[libc][math][c23] Add logf16 C23 math function

libc/config/gpu/entrypoints.txt

@@ -568,6 +568,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.llrintf16
     libc.src.math.llroundf16
     libc.src.math.logbf16
+    libc.src.math.logf16
     libc.src.math.lrintf16
     libc.src.math.lroundf16
     libc.src.math.modff16

libc/config/linux/x86_64/entrypoints.txt

@@ -661,6 +661,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.llrintf16
     libc.src.math.llroundf16
     libc.src.math.logbf16
+    libc.src.math.logf16
     libc.src.math.lrintf16
     libc.src.math.lroundf16
     libc.src.math.modff16

libc/docs/math/index.rst

@@ -310,7 +310,7 @@ Higher Math Functions
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
 | lgamma    |                  |                 |                        |                      |                        | 7.12.8.3               | F.10.5.3                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
-| log       | |check|          | |check|         |                        |                      |                        | 7.12.6.11              | F.10.3.11                  |
+| log       | |check|          | |check|         |                        | |check|              |                        | 7.12.6.11              | F.10.3.11                  |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
 | log10     | |check|          | |check|         |                        |                      |                        | 7.12.6.12              | F.10.3.12                  |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+

libc/spec/stdc.td

@@ -651,6 +651,7 @@ def StdC : StandardSpec<"stdc"> {
 
           FunctionSpec<"log", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"logf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
+          GuardedFunctionSpec<"logf16", RetValSpec<Float16Type>, [ArgSpec<Float16Type>], "LIBC_TYPES_HAS_FLOAT16">,
 
           FunctionSpec<"logb", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
           FunctionSpec<"logbf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,

libc/src/math/CMakeLists.txt

@@ -343,6 +343,7 @@ add_math_entrypoint_object(log2f)
 
 add_math_entrypoint_object(log)
 add_math_entrypoint_object(logf)
+add_math_entrypoint_object(logf16)
 
 add_math_entrypoint_object(logb)
 add_math_entrypoint_object(logbf)

libc/src/math/generic/CMakeLists.txt

@@ -2289,6 +2289,28 @@ add_entrypoint_object(
     -O3
 )
 
+add_entrypoint_object(
+  logf16
+  SRCS
+    logf16.cpp
+  HDRS
+    ../logf16.h
+  DEPENDS
+    .expxf16
+    libc.hdr.errno_macros
+    libc.hdr.fenv_macros
+    libc.src.__support.FPUtil.cast
+    libc.src.__support.FPUtil.except_value_utils
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.polyeval
+    libc.src.__support.macros.optimization
+    libc.src.__support.macros.properties.cpu_features
+  COMPILE_OPTIONS
+    -O3
+)
+
 add_entrypoint_object(
   logb
   SRCS

libc/src/math/generic/expxf16.h

@@ -288,6 +288,34 @@ template <bool IsSinh> LIBC_INLINE float16 eval_sinh_or_cosh(float16 x) {
       lo, half_p_odd * exp2_hi_mid_diff, half_p_even * exp2_hi_mid_sum));
 }
 
