
+  }
+
+  constexpr Frac128 operator-(const Frac128 &other) const {
+    UInt<128> r = UInt<128>(*this) - (UInt<128>(other));
+    return Frac128(r.val);
+  }
+
+  constexpr Frac128 operator*(const Frac128 &other) const {
+    UInt<128> r = UInt<128>::quick_mul_hi(UInt<128>(other));
+    return Frac128(r.val);
+  }
+};
+
+// 1280 + 64 bits of 2/pi, printed using MPFR.
+// Notice that if we store from the highest bytes to lowest bytes, it is
+// essentially having 2/pi in big-endian.  On the other hand, uint64_t type
+// that will be used for computations later are in little-endian.  So a few
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lntue wrote:

I've added proper support for big endian.  Do you mind checking if it works correctly for big endian targets?  Thanks.

About storing in uint64_t, and/or indexing into 2/pi with bit instead of byte, I was looking into it earlier, but the variable adjustment of all 3 64-bit chunks seem to be more costly on armv8m.  Maybe we can try that approach later to see if it can be done better or not.

https://github.com/llvm/llvm-project/pull/184752