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[libc][math][c23] Add logf16 C23 math function
Part of #95250.
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14 files changed

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‎libc/config/gpu/entrypoints.txt‎

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@@ -532,6 +532,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
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libc.src.math.llrintf16
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libc.src.math.llroundf16
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libc.src.math.logbf16
535+
libc.src.math.logf16
535536
libc.src.math.lrintf16
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libc.src.math.lroundf16
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libc.src.math.modff16

‎libc/config/linux/x86_64/entrypoints.txt‎

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@@ -643,6 +643,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
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libc.src.math.llrintf16
644644
libc.src.math.llroundf16
645645
libc.src.math.logbf16
646+
libc.src.math.logf16
646647
libc.src.math.lrintf16
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libc.src.math.lroundf16
648649
libc.src.math.modff16

‎libc/docs/math/index.rst‎

Lines changed: 1 addition & 1 deletion
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@@ -306,7 +306,7 @@ Higher Math Functions
306306
+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
307307
| lgamma | | | | | | 7.12.8.3 | F.10.5.3 |
308308
+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
309-
| log | |check| | |check| | | | | 7.12.6.11 | F.10.3.11 |
309+
| log | |check| | |check| | | |check| | | 7.12.6.11 | F.10.3.11 |
310310
+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+
311311
| log10 | |check| | |check| | | | | 7.12.6.12 | F.10.3.12 |
312312
+-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+----------------------------+

‎libc/spec/stdc.td‎

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@@ -565,6 +565,7 @@ def StdC : StandardSpec<"stdc"> {
565565

566566
FunctionSpec<"log", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
567567
FunctionSpec<"logf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
568+
GuardedFunctionSpec<"logf16", RetValSpec<Float16Type>, [ArgSpec<Float16Type>], "LIBC_TYPES_HAS_FLOAT16">,
568569

569570
FunctionSpec<"logb", RetValSpec<DoubleType>, [ArgSpec<DoubleType>]>,
570571
FunctionSpec<"logbf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,

‎libc/src/math/CMakeLists.txt‎

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@@ -331,6 +331,7 @@ add_math_entrypoint_object(log2f)
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332332
add_math_entrypoint_object(log)
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add_math_entrypoint_object(logf)
334+
add_math_entrypoint_object(logf16)
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335336
add_math_entrypoint_object(logb)
336337
add_math_entrypoint_object(logbf)

‎libc/src/math/generic/CMakeLists.txt‎

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Original file line numberDiff line numberDiff line change
@@ -2210,6 +2210,28 @@ add_entrypoint_object(
22102210
-O3
22112211
)
22122212

2213+
add_entrypoint_object(
2214+
logf16
2215+
SRCS
2216+
logf16.cpp
2217+
HDRS
2218+
../logf16.h
2219+
DEPENDS
2220+
.expxf16
2221+
libc.hdr.errno_macros
2222+
libc.hdr.fenv_macros
2223+
libc.src.__support.CPP.array
2224+
libc.src.__support.FPUtil.except_value_utils
2225+
libc.src.__support.FPUtil.fenv_impl
2226+
libc.src.__support.FPUtil.fp_bits
2227+
libc.src.__support.FPUtil.multiply_add
2228+
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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logb
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SRCS

‎libc/src/math/generic/expxf16.h‎

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@@ -287,6 +287,32 @@ template <bool IsSinh> LIBC_INLINE float16 eval_sinh_or_cosh(float16 x) {
287287
lo, half_p_odd * exp2_hi_mid_diff, half_p_even * exp2_hi_mid_sum));
288288
}
289289

