[libc-commits] [libc] [llvm] [libc][math] Refactor log2f to Header Only. (PR #176527)
via libc-commits
libc-commits at lists.llvm.org
Fri Jan 16 17:31:42 PST 2026
https://github.com/AnonMiraj created https://github.com/llvm/llvm-project/pull/176527
@bassiounix
closes : #176514
>From 909efdb73923f8757eb02ff97217de4a8503671d Mon Sep 17 00:00:00 2001
From: anonmiraj <nabilmalek48 at gmail.com>
Date: Sat, 17 Jan 2026 03:30:44 +0200
Subject: [PATCH] [libc][math] Refactor log2f to Header Only.
---
libc/shared/math.h | 1 +
libc/shared/math/log2f.h | 22 +++
libc/src/__support/math/CMakeLists.txt | 14 ++
libc/src/__support/math/log2f.h | 133 ++++++++++++++++++
libc/src/math/generic/CMakeLists.txt | 9 +-
libc/src/math/generic/log2f.cpp | 113 +--------------
libc/test/shared/CMakeLists.txt | 1 +
libc/test/shared/shared_math_test.cpp | 1 +
.../llvm-project-overlay/libc/BUILD.bazel | 21 ++-
9 files changed, 192 insertions(+), 123 deletions(-)
create mode 100644 libc/shared/math/log2f.h
create mode 100644 libc/src/__support/math/log2f.h
diff --git a/libc/shared/math.h b/libc/shared/math.h
index 017c94f8ad54a..54fc598b979ed 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/log2f.h"
#include "math/logbf.h"
#include "math/logbf128.h"
#include "math/logbf16.h"
diff --git a/libc/shared/math/log2f.h b/libc/shared/math/log2f.h
new file mode 100644
index 0000000000000..29545e7177fb4
--- /dev/null
+++ b/libc/shared/math/log2f.h
@@ -0,0 +1,22 @@
+//===-- Shared log2f 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_LOG2F_H
+#define LLVM_LIBC_SHARED_MATH_LOG2F_H
+#include "shared/libc_common.h"
+#include "src/__support/math/log2f.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::log2f;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_MATH_LOG2F_H
diff --git a/libc/src/__support/math/CMakeLists.txt b/libc/src/__support/math/CMakeLists.txt
index cb03b18678145..ff4bca348d63f 100644
--- a/libc/src/__support/math/CMakeLists.txt
+++ b/libc/src/__support/math/CMakeLists.txt
@@ -1103,6 +1103,20 @@ add_header_library(
libc.src.__support.macros.optimization
)
+add_header_library(
+ log2f
+ HDRS
+ log2f.h
+ DEPENDS
+ libc.src.__support.FPUtil.except_value_utils
+ libc.src.__support.FPUtil.fenv_impl
+ libc.src.__support.FPUtil.fma
+ libc.src.__support.FPUtil.fp_bits
+ libc.src.__support.FPUtil.polyeval
+ libc.src.__support.macros.optimization
+ libc.src.__support.math.common_constants
+)
+
add_header_library(
logbf128
HDRS
diff --git a/libc/src/__support/math/log2f.h b/libc/src/__support/math/log2f.h
new file mode 100644
index 0000000000000..7544afc8fd045
--- /dev/null
+++ b/libc/src/__support/math/log2f.h
@@ -0,0 +1,133 @@
+//===-- Implementation header for log2f ------------------------*- 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_LOG2F_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_LOG2F_H
+
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.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" // LIBC_UNLIKELY
+#include "src/__support/math/common_constants.h" // Lookup table for (1/f)
+
+// This is a correctly-rounded algorithm for log2(x) in single precision with
+// round-to-nearest, tie-to-even mode from the RLIBM project at:
+// https://people.cs.rutgers.edu/~sn349/rlibm
+
+// Step 1 - Range reduction:
+// For x = 2^m * 1.mant, log2(x) = m + log2(1.m)
+// If x is denormal, we normalize it by multiplying x by 2^23 and subtracting
+// m by 23.
+
+// Step 2 - Another range reduction:
+// To compute log(1.mant), let f be the highest 8 bits including the hidden
+// bit, and d be the difference (1.mant - f), i.e. the remaining 16 bits of the
+// mantissa. Then we have the following approximation formula:
+// log2(1.mant) = log2(f) + log2(1.mant / f)
+// = log2(f) + log2(1 + d/f)
+// ~ log2(f) + P(d/f)
+// since d/f is sufficiently small.
+// log2(f) and 1/f are then stored in two 2^7 = 128 entries look-up tables.
