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[libc][math][c23] Add log2f16 C23 math function (#106084)
Part of #95250.
1 parent 0afe6e4 commit 6d347fd

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14 files changed

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14 files changed

+326
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‎libc/config/gpu/entrypoints.txt

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@@ -567,6 +567,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
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libc.src.math.llogbf16
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libc.src.math.llrintf16
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libc.src.math.llroundf16
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libc.src.math.log2f16
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libc.src.math.logbf16
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libc.src.math.logf16
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libc.src.math.lrintf16

‎libc/config/linux/x86_64/entrypoints.txt

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@@ -660,6 +660,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
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libc.src.math.llogbf16
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libc.src.math.llrintf16
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libc.src.math.llroundf16
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libc.src.math.log2f16
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libc.src.math.logbf16
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libc.src.math.logf16
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libc.src.math.lrintf16

‎libc/docs/math/index.rst

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@@ -318,7 +318,7 @@ Higher Math Functions
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+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
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| log1p | |check| | |check| | | | | 7.12.6.14 | F.10.3.14 |
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+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
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| log2 | |check| | |check| | | | | 7.12.6.15 | F.10.3.15 |
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| log2 | |check| | |check| | | |check| | | 7.12.6.15 | F.10.3.15 |
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+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
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| log2p1 | | | | | | 7.12.6.16 | F.10.3.16 |
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+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+

‎libc/spec/stdc.td

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@@ -648,6 +648,7 @@ def StdC : StandardSpec<"stdc"> {
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FunctionSpec<"log2", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
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FunctionSpec<"log2f", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
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GuardedFunctionSpec<"log2f16", RetValSpec<Float16Type>, [ArgSpec<Float16Type>], "LIBC_TYPES_HAS_FLOAT16">,
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FunctionSpec<"log", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
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FunctionSpec<"logf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,

‎libc/src/math/CMakeLists.txt

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@@ -340,6 +340,7 @@ add_math_entrypoint_object(log1pf)
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add_math_entrypoint_object(log2)
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add_math_entrypoint_object(log2f)
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add_math_entrypoint_object(log2f16)
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add_math_entrypoint_object(log)
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add_math_entrypoint_object(logf)

‎libc/src/math/generic/CMakeLists.txt

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@@ -2250,6 +2250,28 @@ add_entrypoint_object(
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-O3
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)
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add_entrypoint_object(
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log2f16
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SRCS
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log2f16.cpp
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HDRS
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../log2f16.h
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DEPENDS
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.expxf16
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libc.hdr.errno_macros
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libc.hdr.fenv_macros
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libc.src.__support.FPUtil.cast
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libc.src.__support.FPUtil.except_value_utils
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libc.src.__support.FPUtil.fenv_impl
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libc.src.__support.FPUtil.fp_bits
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libc.src.__support.FPUtil.multiply_add
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libc.src.__support.FPUtil.polyeval
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libc.src.__support.macros.optimization
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libc.src.__support.macros.properties.cpu_features
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COMPILE_OPTIONS
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-O3
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)
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add_entrypoint_object(
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log
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SRCS

‎libc/src/math/generic/expxf16.h

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@@ -302,6 +302,20 @@ constexpr cpp::array<float, 32> LOGF_F = {
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0x1.41d8fep-1f, 0x1.4a4f86p-1f, 0x1.52a2d2p-1f, 0x1.5ad404p-1f,
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};
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// Generated by Sollya with the following commands:
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// > display = hexadecimal;
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// > for i from 0 to 31 do print(round(log2(1 + i * 2^-5), SG, RN));
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constexpr cpp::array<float, 32> LOG2F_F = {
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0x0p+0f, 0x1.6bad38p-5f, 0x1.663f7p-4f, 0x1.08c588p-3f,
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0x1.5c01a4p-3f, 0x1.acf5e2p-3f, 0x1.fbc16cp-3f, 0x1.24407ap-2f,
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0x1.49a784p-2f, 0x1.6e221cp-2f, 0x1.91bba8p-2f, 0x1.b47ecp-2f,
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0x1.d6753ep-2f, 0x1.f7a856p-2f, 0x1.0c105p-1f, 0x1.1bf312p-1f,
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0x1.2b8034p-1f, 0x1.3abb4p-1f, 0x1.49a784p-1f, 0x1.584822p-1f,
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0x1.66a008p-1f, 0x1.74b1fep-1f, 0x1.82809ep-1f, 0x1.900e62p-1f,
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0x1.9d5dap-1f, 0x1.aa709p-1f, 0x1.b74948p-1f, 0x1.c3e9cap-1f,
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0x1.d053f6p-1f, 0x1.dc899ap-1f, 0x1.e88c6cp-1f, 0x1.f45e08p-1f,
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};
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// Generated by Sollya with the following commands:
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// > display = hexadecimal;
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// > for i from 0 to 31 do print(round(1 / (1 + i * 2^-5), SG, RN));