290+
// Generated by Sollya with the following commands:
291+
// > display = hexadecimal;
292+
// > for i from 0 to 31 do print(round(log(1 + i * 2^-5), SG, RN));
293+
constexpr cpp::array<float, 32> LOGF_F = {
294+
0x0p+0, 0x1.f829bp-6, 0x1.f0a30cp-5, 0x1.6f0d28p-4, 0x1.e27076p-4,
295+
0x1.29553p-3, 0x1.5ff308p-3, 0x1.9525aap-3, 0x1.c8ff7cp-3, 0x1.fb9186p-3,
296+
0x1.1675cap-2, 0x1.2e8e2cp-2, 0x1.4618bcp-2, 0x1.5d1bdcp-2, 0x1.739d8p-2,
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0x1.89a338p-2, 0x1.9f323ep-2, 0x1.b44f78p-2, 0x1.c8ff7cp-2, 0x1.dd46ap-2,
298+
0x1.f128f6p-2, 0x1.02552ap-1, 0x1.0be72ep-1, 0x1.154c3ep-1, 0x1.1e85f6p-1,
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0x1.2795e2p-1, 0x1.307d74p-1, 0x1.393e0ep-1, 0x1.41d8fep-1, 0x1.4a4f86p-1,
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0x1.52a2d2p-1, 0x1.5ad404p-1,
301+
};
302+
303+
// Generated by Sollya with the following commands:
304+
// > display = hexadecimal;
305+
// > for i from 0 to 31 do print(round(1 / (1 + i * 2^-5), SG, RN));
306+
constexpr cpp::array<float, 32> ONE_OVER_F = {
307+
0x1p+0, 0x1.f07c2p-1, 0x1.e1e1e2p-1, 0x1.d41d42p-1, 0x1.c71c72p-1,
308+
0x1.bacf92p-1, 0x1.af286cp-1, 0x1.a41a42p-1, 0x1.99999ap-1, 0x1.8f9c18p-1,
309+
0x1.861862p-1, 0x1.7d05f4p-1, 0x1.745d18p-1, 0x1.6c16c2p-1, 0x1.642c86p-1,
310+
0x1.5c9882p-1, 0x1.555556p-1, 0x1.4e5e0ap-1, 0x1.47ae14p-1, 0x1.414142p-1,
311+
0x1.3b13b2p-1, 0x1.3521dp-1, 0x1.2f684cp-1, 0x1.29e412p-1, 0x1.24924ap-1,
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0x1.1f7048p-1, 0x1.1a7b96p-1, 0x1.15b1e6p-1, 0x1.111112p-1, 0x1.0c9714p-1,
313+
0x1.08421p-1, 0x1.041042p-1,
314+
};
315+
290316
} // namespace LIBC_NAMESPACE_DECL
291317

