[libc-commits] [libc] [llvm] [libc][math] Refactor log2f16 to Header Only. (PR #176526)
via libc-commits
libc-commits at lists.llvm.org
Fri Jan 16 17:26:01 PST 2026
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
Author: Anonmiraj (AnonMiraj)
<details>
<summary>Changes</summary>
@<!-- -->bassiounix
closes : #<!-- -->176513
---
Full diff: https://github.com/llvm/llvm-project/pull/176526.diff
9 Files Affected:
- (modified) libc/shared/math.h (+1)
- (added) libc/shared/math/log2f16.h (+28)
- (modified) libc/src/__support/math/CMakeLists.txt (+18)
- (added) libc/src/__support/math/log2f16.h (+168)
- (modified) libc/src/math/generic/CMakeLists.txt (+2-11)
- (modified) libc/src/math/generic/log2f16.cpp (+2-140)
- (modified) libc/test/shared/CMakeLists.txt (+1)
- (modified) libc/test/shared/shared_math_test.cpp (+1)
- (modified) utils/bazel/llvm-project-overlay/libc/BUILD.bazel (+18-1)
``````````diff
diff --git a/libc/shared/math.h b/libc/shared/math.h
index 017c94f8ad54a..6957d65bdf783 100644
--- a/libc/shared/math.h
+++ b/libc/shared/math.h
@@ -71,6 +71,7 @@
#include "math/ldexpf16.h"
#include "math/llogbf.h"
#include "math/log.h"
+#include "math/log2f16.h"
#include "math/logbf.h"
#include "math/logbf128.h"
#include "math/logbf16.h"
diff --git a/libc/shared/math/log2f16.h b/libc/shared/math/log2f16.h
new file mode 100644
index 0000000000000..f882b77a65a02
--- /dev/null
+++ b/libc/shared/math/log2f16.h
@@ -0,0 +1,28 @@
+//===-- Shared log2f16 function ---------------------------------*- 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_SHARED_MATH_LOG2F16_H
+#define LLVM_LIBC_SHARED_MATH_LOG2F16_H
+
+#include "include/llvm-libc-macros/float16-macros.h"
+#include "shared/libc_common.h"
+#ifdef LIBC_TYPES_HAS_FLOAT16
+
+#include "src/__support/math/log2f16.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::log2f16;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LIBC_TYPES_HAS_FLOAT16
+
+#endif // LLVM_LIBC_SHARED_MATH_LOG2F16_H
diff --git a/libc/src/__support/math/CMakeLists.txt b/libc/src/__support/math/CMakeLists.txt
index cb03b18678145..3b50c95b662e6 100644
--- a/libc/src/__support/math/CMakeLists.txt
+++ b/libc/src/__support/math/CMakeLists.txt
@@ -1103,6 +1103,24 @@ add_header_library(
libc.src.__support.macros.optimization
)
+add_header_library(
+ log2f16
+ HDRS
+ log2f16.h
+ DEPENDS
+ 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
+ libc.src.__support.math.expxf16_utils
+)
+
add_header_library(
logbf128
HDRS
diff --git a/libc/src/__support/math/log2f16.h b/libc/src/__support/math/log2f16.h
new file mode 100644
index 0000000000000..3c540bc6a9dae
--- /dev/null
+++ b/libc/src/__support/math/log2f16.h
@@ -0,0 +1,168 @@
+//===-- Implementation header for log2f16 ------------------------*- 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_LOG2F16_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_LOG2F16_H
+
+#include "include/llvm-libc-macros/float16-macros.h"
+
+#ifdef LIBC_TYPES_HAS_FLOAT16
+
+#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"
+#include "src/__support/math/expxf16_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+#ifdef LIBC_TARGET_CPU_HAS_FMA_FLOAT
+static constexpr size_t N_LOG2F16_EXCEPTS = 2;
+#else
+static constexpr size_t N_LOG2F16_EXCEPTS = 9;
+#endif
+
+static constexpr fputil::ExceptValues<float16, N_LOG2F16_EXCEPTS>
+ LOG2F16_EXCEPTS = {{
+// (input, RZ output, RU offset, RD offset, RN offset)
+#ifndef LIBC_TARGET_CPU_HAS_FMA_FLOAT
+ // x = 0x1.224p-1, log2f16(x) = -0x1.a34p-1 (RZ)
+ {0x3889U, 0xba8dU, 0U, 1U, 0U},
+ // x = 0x1.e34p-1, log2f16(x) = -0x1.558p-4 (RZ)
+ {0x3b8dU, 0xad56U, 0U, 1U, 0U},
+#endif
+ // x = 0x1.e8cp-1, log2f16(x) = -0x1.128p-4 (RZ)
+ {0x3ba3U, 0xac4aU, 0U, 1U, 0U},
+#ifndef LIBC_TARGET_CPU_HAS_FMA_FLOAT
+ // x = 0x1.f98p-1, log2f16(x) = -0x1.2ep-6 (RZ)
+ {0x3be6U, 0xa4b8U, 0U, 1U, 0U},
+ // x = 0x1.facp-1, log2f16(x) = -0x1.e7p-7 (RZ)
