[libc-commits] [libc] [libc][math] Implement atan2f correctly rounded to all rounding modes. (PR #86716)

[via libc-commits]

================
@@ -0,0 +1,299 @@
+//===-- Single-precision atan2f function ----------------------------------===//
+//
+// 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 "src/math/atan2f.h"
+#include "inv_trigf_utils.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/double_double.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/FPUtil/nearest_integer.h"
+#include "src/__support/FPUtil/rounding_mode.h"
+#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
+
+namespace LIBC_NAMESPACE {
+
+namespace {
+
+// Look up tables for accurate pass:
+
+// atan(i/16) with i = 0..16, generated by Sollya with:
+// > for i from 0 to 16 do {
+//     a = round(atan(i/16), D, RN);
+//     b = round(atan(i/16) - a, D, RN);
+//     print("{", b, ",", a, "},");
+//   };
+constexpr fputil::DoubleDouble ATAN_I[17] = {
+    {0.0, 0.0},
+    {-0x1.c934d86d23f1dp-60, 0x1.ff55bb72cfdeap-5},
+    {-0x1.cd37686760c17p-59, 0x1.fd5ba9aac2f6ep-4},
+    {0x1.347b0b4f881cap-58, 0x1.7b97b4bce5b02p-3},
+    {0x1.8ab6e3cf7afbdp-57, 0x1.f5b75f92c80ddp-3},
+    {-0x1.963a544b672d8p-57, 0x1.362773707ebccp-2},
+    {-0x1.c63aae6f6e918p-56, 0x1.6f61941e4def1p-2},
+    {-0x1.24dec1b50b7ffp-56, 0x1.a64eec3cc23fdp-2},
+    {0x1.a2b7f222f65e2p-56, 0x1.dac670561bb4fp-2},
+    {-0x1.d5b495f6349e6p-56, 0x1.0657e94db30dp-1},
+    {-0x1.928df287a668fp-58, 0x1.1e00babdefeb4p-1},
+    {0x1.1021137c71102p-55, 0x1.345f01cce37bbp-1},
+    {0x1.2419a87f2a458p-56, 0x1.4978fa3269ee1p-1},
+    {0x1.0028e4bc5e7cap-57, 0x1.5d58987169b18p-1},
+    {-0x1.8c34d25aadef6p-56, 0x1.700a7c5784634p-1},
+    {-0x1.bf76229d3b917p-56, 0x1.819d0b7158a4dp-1},
+    {0x1.1a62633145c07p-55, 0x1.921fb54442d18p-1},
+};
+
+// Taylor polynomial, generated by Sollya with:
+// > for i from 0 to 8 do {
+//     j = (-1)^(i + 1)/(2*i + 1);
+//     a = round(j, D, RN);
+//     b = round(j - a, D, RN);
+//     print("{", b, ",", a, "},");
+//   };
+constexpr fputil::DoubleDouble COEFFS[9] = {
+    {0.0, 1.0},                                      // 1
+    {-0x1.5555555555555p-56, -0x1.5555555555555p-2}, // -1/3
+    {-0x1.999999999999ap-57, 0x1.999999999999ap-3},  // 1/5
+    {-0x1.2492492492492p-57, -0x1.2492492492492p-3}, // -1/7
+    {0x1.c71c71c71c71cp-58, 0x1.c71c71c71c71cp-4},   // 1/9
+    {0x1.745d1745d1746p-59, -0x1.745d1745d1746p-4},  // -1/11
+    {-0x1.3b13b13b13b14p-58, 0x1.3b13b13b13b14p-4},  // 1/13
+    {-0x1.1111111111111p-60, -0x1.1111111111111p-4}, // -1/15
+    {0x1.e1e1e1e1e1e1ep-61, 0x1.e1e1e1e1e1e1ep-5},   // 1/17
+};
+
+// Veltkamp's splitting of a double precision into hi + lo, where the hi part is
+// slightly smaller than an even split, so that the product of
+//   hi * s * k is exact,
+// where:
+//   s is single precsion,
+//   0 < k < 16 is an integer.
+// This is used when FMA instruction is not available.
+[[maybe_unused]] LIBC_INLINE constexpr fputil::DoubleDouble split_d(double a) {
----------------
lntue wrote:

I want them to be `inline`, but using the same macro so that it's less confusing and mix-matched between `inline` and `LIBC_INLINE` in our code base.

https://github.com/llvm/llvm-project/pull/86716