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from 0 to 31 do print(round(log(1 + i * 2^-5), SG, RN));
+constexpr cpp::array<float, 32> LOGF_F = {
+    0x0p+0f,        0x1.f829bp-6f,  0x1.f0a30cp-5f, 0x1.6f0d28p-4f,
+    0x1.e27076p-4f, 0x1.29553p-3f,  0x1.5ff308p-3f, 0x1.9525aap-3f,
+    0x1.c8ff7cp-3f, 0x1.fb9186p-3f, 0x1.1675cap-2f, 0x1.2e8e2cp-2f,
+    0x1.4618bcp-2f, 0x1.5d1bdcp-2f, 0x1.739d8p-2f,  0x1.89a338p-2f,
+    0x1.9f323ep-2f, 0x1.b44f78p-2f, 0x1.c8ff7cp-2f, 0x1.dd46ap-2f,
+    0x1.f128f6p-2f, 0x1.02552ap-1f, 0x1.0be72ep-1f, 0x1.154c3ep-1f,
+    0x1.1e85f6p-1f, 0x1.2795e2p-1f, 0x1.307d74p-1f, 0x1.393e0ep-1f,
+    0x1.41d8fep-1f, 0x1.4a4f86p-1f, 0x1.52a2d2p-1f, 0x1.5ad404p-1f,
+};
+
+// Generated by Sollya with the following commands:
+//   > display = hexadecimal;
+//   > for i from 0 to 31 do print(round(1 / (1 + i * 2^-5), SG, RN));
+constexpr cpp::array<float, 32> ONE_OVER_F_F = {
+    0x1p+0f,        0x1.f07c2p-1f,  0x1.e1e1e2p-1f, 0x1.d41d42p-1f,
+    0x1.c71c72p-1f, 0x1.bacf92p-1f, 0x1.af286cp-1f, 0x1.a41a42p-1f,
+    0x1.99999ap-1f, 0x1.8f9c18p-1f, 0x1.861862p-1f, 0x1.7d05f4p-1f,
+    0x1.745d18p-1f, 0x1.6c16c2p-1f, 0x1.642c86p-1f, 0x1.5c9882p-1f,
+    0x1.555556p-1f, 0x1.4e5e0ap-1f, 0x1.47ae14p-1f, 0x1.414142p-1f,
+    0x1.3b13b2p-1f, 0x1.3521dp-1f,  0x1.2f684cp-1f, 0x1.29e412p-1f,
+    0x1.24924ap-1f, 0x1.1f7048p-1f, 0x1.1a7b96p-1f, 0x1.15b1e6p-1f,
+    0x1.111112p-1f, 0x1.0c9714p-1f, 0x1.08421p-1f,  0x1.041042p-1f,
+};
+
 } // namespace LIBC_NAMESPACE_DECL
 