‎libc/src/math/generic/log2f16.cpp

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@@ -0,0 +1,149 @@
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//===-- Half-precision log2(x) function -----------------------------------===//
2+
//
3+
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4+
// See https://llvm.org/LICENSE.txt for license information.
5+
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
8+
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#include "src/math/log2f16.h"
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#include "expxf16.h"
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#include "hdr/errno_macros.h"
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#include "hdr/fenv_macros.h"
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#include "src/__support/FPUtil/FEnvImpl.h"
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#include "src/__support/FPUtil/FPBits.h"
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#include "src/__support/FPUtil/PolyEval.h"
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#include "src/__support/FPUtil/cast.h"
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#include "src/__support/FPUtil/except_value_utils.h"
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#include "src/__support/FPUtil/multiply_add.h"
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#include "src/__support/common.h"
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#include "src/__support/macros/config.h"
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#include "src/__support/macros/optimization.h"
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#include "src/__support/macros/properties/cpu_features.h"
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namespace LIBC_NAMESPACE_DECL {
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#ifdef LIBC_TARGET_CPU_HAS_FMA
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static constexpr size_t N_LOG2F16_EXCEPTS = 2;
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#else
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static constexpr size_t N_LOG2F16_EXCEPTS = 9;
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#endif
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static constexpr fputil::ExceptValues<float16, N_LOG2F16_EXCEPTS>
33+
LOG2F16_EXCEPTS = {{
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// (input, RZ output, RU offset, RD offset, RN offset)
35+
#ifndef LIBC_TARGET_CPU_HAS_FMA
36+
// x = 0x1.224p-1, log2f16(x) = -0x1.a34p-1 (RZ)
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{0x3889U, 0xba8dU, 0U, 1U, 0U},
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// x = 0x1.e34p-1, log2f16(x) = -0x1.558p-4 (RZ)
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{0x3b8dU, 0xad56U, 0U, 1U, 0U},
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#endif
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// x = 0x1.e8cp-1, log2f16(x) = -0x1.128p-4 (RZ)
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{0x3ba3U, 0xac4aU, 0U, 1U, 0U},
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#ifndef LIBC_TARGET_CPU_HAS_FMA
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// x = 0x1.f98p-1, log2f16(x) = -0x1.2ep-6 (RZ)
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{0x3be6U, 0xa4b8U, 0U, 1U, 0U},
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// x = 0x1.facp-1, log2f16(x) = -0x1.e7p-7 (RZ)
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{0x3bebU, 0xa39cU, 0U, 1U, 1U},
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#endif
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// x = 0x1.fb4p-1, log2f16(x) = -0x1.b88p-7 (RZ)
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{0x3bedU, 0xa2e2U, 0U, 1U, 1U},
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#ifndef LIBC_TARGET_CPU_HAS_FMA
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// x = 0x1.fecp-1, log2f16(x) = -0x1.cep-9 (RZ)
53+
{0x3bfbU, 0x9b38U, 0U, 1U, 1U},
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// x = 0x1.ffcp-1, log2f16(x) = -0x1.714p-11 (RZ)
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{0x3bffU, 0x91c5U, 0U, 1U, 1U},
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// x = 0x1.224p+0, log2f16(x) = 0x1.72cp-3 (RZ)
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{0x3c89U, 0x31cbU, 1U, 0U, 1U},
58+
#endif
59+
}};
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LLVM_LIBC_FUNCTION(float16, log2f16, (float16 x)) {
62+
using FPBits = fputil::FPBits<float16>;
63+
FPBits x_bits(x);
64+
65+
uint16_t x_u = x_bits.uintval();
66+
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// If x <= 0, or x is 1, or x is +inf, or x is NaN.
68+
if (LIBC_UNLIKELY(x_u == 0U || x_u == 0x3c00U || x_u >= 0x7c00U)) {
69+
// log2(NaN) = NaN
70+
if (x_bits.is_nan()) {
71+
if (x_bits.is_signaling_nan()) {
72+
fputil::raise_except_if_required(FE_INVALID);
73+
return FPBits::quiet_nan().get_val();
74+
}
75+
76+
return x;
77+
}
78+
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// log2(+/-0) = −inf
80+
if ((x_u & 0x7fffU) == 0U) {
81+
fputil::raise_except_if_required(FE_DIVBYZERO);
82+
return FPBits::inf(Sign::NEG).get_val();
83+
}
84+
85+
if (x_u == 0x3c00U)
86+
return FPBits::zero().get_val();
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88+
// When x < 0.
89+
if (x_u > 0x8000U) {
90+
fputil::set_errno_if_required(EDOM);
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fputil::raise_except_if_required(FE_INVALID);