+
+// Step 3 - Polynomial approximation:
+// To compute P(d/f), we use a single degree-5 polynomial in double precision
+// which provides correct rounding for all but few exception values.
+// For more detail about how this polynomial is obtained, please refer to the
+// papers:
+// Lim, J. and Nagarakatte, S., "One Polynomial Approximation to Produce
+// Correctly Rounded Results of an Elementary Function for Multiple
+// Representations and Rounding Modes", Proceedings of the 49th ACM SIGPLAN
+// Symposium on Principles of Programming Languages (POPL-2022), Philadelphia,
+// USA, Jan. 16-22, 2022.
+// https://people.cs.rutgers.edu/~sn349/papers/rlibmall-popl-2022.pdf
+// Aanjaneya, M., Lim, J., and Nagarakatte, S., "RLibm-Prog: Progressive
+// Polynomial Approximations for Fast Correctly Rounded Math Libraries",
+// Dept. of Comp. Sci., Rutgets U., Technical Report DCS-TR-758, Nov. 2021.
+// https://arxiv.org/pdf/2111.12852.pdf.
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+LIBC_INLINE static constexpr float log2f(float x) {
+ using namespace common_constants_internal;
+ using FPBits = typename fputil::FPBits<float>;
+
+ FPBits xbits(x);
+ uint32_t x_u = xbits.uintval();
+
+ // Hard to round value(s).
+ using fputil::round_result_slightly_up;
+
+ int m = -FPBits::EXP_BIAS;
+
+ // log2(1.0f) = 0.0f.
+ if (LIBC_UNLIKELY(x_u == 0x3f80'0000U))
+ return 0.0f;
+
+ // Exceptional inputs.
+ if (LIBC_UNLIKELY(x_u < FPBits::min_normal().uintval() ||
+ x_u > FPBits::max_normal().uintval())) {
+ if (x == 0.0f) {
+ fputil::set_errno_if_required(ERANGE);
+ fputil::raise_except_if_required(FE_DIVBYZERO);
+ return FPBits::inf(Sign::NEG).get_val();
+ }
+ if (xbits.is_neg() && !xbits.is_nan()) {
+ fputil::set_errno_if_required(EDOM);
+ fputil::raise_except_if_required(FE_INVALID);
+ return FPBits::quiet_nan().get_val();
+ }
+ if (xbits.is_inf_or_nan()) {
+ return x;
+ }
+ // Normalize denormal inputs.
+ xbits = FPBits(xbits.get_val() * 0x1.0p23f);
+ m -= 23;
+ }
+
+ m += xbits.get_biased_exponent();
+ int index = xbits.get_mantissa() >> 16;
+ // Set bits to 1.m
+ xbits.set_biased_exponent(0x7F);
+
+ float u = xbits.get_val();
+#ifdef LIBC_TARGET_CPU_HAS_FMA_FLOAT
+ double v =
+ static_cast<double>(fputil::multiply_add(u, R[index], -1.0f)); // Exact.
+#else
+ double v =
+ fputil::multiply_add(static_cast<double>(u), RD[index], -1.0); // Exact
+#endif // LIBC_TARGET_CPU_HAS_FMA_FLOAT
+
+ double extra_factor = static_cast<double>(m) + LOG2_R[index];
+
+ // Degree-5 polynomial approximation of log2 generated by Sollya with:
+ // > P = fpminimax(log2(1 + x)/x, 4, [|1, D...|], [-2^-8, 2^-7]);
+ constexpr double COEFFS[5] = {0x1.71547652b8133p0, -0x1.71547652d1e33p-1,
+ 0x1.ec70a098473dep-2, -0x1.7154c5ccdf121p-2,
+ 0x1.2514fd90a130ap-2};
+
+ double vsq = v * v; // Exact
+ double c0 = fputil::multiply_add(v, COEFFS[0], extra_factor);
+ double c1 = fputil::multiply_add(v, COEFFS[2], COEFFS[1]);
+ double c2 = fputil::multiply_add(v, COEFFS[4], COEFFS[3]);
+
+ double r = fputil::polyeval(vsq, c0, c1, c2);
+
+ return static_cast<float>(r);
+}
+
+} // namespace math
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_MATH_LOG2F_H
diff --git a/libc/src/math/generic/CMakeLists.txt b/libc/src/math/generic/CMakeLists.txt
index e4ad6fcafeaf0..aa6c3f076eb35 100644
--- a/libc/src/math/generic/CMakeLists.txt
+++ b/libc/src/math/generic/CMakeLists.txt
@@ -2066,13 +2066,8 @@ add_entrypoint_object(
HDRS
../log2f.h
DEPENDS
- libc.src.__support.FPUtil.except_value_utils
- libc.src.__support.FPUtil.fenv_impl
- libc.src.__support.FPUtil.fp_bits
- libc.src.__support.FPUtil.fma
