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[libc][math][c23] Add logf16 C23 math function (#106072)
Part of #95250.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -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 | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
|
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| #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" | ||
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| namespace LIBC_NAMESPACE_DECL { | ||
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| #ifdef LIBC_TARGET_CPU_HAS_FMA | ||
| static constexpr size_t N_LOGF16_EXCEPTS = 5; | ||
| #else | ||
| static constexpr size_t N_LOGF16_EXCEPTS = 11; | ||
| #endif | ||
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| 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 | ||
| }}; | ||
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| LLVM_LIBC_FUNCTION(float16, logf16, (float16 x)) { | ||
| using FPBits = fputil::FPBits<float16>; | ||
| FPBits x_bits(x); | ||
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| uint16_t x_u = x_bits.uintval(); | ||
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| // 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(); | ||
| } | ||
|
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| return x; | ||
| } | ||
|
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| // log(+/-0) = −inf | ||
| if ((x_u & 0x7fffU) == 0U) { | ||
| fputil::raise_except_if_required(FE_DIVBYZERO); | ||
| return FPBits::inf(Sign::NEG).get_val(); | ||
| } | ||
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| if (x_u == 0x3c00U) | ||
| return FPBits::zero().get_val(); | ||
|
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| // 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(); | ||
| } | ||
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| // log(+inf) = +inf | ||
| return FPBits::inf().get_val(); | ||
| } | ||
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| if (auto r = LOGF16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value())) | ||
| return r.value(); | ||
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| // 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. | ||
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| int m = -FPBits::EXP_BIAS; | ||
|
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| // 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; | ||
| } | ||
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| 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; | ||
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| // 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); | ||
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| // 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)); | ||
| } | ||
|
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||
| } // namespace LIBC_NAMESPACE_DECL |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -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 | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
|
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| #ifndef LLVM_LIBC_SRC_MATH_LOGF16_H | ||
| #define LLVM_LIBC_SRC_MATH_LOGF16_H | ||
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| #include "src/__support/macros/config.h" | ||
| #include "src/__support/macros/properties/types.h" | ||
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| namespace LIBC_NAMESPACE_DECL { | ||
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| float16 logf16(float16 x); | ||
|
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| } // namespace LIBC_NAMESPACE_DECL | ||
|
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| #endif // LLVM_LIBC_SRC_MATH_LOGF16_H |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -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 | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
|
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| #include "src/math/logf16.h" | ||
| #include "test/UnitTest/FPMatcher.h" | ||
| #include "test/UnitTest/Test.h" | ||
| #include "utils/MPFRWrapper/MPFRUtils.h" | ||
|
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| using LlvmLibcLogf16Test = LIBC_NAMESPACE::testing::FPTest<float16>; | ||
|
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| namespace mpfr = LIBC_NAMESPACE::testing::mpfr; | ||
|
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| // Range: [0, Inf]; | ||
| static constexpr uint16_t POS_START = 0x0000U; | ||
| static constexpr uint16_t POS_STOP = 0x7c00U; | ||
|
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| // Range: [-Inf, 0]; | ||
| static constexpr uint16_t NEG_START = 0x8000U; | ||
| static constexpr uint16_t NEG_STOP = 0xfc00U; | ||
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| 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); | ||
| } | ||
| } | ||
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| 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); | ||
| } | ||
| } |
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