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return FPBits::quiet_nan().get_val();
93+
}
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// log2(+inf) = +inf
96+
return FPBits::inf().get_val();
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}
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if (auto r = LOG2F16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
100+
return r.value();
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// To compute log2(x), we perform the following range reduction:
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// x = 2^m * 1.mant,
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// log2(x) = m + log2(1.mant).
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// To compute log2(1.mant), let f be the highest 6 bits including the hidden
106+
// bit, and d be the difference (1.mant - f), i.e., the remaining 5 bits of
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// the mantissa, then:
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// log2(1.mant) = log2(f) + log2(1.mant / f)
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// = log2(f) + log2(1 + d/f)
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// since d/f is sufficiently small.
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// We store log2(f) and 1/f in the lookup tables LOG2F_F and ONE_OVER_F_F
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// respectively.
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int m = -FPBits::EXP_BIAS;
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// When x is subnormal, normalize it.
117+
if ((x_u & FPBits::EXP_MASK) == 0U) {
118+
// Can't pass an integer to fputil::cast directly.
119+
constexpr float NORMALIZE_EXP = 1U << FPBits::FRACTION_LEN;
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x_bits = FPBits(x_bits.get_val() * fputil::cast<float16>(NORMALIZE_EXP));
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x_u = x_bits.uintval();
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m -= FPBits::FRACTION_LEN;
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}
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uint16_t mant = x_bits.get_mantissa();
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// Leading 10 - 5 = 5 bits of the mantissa.
127+
int f = mant >> 5;
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// Unbiased exponent.
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m += x_u >> FPBits::FRACTION_LEN;
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// Set bits to 1.mant instead of 2^m * 1.mant.
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x_bits.set_biased_exponent(FPBits::EXP_BIAS);
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float mant_f = x_bits.get_val();
134+
// v = 1.mant * 1/f - 1 = d/f
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float v = fputil::multiply_add(mant_f, ONE_OVER_F_F[f], -1.0f);
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// Degree-3 minimax polynomial generated by Sollya with the following
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// commands:
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// > display = hexadecimal;
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// > P = fpminimax(log2(1 + x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
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// > x * P;
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float log2p1_d_over_f =
143+
v * fputil::polyeval(v, 0x1.715476p+0f, -0x1.71771ap-1f, 0x1.ecb38ep-2f);
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// log2(1.mant) = log2(f) + log2(1 + d/f)
145+
float log2_1_mant = LOG2F_F[f] + log2p1_d_over_f;
146+
return fputil::cast<float16>(static_cast<float>(m) + log2_1_mant);
147+
}
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} // namespace LIBC_NAMESPACE_DECL

‎libc/src/math/generic/logf16.cpp

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@@ -115,7 +115,7 @@ LLVM_LIBC_FUNCTION(float16, logf16, (float16 x)) {
115115
// log(1.mant) = log(f) + log(1.mant / f)
116116
// = log(f) + log(1 + d/f)
117117
// since d/f is sufficiently small.
118-
// We store log(f) and 1/f in the lookup tables LOGF_F and ONE_OVER_F
118+
// We store log(f) and 1/f in the lookup tables LOGF_F and ONE_OVER_F_F
119119
// respectively.
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121121
int m = -FPBits::EXP_BIAS;

‎libc/src/math/log2f16.h

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//===-- Implementation header for log2f16 -----------------------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4+
// See https://llvm.org/LICENSE.txt for license information.
5+
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_LIBC_SRC_MATH_LOG2F16_H
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#define LLVM_LIBC_SRC_MATH_LOG2F16_H
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#include "src/__support/macros/config.h"
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#include "src/__support/macros/properties/types.h"
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namespace LIBC_NAMESPACE_DECL {
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float16 log2f16(float16 x);
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} // namespace LIBC_NAMESPACE_DECL
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#endif // LLVM_LIBC_SRC_MATH_LOG2F16_H

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