+ {0x3bebU, 0xa39cU, 0U, 1U, 1U},
+#endif
+ // x = 0x1.fb4p-1, log2f16(x) = -0x1.b88p-7 (RZ)
+ {0x3bedU, 0xa2e2U, 0U, 1U, 1U},
+#ifndef LIBC_TARGET_CPU_HAS_FMA_FLOAT
+ // x = 0x1.fecp-1, log2f16(x) = -0x1.cep-9 (RZ)
+ {0x3bfbU, 0x9b38U, 0U, 1U, 1U},
+ // x = 0x1.ffcp-1, log2f16(x) = -0x1.714p-11 (RZ)
+ {0x3bffU, 0x91c5U, 0U, 1U, 1U},
+ // x = 0x1.224p+0, log2f16(x) = 0x1.72cp-3 (RZ)
+ {0x3c89U, 0x31cbU, 1U, 0U, 1U},
+#endif
+ }};
+#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+
+LLVM_LIBC_FUNCTION(float16, log2f16, (float16 x)) {
+ using namespace math::expxf16_internal;
+ 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)) {
+ // log2(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;
+ }
+
+ // log2(+/-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();
+ }
+
+ // log2(+inf) = +inf
+ return FPBits::inf().get_val();
+ }
+
+#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+ if (auto r = LOG2F16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+ return r.value();
+#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+
+ // To compute log2(x), we perform the following range reduction:
+ // x = 2^m * 1.mant,
+ // log2(x) = m + log2(1.mant).
+ // To compute log2(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:
+ // log2(1.mant) = log2(f) + log2(1.mant / f)
+ // = log2(f) + log2(1 + d/f)
+ // since d/f is sufficiently small.
+ // We store log2(f) and 1/f in the lookup tables LOG2F_F and ONE_OVER_F_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(log2(1 + x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
+ // > x * P;
+ float log2p1_d_over_f =
+ v * fputil::polyeval(v, 0x1.715476p+0f, -0x1.71771ap-1f, 0x1.ecb38ep-2f);
+ // log2(1.mant) = log2(f) + log2(1 + d/f)
+ float log2_1_mant = LOG2F_F[f] + log2p1_d_over_f;
+ return fputil::cast<float16>(static_cast<float>(m) + log2_1_mant);
+}
+
+} // namespace math
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LIBC_TYPES_HAS_FLOAT16
+
+#endif // LLVM_LIBC_SRC___SUPPORT_MATH_LOG2F16_H
diff --git a/libc/src/math/generic/CMakeLists.txt b/libc/src/math/generic/CMakeLists.txt
index e4ad6fcafeaf0..4b5ca4e6b5ec4 100644
--- a/libc/src/math/generic/CMakeLists.txt
+++ b/libc/src/math/generic/CMakeLists.txt
@@ -2082,17 +2082,8 @@ add_entrypoint_object(
HDRS
../log2f16.h
DEPENDS
- 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
- libc.src.__support.math.expxf16_utils
+ libc.src.__support.math.log2f16
+ libc.src.errno.errno
)
add_entrypoint_object(
diff --git a/libc/src/math/generic/log2f16.cpp b/libc/src/math/generic/log2f16.cpp
index 5b60323aabf35..5b8490c886639 100644
--- a/libc/src/math/generic/log2f16.cpp
+++ b/libc/src/math/generic/log2f16.cpp
@@ -7,148 +7,10 @@
//===----------------------------------------------------------------------===//
#include "src/math/log2f16.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"
-#include "src/__support/math/expxf16_utils.h"
+#include "src/__support/math/log2f16.h"
namespace LIBC_NAMESPACE_DECL {
-#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-#ifdef LIBC_TARGET_CPU_HAS_FMA_FLOAT
-static constexpr size_t N_LOG2F16_EXCEPTS = 2;
-#else
-static constexpr size_t N_LOG2F16_EXCEPTS = 9;
-#endif
-
-static constexpr fputil::ExceptValues<float16, N_LOG2F16_EXCEPTS>
- LOG2F16_EXCEPTS = {{
-// (input, RZ output, RU offset, RD offset, RN offset)
-#ifndef LIBC_TARGET_CPU_HAS_FMA_FLOAT
- // x = 0x1.224p-1, log2f16(x) = -0x1.a34p-1 (RZ)
- {0x3889U, 0xba8dU, 0U, 1U, 0U},
- // x = 0x1.e34p-1, log2f16(x) = -0x1.558p-4 (RZ)
- {0x3b8dU, 0xad56U, 0U, 1U, 0U},
-#endif
- // x = 0x1.e8cp-1, log2f16(x) = -0x1.128p-4 (RZ)
- {0x3ba3U, 0xac4aU, 0U, 1U, 0U},
-#ifndef LIBC_TARGET_CPU_HAS_FMA_FLOAT
- // x = 0x1.f98p-1, log2f16(x) = -0x1.2ep-6 (RZ)
- {0x3be6U, 0xa4b8U, 0U, 1U, 0U},
- // x = 0x1.facp-1, log2f16(x) = -0x1.e7p-7 (RZ)
- {0x3bebU, 0xa39cU, 0U, 1U, 1U},
-#endif