 #endif // LLVM_LIBC_SRC_MATH_GENERIC_EXPXF16_H

libc/src/math/generic/logf16.cpp

@@ -0,0 +1,157 @@
+//===-- Half-precision log(x) 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/logf16.h"
+#include "expxf16.h"
+#include "hdr/errno_macros.h"
+#include "hdr/fenv_macros.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.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/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+#include "src/__support/macros/properties/cpu_features.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+#ifdef LIBC_TARGET_CPU_HAS_FMA
+static constexpr size_t N_LOGF16_EXCEPTS = 5;
+#else
+static constexpr size_t N_LOGF16_EXCEPTS = 11;
+#endif
+
+static constexpr fputil::ExceptValues<float16, N_LOGF16_EXCEPTS>
+    LOGF16_EXCEPTS = {{
+// (input, RZ output, RU offset, RD offset, RN offset)
+#ifndef LIBC_TARGET_CPU_HAS_FMA
+        // x = 0x1.61cp-13, logf16(x) = -0x1.16p+3 (RZ)
+        {0x0987U, 0xc858U, 0U, 1U, 0U},
+        // x = 0x1.f2p-12, logf16(x) = -0x1.e98p+2 (RZ)
+        {0x0fc8U, 0xc7a6U, 0U, 1U, 1U},
+#endif
+        // x = 0x1.4d4p-9, logf16(x) = -0x1.7e4p+2 (RZ)
+        {0x1935U, 0xc5f9U, 0U, 1U, 0U},
+        // x = 0x1.5ep-8, logf16(x) = -0x1.4ecp+2 (RZ)
+        {0x1d78U, 0xc53bU, 0U, 1U, 0U},
+#ifndef LIBC_TARGET_CPU_HAS_FMA
+        // x = 0x1.fdp-1, logf16(x) = -0x1.81p-8 (RZ)
+        {0x3bf4U, 0x9e04U, 0U, 1U, 1U},
+        // x = 0x1.fep-1, logf16(x) = -0x1.008p-8 (RZ)
+        {0x3bf8U, 0x9c02U, 0U, 1U, 0U},
+#endif
+        // x = 0x1.ffp-1, logf16(x) = -0x1.004p-9 (RZ)
+        {0x3bfcU, 0x9801U, 0U, 1U, 0U},
+        // x = 0x1.ff8p-1, logf16(x) = -0x1p-10 (RZ)
+        {0x3bfeU, 0x9400U, 0U, 1U, 1U},
+#ifdef LIBC_TARGET_CPU_HAS_FMA
+        // x = 0x1.4c4p+1, logf16(x) = 0x1.e84p-1 (RZ)
+        {0x4131U, 0x3ba1U, 1U, 0U, 1U},
+#else
+        // x = 0x1.75p+2, logf16(x) = 0x1.c34p+0 (RZ)
+        {0x45d4U, 0x3f0dU, 1U, 0U, 0U},
+        // x = 0x1.75p+2, logf16(x) = 0x1.c34p+0 (RZ)
+        {0x45d4U, 0x3f0dU, 1U, 0U, 0U},
+        // x = 0x1.d5p+9, logf16(x) = 0x1.b5cp+2 (RZ)
+        {0x6354U, 0x46d7U, 1U, 0U, 1U},
+#endif
+    }};
+
+LLVM_LIBC_FUNCTION(float16, logf16, (float16 x)) {
+  using FPBits = fputil::FPBits<float16>;
+  FPBits x_bits(x);
+
+  uint16_t x_u = x_bits.uintval();
+
+  // If x <= 0, or x is 1, or x is +inf, or x is NaN.
+  if (LIBC_UNLIKELY(x_u == 0U || x_u == 0x3c00U || x_u >= 0x7c00U)) {
+    // log(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;
+    }
+
+    // log(+/-0) = −inf
+    if ((x_u & 0x7fffU) == 0U) {
+      fputil::raise_except_if_required(FE_DIVBYZERO);
+      return FPBits::inf(Sign::NEG).get_val();
+    }
+
+    if (x_u == 0x3c00U)
+      return FPBits::zero().get_val();
+
+    // When x < 0.
+    if (x_u > 0x8000U) {
+      fputil::set_errno_if_required(EDOM);
+      fputil::raise_except_if_required(FE_INVALID);
+      return FPBits::quiet_nan().get_val();
+    }
+
+    // log(+inf) = +inf
+    return FPBits::inf().get_val();
+  }
+
+  if (auto r = LOGF16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+    return r.value();
+
+  // To compute log(x), we perform the following range reduction:
+  //   x = 2^m * 1.mant,
+  //   log(x) = m * log(2) + log(1.mant).
+  // To compute log(1.mant), let f be the highest 6 bits including the hidden
+  // bit, and d be the difference (1.mant - f), i.e., the remaining 5 bits of
+  // the mantissa, then:
+  //   log(1.mant) = log(f) + log(1.mant / f)
+  //               = log(f) + log(1 + d/f)
+  // since d/f is sufficiently small.
+  // We store log(f) and 1/f in the lookup tables LOGF_F and ONE_OVER_F
+  // respectively.
+
+  int m = -FPBits::EXP_BIAS;
+
+  // When x is subnormal, normalize it.
+  if ((x_u & FPBits::EXP_MASK) == 0U) {
+    // Can't pass an integer to fputil::cast directly.
+    constexpr float NORMALIZE_EXP = 1U << FPBits::FRACTION_LEN;
+    x_bits = FPBits(x_bits.get_val() * fputil::cast<float16>(NORMALIZE_EXP));
+    x_u = x_bits.uintval();
+    m -= FPBits::FRACTION_LEN;
+  }
+
+  uint16_t mant = x_bits.get_mantissa();
+  // Leading 10 - 5 = 5 bits of the mantissa.
+  int f = mant >> 5;
+  // Unbiased exponent.
+  m += x_u >> FPBits::FRACTION_LEN;
+
+  // Set bits to 1.mant instead of 2^m * 1.mant.
+  x_bits.set_biased_exponent(FPBits::EXP_BIAS);
+  float mant_f = x_bits.get_val();
+  // v = 1.mant * 1/f - 1 = d/f
+  float v = fputil::multiply_add(mant_f, ONE_OVER_F_F[f], -1.0f);
+
+  // Degree-3 minimax polynomial generated by Sollya with the following
+  // commands:
+  //   > display = hexadecimal;
+  //   > P = fpminimax(log(1 + x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
+  //   > x * P;
+  float log1p_d_over_f =
+      v * fputil::polyeval(v, 0x1p+0f, -0x1.001804p-1f, 0x1.557ef6p-2f);
+  // log(1.mant) = log(f) + log(1 + d/f)
+  float log_1_mant = LOGF_F[f] + log1p_d_over_f;
+  return fputil::cast<float16>(
+      fputil::multiply_add(static_cast<float>(m), LOGF_2, log_1_mant));
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/math/logf16.h