292318
#endif // LLVM_LIBC_SRC_MATH_GENERIC_EXPXF16_H

‎libc/src/math/generic/logf16.cpp‎

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@@ -0,0 +1,158 @@
1+
//===-- Half-precision log(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
6+
//
7+
//===----------------------------------------------------------------------===//
8+
9+
#include "src/math/logf16.h"
10+
#include "expxf16.h"
11+
#include "hdr/errno_macros.h"
12+
#include "hdr/fenv_macros.h"
13+
#include "src/__support/CPP/array.h"
14+
#include "src/__support/FPUtil/FEnvImpl.h"
15+
#include "src/__support/FPUtil/FPBits.h"
16+
#include "src/__support/FPUtil/PolyEval.h"
17+
#include "src/__support/FPUtil/except_value_utils.h"
18+
#include "src/__support/FPUtil/multiply_add.h"
19+
#include "src/__support/common.h"
20+
#include "src/__support/macros/config.h"
21+
#include "src/__support/macros/optimization.h"
22+
#include "src/__support/macros/properties/cpu_features.h"
23+
24+
namespace LIBC_NAMESPACE_DECL {
25+
26+
#ifdef LIBC_TARGET_CPU_HAS_FMA
27+
static constexpr size_t N_LOGF16_EXCEPTS = 5;
28+
#else
29+
static constexpr size_t N_LOGF16_EXCEPTS = 11;
30+
#endif
31+
32+
static constexpr fputil::ExceptValues<float16, N_LOGF16_EXCEPTS>
33+
LOGF16_EXCEPTS = {{
34+
// (input, RZ output, RU offset, RD offset, RN offset)
35+
#ifndef LIBC_TARGET_CPU_HAS_FMA
36+
// x = 0x1.61cp-13, logf16(x) = -0x1.16p+3 (RZ)
37+
{0x0987U, 0xc858U, 0U, 1U, 0U},
38+
// x = 0x1.f2p-12, logf16(x) = -0x1.e98p+2 (RZ)
39+
{0x0fc8U, 0xc7a6U, 0U, 1U, 1U},
40+
// x = 0x1.4d4p-9, logf16(x) = -0x1.7e4p+2 (RZ)
41+
#endif
42+
// x = 0x1.4d4p-9, logf16(x) = -0x1.7e4p+2 (RZ)
43+
{0x1935U, 0xc5f9U, 0U, 1U, 0U},
44+
// x = 0x1.5ep-8, logf16(x) = -0x1.4ecp+2 (RZ)
45+
{0x1d78U, 0xc53bU, 0U, 1U, 0U},
46+
#ifndef LIBC_TARGET_CPU_HAS_FMA
47+
// x = 0x1.fdp-1, logf16(x) = -0x1.81p-8 (RZ)
48+
{0x3bf4U, 0x9e04U, 0U, 1U, 1U},
49+
// x = 0x1.fep-1, logf16(x) = -0x1.008p-8 (RZ)
50+
{0x3bf8U, 0x9c02U, 0U, 1U, 0U},
51+
#endif
52+
// x = 0x1.ffp-1, logf16(x) = -0x1.004p-9 (RZ)
53+
{0x3bfcU, 0x9801U, 0U, 1U, 0U},
54+
// x = 0x1.ff8p-1, logf16(x) = -0x1p-10 (RZ)
55+
{0x3bfeU, 0x9400U, 0U, 1U, 1U},
56+
#ifdef LIBC_TARGET_CPU_HAS_FMA
57+
// x = 0x1.4c4p+1, logf16(x) = 0x1.e84p-1 (RZ)
58+
{0x4131U, 0x3ba1U, 1U, 0U, 1U},
59+
#else
60+
// x = 0x1.75p+2, logf16(x) = 0x1.c34p+0 (RZ)
61+
{0x45d4U, 0x3f0dU, 1U, 0U, 0U},
62+
// x = 0x1.75p+2, logf16(x) = 0x1.c34p+0 (RZ)
63+
{0x45d4U, 0x3f0dU, 1U, 0U, 0U},
64+
// x = 0x1.d5p+9, logf16(x) = 0x1.b5cp+2 (RZ)
65+
{0x6354U, 0x46d7U, 1U, 0U, 1U},
66+
#endif
67+
}};
68+
69+
LLVM_LIBC_FUNCTION(float16, logf16, (float16 x)) {
70+
using FPBits = fputil::FPBits<float16>;
71+
FPBits x_bits(x);
72+
73+
uint16_t x_u = x_bits.uintval();
74+
75+
// If x <= 0, or x is 1, or x is +inf, or x is NaN.
76+
if (LIBC_UNLIKELY(x_u == 0U || x_u == 0x3c00U || x_u >= 0x7c00U)) {
77+
// log(NaN) = NaN
78+
if (x_bits.is_nan()) {
79+
if (x_bits.is_signaling_nan()) {
80+
fputil::raise_except_if_required(FE_INVALID);
81+
return FPBits::quiet_nan().get_val();
82+
}
83+
84+
return x;
85+
}
86+
87+
// log(+/-0) = −inf
88+
if ((x_u & 0x7fffU) == 0U) {
89+
fputil::raise_except_if_required(FE_DIVBYZERO);
90+
return FPBits::inf(Sign::NEG).get_val();
91+
}
92+
93+
if (x_u == 0x3c00U)
94+
return FPBits::zero().get_val();
95+
96+
// When x < 0.
97+
if (x_u > 0x8000) {
98+
fputil::set_errno_if_required(EDOM);
99+
fputil::raise_except_if_required(FE_INVALID);
100+
return FPBits::quiet_nan().get_val();
101+
}
102+
103+
// log(+inf) = +inf
104+
return FPBits::inf().get_val();
105+
}
106+
107+
if (auto r = LOGF16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
108+
return r.value();
109+
110+
// To compute log(x), we perform the following range reduction:
111+
// x = 2^m * 1.mant,
112+
// log(x) = m * log(2) + log(1.mant).
113+
// To compute log(1.mant), let f be the highest 6 bits including the hidden
114+
// bit, and d be the difference (1.mant - f), i.e., the remaining 5 bits of
115+
// the mantissa, then:
116+
// log(1.mant) = log(f) + log(1.mant / f)
117+
// = log(f) + log(1 + d/f)
118+
// since d/f is sufficiently small.
119+
// We store log(f) and 1/f in the lookup tables LOGF_F and ONE_OVER_F
120+
// respectively.
121+
122+
int m = -FPBits::EXP_BIAS;
123+
124+
// When x is subnormal.
125+
if ((x_u & FPBits::EXP_MASK) == 0U) {
126+
// Normalize x.
127+
x_bits = FPBits(x_bits.get_val() *
128+
static_cast<float16>((1U << FPBits::FRACTION_LEN)));
129+
x_u = x_bits.uintval();
130+
m -= FPBits::FRACTION_LEN;
131+
}
132+
133+
uint16_t mant = x_bits.get_mantissa();
134+
// Leading 10 - 5 = 5 bits of the mantissa.
135+
int f = mant >> 5;
136+
// Unbiased exponent.
137+
m += x_u >> FPBits::FRACTION_LEN;
138+
139+
// Set bits to 1.mant instead of 2^m * 1.mant.
140+
x_bits.set_biased_exponent(FPBits::EXP_BIAS);
141+
float mant_f = x_bits.get_val();
142+
// v = 1.mant * 1/f - 1 = d/f
143+
float v = fputil::multiply_add(mant_f, ONE_OVER_F[f], -1.0f);
144+
145+
// Degree-3 minimax polynomial generated by Sollya with the following
146+
// commands:
147+
// > display = hexadecimal;
148+
// > P = fpminimax(log(1 + x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
149+
// > x * P;
150+
float log1p_d_over_f =
151+
v * fputil::polyeval(v, 0x1p+0f, -0x1.001804p-1f, 0x1.557ef6p-2f);
152+
// log(1.mant) = log(f) + log(1 + d/f)
153+
float log_1_mant = LOGF_F[f] + log1p_d_over_f;
154+
return static_cast<float16>(
155+
fputil::multiply_add(static_cast<float>(m), LOGF_2, log_1_mant));
156+
}
157+
158+
} // namespace LIBC_NAMESPACE_DECL