- // x = 0x1.fb4p-1, log2f16(x) = -0x1.b88p-7 (RZ)
- {0x3bedU, 0xa2e2U, 0U, 1U, 1U},
-#ifndef LIBC_TARGET_CPU_HAS_FMA_FLOAT
- // x = 0x1.fecp-1, log2f16(x) = -0x1.cep-9 (RZ)
- {0x3bfbU, 0x9b38U, 0U, 1U, 1U},
- // x = 0x1.ffcp-1, log2f16(x) = -0x1.714p-11 (RZ)
- {0x3bffU, 0x91c5U, 0U, 1U, 1U},
- // x = 0x1.224p+0, log2f16(x) = 0x1.72cp-3 (RZ)
- {0x3c89U, 0x31cbU, 1U, 0U, 1U},
-#endif
- }};
-#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-
-LLVM_LIBC_FUNCTION(float16, log2f16, (float16 x)) {
- using namespace math::expxf16_internal;
- 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)) {
- // log2(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;
- }
-
- // log2(+/-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();
- }
-
- // log2(+inf) = +inf
- return FPBits::inf().get_val();
- }
-
-#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
- if (auto r = LOG2F16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
- return r.value();
-#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-
- // To compute log2(x), we perform the following range reduction:
- // x = 2^m * 1.mant,
- // log2(x) = m + log2(1.mant).
- // To compute log2(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:
- // log2(1.mant) = log2(f) + log2(1.mant / f)
- // = log2(f) + log2(1 + d/f)
- // since d/f is sufficiently small.
- // We store log2(f) and 1/f in the lookup tables LOG2F_F and ONE_OVER_F_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(log2(1 + x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
- // > x * P;
- float log2p1_d_over_f =
- v * fputil::polyeval(v, 0x1.715476p+0f, -0x1.71771ap-1f, 0x1.ecb38ep-2f);
- // log2(1.mant) = log2(f) + log2(1 + d/f)
- float log2_1_mant = LOG2F_F[f] + log2p1_d_over_f;
- return fputil::cast<float16>(static_cast<float>(m) + log2_1_mant);
-}
+LLVM_LIBC_FUNCTION(float16, log2f16, (float16 x)) { return math::log2f16(x); }
} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/shared/CMakeLists.txt b/libc/test/shared/CMakeLists.txt
index 61625c0bc0444..1ac1e7b12f3b5 100644
--- a/libc/test/shared/CMakeLists.txt
+++ b/libc/test/shared/CMakeLists.txt
@@ -73,4 +73,5 @@ add_fp_unittest(
libc.src.__support.math.rsqrtf
libc.src.__support.math.rsqrtf16
libc.src.__support.math.sin
+ libc.src.__support.math.log2f16
)
diff --git a/libc/test/shared/shared_math_test.cpp b/libc/test/shared/shared_math_test.cpp
index 49047631fb227..234d9842d8142 100644
--- a/libc/test/shared/shared_math_test.cpp
+++ b/libc/test/shared/shared_math_test.cpp
@@ -42,6 +42,7 @@ TEST(LlvmLibcSharedMathTest, AllFloat16) {
EXPECT_EQ(exponent, 5);
EXPECT_EQ(0, LIBC_NAMESPACE::shared::ilogbf16(1.0f16));
+ EXPECT_FP_EQ(0x1p+0f16, LIBC_NAMESPACE::shared::log2f16(2.0f16));
EXPECT_FP_EQ(0x0p+0f16, LIBC_NAMESPACE::shared::logbf16(1.0f16));
EXPECT_FP_EQ(0x1.921fb6p+0f16, LIBC_NAMESPACE::shared::acosf16(0.0f16));
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
index 3d9491e8baa0c..ac47d4cd256ce 100644
--- a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -2932,6 +2932,22 @@ libc_support_library(
],
)
+libc_support_library(
+ name = "__support_math_log2f16",
+ hdrs = ["src/__support/math/log2f16.h"],
+ deps = [
+ ":__support_fputil_cast",
+ ":__support_fputil_except_value_utils",
+ ":__support_fputil_fenv_impl",
+ ":__support_fputil_fp_bits",
+ ":__support_fputil_multiply_add",
+ ":__support_fputil_polyeval",
+ ":__support_macros_optimization",
+ ":__support_macros_properties",
+ ":__support_math_expxf16_utils",
+ ],
+)
+
libc_support_library(
name = "__support_math_logbf",
hdrs = ["src/__support/math/logbf.h"],
@@ -4609,7 +4625,8 @@ libc_math_function(
libc_math_function(
name = "log2f16",
additional_deps = [
- ":__support_math_expxf16_utils",
+ ":__support_math_log2f16",
+ ":errno",
],
)
``````````
</details>
https://github.com/llvm/llvm-project/pull/176526
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