- libc.src.__support.FPUtil.polyeval
- libc.src.__support.macros.optimization
- libc.src.__support.math.common_constants
+ libc.src.__support.math.log2f
+ libc.src.errno.errno
)
add_entrypoint_object(
diff --git a/libc/src/math/generic/log2f.cpp b/libc/src/math/generic/log2f.cpp
index 7353f0302ab3e..9c9d34ca3ddd3 100644
--- a/libc/src/math/generic/log2f.cpp
+++ b/libc/src/math/generic/log2f.cpp
@@ -7,119 +7,10 @@
//===----------------------------------------------------------------------===//
#include "src/math/log2f.h"
-#include "src/__support/FPUtil/FEnvImpl.h"
-#include "src/__support/FPUtil/FPBits.h"
-#include "src/__support/FPUtil/PolyEval.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" // LIBC_UNLIKELY
-#include "src/__support/math/common_constants.h" // Lookup table for (1/f)
-
-// This is a correctly-rounded algorithm for log2(x) in single precision with
-// round-to-nearest, tie-to-even mode from the RLIBM project at:
-// https://people.cs.rutgers.edu/~sn349/rlibm
-
-// Step 1 - Range reduction:
-// For x = 2^m * 1.mant, log2(x) = m + log2(1.m)
-// If x is denormal, we normalize it by multiplying x by 2^23 and subtracting
-// m by 23.
-
-// Step 2 - Another range reduction:
-// To compute log(1.mant), let f be the highest 8 bits including the hidden
-// bit, and d be the difference (1.mant - f), i.e. the remaining 16 bits of the
-// mantissa. Then we have the following approximation formula:
-// log2(1.mant) = log2(f) + log2(1.mant / f)
-// = log2(f) + log2(1 + d/f)
-// ~ log2(f) + P(d/f)
-// since d/f is sufficiently small.
-// log2(f) and 1/f are then stored in two 2^7 = 128 entries look-up tables.
-
-// Step 3 - Polynomial approximation:
-// To compute P(d/f), we use a single degree-5 polynomial in double precision
-// which provides correct rounding for all but few exception values.
-// For more detail about how this polynomial is obtained, please refer to the
-// papers:
-// Lim, J. and Nagarakatte, S., "One Polynomial Approximation to Produce
-// Correctly Rounded Results of an Elementary Function for Multiple
-// Representations and Rounding Modes", Proceedings of the 49th ACM SIGPLAN
-// Symposium on Principles of Programming Languages (POPL-2022), Philadelphia,
-// USA, Jan. 16-22, 2022.
-// https://people.cs.rutgers.edu/~sn349/papers/rlibmall-popl-2022.pdf
-// Aanjaneya, M., Lim, J., and Nagarakatte, S., "RLibm-Prog: Progressive
-// Polynomial Approximations for Fast Correctly Rounded Math Libraries",
-// Dept. of Comp. Sci., Rutgets U., Technical Report DCS-TR-758, Nov. 2021.
-// https://arxiv.org/pdf/2111.12852.pdf.
+#include "src/__support/math/log2f.h"
namespace LIBC_NAMESPACE_DECL {
-LLVM_LIBC_FUNCTION(float, log2f, (float x)) {
- using namespace common_constants_internal;
- using FPBits = typename fputil::FPBits<float>;
-
- FPBits xbits(x);
- uint32_t x_u = xbits.uintval();
-
- // Hard to round value(s).
- using fputil::round_result_slightly_up;
-
- int m = -FPBits::EXP_BIAS;
-
- // log2(1.0f) = 0.0f.
- if (LIBC_UNLIKELY(x_u == 0x3f80'0000U))
- return 0.0f;
-
- // Exceptional inputs.
- if (LIBC_UNLIKELY(x_u < FPBits::min_normal().uintval() ||
- x_u > FPBits::max_normal().uintval())) {
- if (x == 0.0f) {
- fputil::set_errno_if_required(ERANGE);
- fputil::raise_except_if_required(FE_DIVBYZERO);
- return FPBits::inf(Sign::NEG).get_val();
- }
- if (xbits.is_neg() && !xbits.is_nan()) {
- fputil::set_errno_if_required(EDOM);
- fputil::raise_except_if_required(FE_INVALID);
- return FPBits::quiet_nan().get_val();
- }
- if (xbits.is_inf_or_nan()) {
- return x;
- }
- // Normalize denormal inputs.
