[libc-commits] [libc] [llvm] [libc][math][c++23] Add expbf16 math function (PR #161919)

[via libc-commits]

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@@ -0,0 +1,209 @@
+//===-- Implementation header for expbf16 -----------------------*- 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_EXPBF16_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_EXPBF16_H
+
+#include "hdr/errno_macros.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/bfloat16.h"
+#include "src/__support/FPUtil/cast.h"
+#include "src/__support/FPUtil/except_value_utils.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/config.h"
+#include "src/__support/macros/optimization.h"
+#include "src/__support/macros/properties/types.h"
+
+// bfloat16 expbf16(bfloat16 x);
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+namespace expbf16_internal {
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from -96 to 88 by 8 do print(i, round(exp(i), SG, RN) @ "f,");
+LIBC_INLINE_VAR constexpr float EXP_HI[24] = {
+    0x1.6a4p-139f,   0x1.07b71p-127f, 0x1.7fd974p-116f, 0x1.175afp-104f,
+    0x1.969d48p-93f, 0x1.27ec46p-81f, 0x1.aebabap-70f,  0x1.397924p-58f,
+    0x1.c8465p-47f,  0x1.4c1078p-35f, 0x1.e355bcp-24f,  0x1.5fc21p-12f,
+    0x1p0f,          0x1.749ea8p11f,  0x1.0f2ebep23f,   0x1.8ab7fcp34f,
+    0x1.1f43fcp46f,  0x1.a220d4p57f,  0x1.304d6ap69f,   0x1.baed16p80f,
+    0x1.425982p92f,  0x1.d531d8p103f, 0x1.55779cp115f,  0x1.f1056ep126f,
+};
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from 0 to 7.75 by 0.25 do print(round(exp(i), SG, RN) @ "f,");
+LIBC_INLINE_VAR constexpr float EXP_MID[32] = {
+    0x1p0f,         0x1.48b5e4p0f,  0x1.a61298p0f,  0x1.0ef9dcp1f,
+    0x1.5bf0a8p1f,  0x1.bec38ep1f,  0x1.1ed3fep2f,  0x1.704b6ap2f,
+    0x1.d8e64cp2f,  0x1.2f9b88p3f,  0x1.85d6fep3f,  0x1.f4907p3f,
+    0x1.415e5cp4f,  0x1.9ca53cp4f,  0x1.08ec72p5f,  0x1.542b2ep5f,
+    0x1.b4c902p5f,  0x1.186bf2p6f,  0x1.68118ap6f,  0x1.ce564ep6f,
+    0x1.28d38ap7f,  0x1.7d21eep7f,  0x1.e96244p7f,  0x1.3a30dp8f,
+    0x1.936dc6p8f,  0x1.0301a4p9f,  0x1.4c9222p9f,  0x1.ab0786p9f,
+    0x1.122886p10f, 0x1.6006b6p10f, 0x1.c402b6p10f, 0x1.223252p11f,
+};
+
+} // namespace expbf16_internal
+
+LIBC_INLINE bfloat16 expbf16(bfloat16 x) {
+  using FPBits = fputil::FPBits<bfloat16>;
+  FPBits x_bits(x);
+
+  uint16_t x_u = x_bits.uintval();
+  uint16_t x_abs = x_u & 0x7fffU;
+
+  // 0 <= |x| <= 2^(-3), or |x| >= 89, or x is NaN
+  if (LIBC_UNLIKELY(x_abs <= 0x3e00U || x_abs >= 0x42b2U)) {
+
+    // exp(NaN) = NaN
+    if (x_bits.is_nan()) {
+      if (x_bits.is_signaling_nan()) {
+        fputil::raise_except_if_required(FE_INVALID);
+        return FPBits::quiet_nan().get_val();
+      }
+
+      return x;
+    }
+
+    // if x >= 89
+    if (x_bits.is_pos() && x_abs >= 0x42b2U) {
+      // exp(inf) = inf
+      if (x_bits.is_inf())
+        return x;
+
+#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
+      fputil::set_errno_if_required(ERANGE);
+      fputil::raise_except_if_required(FE_OVERFLOW);
+      return FPBits::inf().get_val();
+#else
+      switch (fputil::quick_get_round()) {
+      case FE_TONEAREST:
+      case FE_UPWARD:
+        fputil::set_errno_if_required(ERANGE);
+        fputil::raise_except_if_required(FE_OVERFLOW);
+        return FPBits::inf().get_val();
+      default:
+        return FPBits::max_normal().get_val();
+      }
+#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
+    }
+
+    // x <= -92.5
+    if (x_u >= 0xc2b9U) {
+      // exp(-inf) = +0
+      if (x_bits.is_inf())
+        return FPBits::zero().get_val();
+
+      fputil::set_errno_if_required(ERANGE);
+      fputil::raise_except_if_required(FE_UNDERFLOW | FE_INEXACT);
+
+#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
+      return FPBits::zero().get_val();
+#else
+      switch (fputil::quick_get_round()) {
+      case FE_UPWARD:
+      case FE_TONEAREST:
+        if (LIBC_UNLIKELY(x_u == 0xc2b9U))
+          return FPBits::min_subnormal().get_val();
+        return FPBits::zero().get_val();
+      default:
+        return FPBits::zero().get_val();
+      }
+#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
+    }
+
+    // exp(0) = 1
+    if (x_bits.is_zero())
+      return bfloat16(1.0f);
+
+    // 0 < |x| <= 2^(-3)
+    if (x_abs <= 0x3e00U) {
+      float xf = static_cast<float>(x);
+      // Degree-3 minimax polynomial generated by Sollya with the following
+      // commands:
+      //   > display = hexadecimal;
+      //   > P = fpminimax(expm1(x)/x, 2, [|SG...|], [-2^-7, 2^-7]);
+      //   > 1 + x * P;
+      //   0x1p0 + x * (0x1p0 + x * (0x1.00004p-1 + x * 0x1.555578p-3))
----------------
Sukumarsawant wrote:

added in comments below, the `0x1.555778p-3` instead of `0x1.5555778p-3` error which was there in two places

https://github.com/llvm/llvm-project/pull/161919