[libc-commits] [libc] [libc] Add sinpif16 function (PR #110994)

[via libc-commits]

================
@@ -0,0 +1,197 @@
+//===-- Half-precision sinpif 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/sinpif16.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/FPUtil/nearest_integer.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+
+// TODO: Should probably create a new file; sincospif16_utils.h
+// To store the following helper functions and constants.
+// I'd defer to @lntue for suggestions regarding that
+
+// HELPER_START
+namespace LIBC_NAMESPACE_DECL {
+
+constexpr float PI_OVER_32 = 0x1.921fb6p-4f;
+
+// In Sollya generate 10 coeffecients for a degree-9 chebyshev polynomial
+// approximating the sine function in [-pi / 32, pi / 32] with the following
+// commands:
+// > prec=24;
+// > TL = chebyshevform(sin(x), 9, [-pi / 32, pi / 32]);
+// > TL[0];
+const float SIN_COEFF[10] = {
+    0x1.d333p-26, 0x1.000048p0, -0x1.a5d2p-14, -0x1.628588p-3, 0x1.c1eep-5,
+    0x1.4455p1,   -0x1.317a8p3, -0x1.6bb9p8,   0x1.00ef8p9,    0x1.0edcp14};
+// In Sollya generate 10 coefficients for a degree-9 chebyshev polynomial
+// approximating the sine function in [-pi/32, pi/32] with the following
+// commands:
+// > prec = 24;
+// > TL = chebyshevform(cos(x), 9, [-pi / 32, pi / 32]);
+// > TL[0];
+const float COS_COEFF[10] = {
+    0x1.000006p0, 0x1.e1eap-15, -0x1.0071p-1, -0x1.3b56p-4, 0x1.f3dfp-2,
+    0x1.ccbap4,   -0x1.3034p6,  -0x1.f817p11, 0x1.fc59p11,  0x1.7079p17};
+// Lookup table for sin(k * pi / 32) with k = 0, ..., 63.
+// Table is generated with Sollya as follows:
+// > display = hexadecimmal;
+// > prec = 24;
+// > for k from 0 to 63 do {sin(k * pi/32);};
+
+const float SIN_K_PI_OVER_32[64] = {0,
+                                    0x1.917a6cp-4,
+                                    0x1.8f8b84p-3,
+                                    0x1.294062p-2,
+                                    0x1.87de2ap-2,
+                                    0x1.e2b5d4p-2,
+                                    0x1.1c73b4p-1,
+                                    0x1.44cf32p-1,
+                                    0x1.6a09e6p-1,
+                                    0x1.8bc806p-1,
+                                    0x1.a9b662p-1,
+                                    0x1.c38b3p-1,
+                                    0x1.d906bcp-1,
+                                    0x1.e9f416p-1,
+                                    0x1.f6297cp-1,
+                                    0x1.fd88dap-1,
+                                    0x1p0,
+                                    0x1.fd88dap-1,
+                                    0x1.f6297cp-1,
+                                    0x1.e9f416p-1,
+                                    0x1.d906bcp-1,
+                                    0x1.c38b3p-1,
+                                    0x1.a9b662p-1,
+                                    0x1.8bc806p-1,
+                                    0x1.6a09e6p-1,
+                                    0x1.44cf32p-1,
+                                    0x1.1c73b4p-1,
+                                    0x1.e2b5d4p-2,
+                                    0x1.87de2ap-2,
+                                    0x1.294062p-2,
+                                    0x1.8f8b84p-3,
+                                    0x1.917a6cp-4,
+                                    0,
+                                    -0x1.917a6cp-4,
+                                    -0x1.8f8b84p-3,
+                                    -0x1.294062p-2,
+                                    -0x1.87de2ap-2,
+                                    -0x1.e2b5d4p-2,
+                                    -0x1.1c73b4p-1,
+                                    -0x1.44cf32p-1,
+                                    -0x1.6a09e6p-1,
+                                    -0x1.8bc806p-1,
+                                    -0x1.a9b662p-1,
+                                    -0x1.c38b3p-1,
+                                    -0x1.d906bcp-1,
+                                    -0x1.e9f416p-1,
+                                    -0x1.f6297ep-1,
+                                    -0x1.fd88dap-1,
+                                    -0x1p0,
+                                    -0x1.fd88dap-1,
+                                    -0x1.f6297cp-1,
+                                    -0x1.e9f416p-1,
+                                    -0x1.d906bcp-1,
+                                    -0x1.c38b3p-1,
+                                    -0x1.a9b662p-1,
+                                    -0x1.8bc806p-1,
+                                    -0x1.6a09e6p-1,
+                                    -0x1.44cf32p-1,
+                                    -0x1.1c73b4p-1,
+                                    -0x1.e2b5d4p-2,
+                                    -0x1.87de2ap-2,
+                                    -0x1.294062p-2,
+                                    -0x1.8f8b84p-3,
+                                    -0x1.917a6cp-4};
+
+int32_t range_reduction(float x, float &y) {
+  float kf = fputil::nearest_integer(x * 32);
+  y = fputil::multiply_add<float>(x, 32.0, -kf);
+
+  return static_cast<int32_t>(kf);
+}
----------------
wldfngrs wrote:

I did consider that. However, the already existent range_reduction function for double parameter is implemented similarly. I only wrote a float reimplementation for consistency with the intermediate computation floating point size used for the sinpif16 function; float.

https://github.com/llvm/llvm-project/blob/02dd6b1014c708591fa0e8d46efb328c513a86e7/libc/src/math/generic/sincosf_utils.h#L109

```
static LIBC_INLINE int64_t range_reduction_sincospi(double x, double &y) {
  double kd = fputil::nearest_integer(x * 32);
  y = fputil::multiply_add<double>(x, 32.0, -kd);

  return static_cast<int64_t>(kd);
}
```

https://github.com/llvm/llvm-project/pull/110994