- xbits = FPBits(xbits.get_val() * 0x1.0p23f);
- m -= 23;
- }
-
- m += xbits.get_biased_exponent();
- int index = xbits.get_mantissa() >> 16;
- // Set bits to 1.m
- xbits.set_biased_exponent(0x7F);
-
- float u = xbits.get_val();
- double v;
-#ifdef LIBC_TARGET_CPU_HAS_FMA_FLOAT
- v = static_cast<double>(fputil::multiply_add(u, R[index], -1.0f)); // Exact.
-#else
- v = fputil::multiply_add(static_cast<double>(u), RD[index], -1.0); // Exact
-#endif // LIBC_TARGET_CPU_HAS_FMA_FLOAT
-
- double extra_factor = static_cast<double>(m) + LOG2_R[index];
-
- // Degree-5 polynomial approximation of log2 generated by Sollya with:
- // > P = fpminimax(log2(1 + x)/x, 4, [|1, D...|], [-2^-8, 2^-7]);
- constexpr double COEFFS[5] = {0x1.71547652b8133p0, -0x1.71547652d1e33p-1,
- 0x1.ec70a098473dep-2, -0x1.7154c5ccdf121p-2,
- 0x1.2514fd90a130ap-2};
-
- double vsq = v * v; // Exact
- double c0 = fputil::multiply_add(v, COEFFS[0], extra_factor);
- double c1 = fputil::multiply_add(v, COEFFS[2], COEFFS[1]);
- double c2 = fputil::multiply_add(v, COEFFS[4], COEFFS[3]);
-
- double r = fputil::polyeval(vsq, c0, c1, c2);
-
- return static_cast<float>(r);
-}
+LLVM_LIBC_FUNCTION(float, log2f, (float x)) { return math::log2f(x); }
} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/shared/CMakeLists.txt b/libc/test/shared/CMakeLists.txt
index 61625c0bc0444..2befd513aa763 100644
--- a/libc/test/shared/CMakeLists.txt
+++ b/libc/test/shared/CMakeLists.txt
@@ -63,6 +63,7 @@ add_fp_unittest(
libc.src.__support.math.ilogbf16
libc.src.__support.math.ilogbl
libc.src.__support.math.log
+ libc.src.__support.math.log2f
libc.src.__support.math.logbf
libc.src.__support.math.logbf128
libc.src.__support.math.logbf16
diff --git a/libc/test/shared/shared_math_test.cpp b/libc/test/shared/shared_math_test.cpp
index 49047631fb227..f03980a6a9f65 100644
--- a/libc/test/shared/shared_math_test.cpp
+++ b/libc/test/shared/shared_math_test.cpp
@@ -80,6 +80,7 @@ TEST(LlvmLibcSharedMathTest, AllFloat) {
ASSERT_FP_EQ(float(-1 * (8 << 5)), LIBC_NAMESPACE::shared::ldexpf(-8.0f, 5));
EXPECT_EQ(long(0), LIBC_NAMESPACE::shared::llogbf(1.0f));
+ EXPECT_FP_EQ(0x1p+0f, LIBC_NAMESPACE::shared::log2f(2.0f));
EXPECT_FP_EQ(0x0p+0f, LIBC_NAMESPACE::shared::logbf(1.0f));
EXPECT_FP_EQ(0x1p+0f, LIBC_NAMESPACE::shared::rsqrtf(1.0f));
}
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
index 3d9491e8baa0c..80c9f7cad9e27 100644
--- a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -2932,6 +2932,20 @@ libc_support_library(
],
)
+libc_support_library(
+ name = "__support_math_log2f",
+ hdrs = ["src/__support/math/log2f.h"],
+ deps = [
+ ":__support_fputil_except_value_utils",
+ ":__support_fputil_fenv_impl",
+ ":__support_fputil_fma",
+ ":__support_fputil_fp_bits",
+ ":__support_fputil_polyeval",
+ ":__support_macros_optimization",
+ ":__support_math_common_constants",
+ ],
+)
+
libc_support_library(
name = "__support_math_logbf",
hdrs = ["src/__support/math/logbf.h"],
@@ -4598,11 +4612,8 @@ libc_math_function(
libc_math_function(
name = "log2f",
additional_deps = [
- ":__support_fputil_fma",
- ":__support_fputil_multiply_add",
- ":__support_fputil_polyeval",
- ":__support_macros_optimization",
- ":__support_math_common_constants",
+ ":__support_math_log2f",
+ ":errno",
],
)
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