‎libc/src/math/logf16.h‎

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@@ -0,0 +1,21 @@
1+
//===-- Implementation header for logf16 ------------------------*- C++ -*-===//
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
6+
//
7+
//===----------------------------------------------------------------------===//
8+
9+
#ifndef LLVM_LIBC_SRC_MATH_LOGF16_H
10+
#define LLVM_LIBC_SRC_MATH_LOGF16_H
11+
12+
#include "src/__support/macros/config.h"
13+
#include "src/__support/macros/properties/types.h"
14+
15+
namespace LIBC_NAMESPACE_DECL {
16+
17+
float16 logf16(float16 x);
18+
19+
} // namespace LIBC_NAMESPACE_DECL
20+
21+
#endif // LLVM_LIBC_SRC_MATH_LOGF16_H

‎libc/test/UnitTest/FPMatcher.h‎

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Original file line numberDiff line numberDiff line change
@@ -266,4 +266,17 @@ template <typename T> struct FPTest : public Test {
266266
EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_MODE( \
267267
(expected), (actual), (expected_except), RoundingMode::TowardZero)
268268

269+
#define EXPECT_FP_EQ_WITH_EXCEPTION_ALL_ROUNDING(expected, actual, \
270+
expected_except) \
271+
do { \
272+
EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_NEAREST((expected), (actual), \
273+
(expected_except)); \
274+
EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_UPWARD((expected), (actual), \
275+
(expected_except)); \
276+
EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_DOWNWARD((expected), (actual), \
277+
(expected_except)); \
278+
EXPECT_FP_EQ_WITH_EXCEPTION_ROUNDING_TOWARD_ZERO((expected), (actual), \
279+
(expected_except)); \
280+
} while (0)
281+
269282
#endif // LLVM_LIBC_TEST_UNITTEST_FPMATCHER_H

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