@@ -0,0 +1,21 @@
+//===-- Implementation header for logf16 ------------------------*- 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_MATH_LOGF16_H
+#define LLVM_LIBC_SRC_MATH_LOGF16_H
+
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/types.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+float16 logf16(float16 x);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_MATH_LOGF16_H

libc/test/UnitTest/FPMatcher.h

@@ -374,4 +374,17 @@ template <typename T> struct FPTest : public Test {
   EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_MODE(                                   \
       (expected), (actual), (expected_except), RoundingMode::TowardZero)
 
+#define EXPECT_FP_EQ_WITH_EXCEPTION_ALL_ROUNDING(expected, actual,             \
+                                                 expected_except)              \
+  do {                                                                         \
+    EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_NEAREST((expected), (actual),         \
+                                                 (expected_except));           \
+    EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_UPWARD((expected), (actual),          \
+                                                (expected_except));            \
+    EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_DOWNWARD((expected), (actual),        \
+                                                  (expected_except));          \
+    EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_TOWARD_ZERO((expected), (actual),     \
+                                                     (expected_except));       \
+  } while (0)
+
 #endif // LLVM_LIBC_TEST_UNITTEST_FPMATCHER_H

libc/test/src/math/CMakeLists.txt

@@ -1772,6 +1772,17 @@ add_fp_unittest(
     libc.src.__support.FPUtil.fp_bits
 )
 
+add_fp_unittest(
+  logf16_test
+  NEED_MPFR
+  SUITE
+    libc-math-unittests
+  SRCS
+    logf16_test.cpp
+  DEPENDS
+    libc.src.math.logf16
+)
+
 add_fp_unittest(
 log2_test
  NEED_MPFR

libc/test/src/math/logf16_test.cpp

@@ -0,0 +1,40 @@
+//===-- Exhaustive test for logf16 ----------------------------------------===//
+//
+// 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/logf16.h"
+#include "test/UnitTest/FPMatcher.h"
+#include "test/UnitTest/Test.h"
+#include "utils/MPFRWrapper/MPFRUtils.h"
+
+using LlvmLibcLogf16Test = LIBC_NAMESPACE::testing::FPTest<float16>;
+
+namespace mpfr = LIBC_NAMESPACE::testing::mpfr;
+
+// Range: [0, Inf];
+static constexpr uint16_t POS_START = 0x0000U;
+static constexpr uint16_t POS_STOP = 0x7c00U;
+
+// Range: [-Inf, 0];
+static constexpr uint16_t NEG_START = 0x8000U;
+static constexpr uint16_t NEG_STOP = 0xfc00U;
+
+TEST_F(LlvmLibcLogf16Test, PositiveRange) {
+  for (uint16_t v = POS_START; v <= POS_STOP; ++v) {
+    float16 x = FPBits(v).get_val();
+    EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Log, x,
+                                   LIBC_NAMESPACE::logf16(x), 0.5);
+  }
+}
+
+TEST_F(LlvmLibcLogf16Test, NegativeRange) {
+  for (uint16_t v = NEG_START; v <= NEG_STOP; ++v) {
+    float16 x = FPBits(v).get_val();
+    EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Log, x,
+                                   LIBC_NAMESPACE::logf16(x), 0.5);
+  }
+}

libc/test/src/math/smoke/CMakeLists.txt

@@ -3558,6 +3558,19 @@ add_fp_unittest(
     libc.src.__support.FPUtil.fp_bits
 )
 
+add_fp_unittest(
+  logf16_test
+  SUITE
+    libc-math-smoke-tests
+  SRCS
+    logf16_test.cpp
+  DEPENDS
+    libc.hdr.fenv_macros
+    libc.src.errno.errno
+    libc.src.math.logf16
+    libc.src.__support.FPUtil.cast
+)
+
 add_fp_unittest(
   log2_test
   SUITE