[libc-commits] [libc] [llvm] [libc][math] Refactor llogb to Header Only (PR #175524)
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Mon Jan 12 04:05:41 PST 2026
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
Author: Vedant Neve (0bVdnt)
<details>
<summary>Changes</summary>
Refactors the llogb math function to be header-only and constexpr.
This moves the implementation to src/__support/math/llogb.h and updates the entrypoint to be a wrapper.
Fixes #<!-- -->175360
---
Full diff: https://github.com/llvm/llvm-project/pull/175524.diff
7 Files Affected:
- (modified) libc/shared/math.h (+1)
- (added) libc/shared/math/llogb.h (+23)
- (modified) libc/src/__support/math/CMakeLists.txt (+11)
- (added) libc/src/__support/math/llogb.h (+64)
- (modified) libc/src/math/generic/CMakeLists.txt (+2-1)
- (modified) libc/src/math/generic/llogb.cpp (+3-3)
- (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 7fb4c43f509c4..49dbe5ff9027e 100644
--- a/libc/shared/math.h
+++ b/libc/shared/math.h
@@ -63,6 +63,7 @@
#include "math/ldexpf.h"
#include "math/ldexpf128.h"
#include "math/ldexpf16.h"
+#include "math/llogb.h"
#include "math/rsqrtf.h"
#include "math/rsqrtf16.h"
#include "math/sin.h"
diff --git a/libc/shared/math/llogb.h b/libc/shared/math/llogb.h
new file mode 100644
index 0000000000000..9d650ff370539
--- /dev/null
+++ b/libc/shared/math/llogb.h
@@ -0,0 +1,23 @@
+//===-- Shared llogb 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_LLOGB_H
+#define LLVM_LIBC_SHARED_MATH_LLOGB_H
+
+#include "shared/libc_common.h"
+#include "src/__support/math/llogb.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::llogb;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_MATH_LLOGB_H
diff --git a/libc/src/__support/math/CMakeLists.txt b/libc/src/__support/math/CMakeLists.txt
index 741da7432c94f..f2e344a19e75d 100644
--- a/libc/src/__support/math/CMakeLists.txt
+++ b/libc/src/__support/math/CMakeLists.txt
@@ -924,6 +924,17 @@ add_header_library(
libc.src.__support.macros.optimization
)
+add_header_library(
+ llogb
+ HDRS
+ llogb.h
+ DEPENDS
+ libc.src.__support.CPP.bit
+ libc.src.__support.FPUtil.fp_bits
+ libc.src.__support.macros.config
+ libc.src.__support.macros.optimization
+)
+
add_header_library(
range_reduction_double
HDRS
diff --git a/libc/src/__support/math/llogb.h b/libc/src/__support/math/llogb.h
new file mode 100644
index 0000000000000..b77ee11f9a411
--- /dev/null
+++ b/libc/src/__support/math/llogb.h
@@ -0,0 +1,64 @@
+//===-- Implementation header for llogb --------------------------*- 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_LLOGB_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_LLOGB_H
+
+#include "src/__support/CPP/bit.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
+#include <limits.h>
+
+namespace LIBC_NAMESPACE_DECL {
+namespace math {
+
+LIBC_INLINE constexpr long llogb(double x) {
+ using FPBits = fputil::FPBits<double>;
+ FPBits bits(x);
+
+ if (LIBC_UNLIKELY(bits.is_zero())) {
+ return -LONG_MAX - 1; // FP_LLOGB0
+ }
+
+ if (LIBC_UNLIKELY(bits.is_inf())) {
+ return LONG_MAX;
+ }
+
+ if (LIBC_UNLIKELY(bits.is_nan())) {
+#ifdef __FP_LOGBNAN_MIN
+ return -LONG_MAX - 1;
+#else
+ return LONG_MAX; // FP_LLOGBNAN
+#endif
+ }
+
+ int biased_exp = bits.get_biased_exponent();
+
+ // Handle normal numbers
+ if (biased_exp > 0) {
+ return biased_exp - FPBits::EXP_BIAS;
+ }
+
+ // Handle subnormal numbers
+ // Formula: log2(mantissa * 2^-1074) = log2(mantissa) - 1074
+ uint64_t mantissa = bits.get_mantissa();
+
+ // Define constants
+ constexpr int EXP_ADJUSTMENT = FPBits::EXP_BIAS + FPBits::FRACTION_LEN - 1;
+ constexpr int STORAGE_MSB = (sizeof(uint64_t) * CHAR_BIT) - 1;
+
+ // log2(mantissa) = STORAGE_MSB - countl_zero(mantissa)
+ return (STORAGE_MSB - cpp::countl_zero(mantissa)) - EXP_ADJUSTMENT;
+}
+
+} // namespace math
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_MATH_LLOGB_H
diff --git a/libc/src/math/generic/CMakeLists.txt b/libc/src/math/generic/CMakeLists.txt
index 9c0da076b6cf0..aabd6899ffb02 100644
--- a/libc/src/math/generic/CMakeLists.txt
+++ b/libc/src/math/generic/CMakeLists.txt
@@ -1828,7 +1828,8 @@ add_entrypoint_object(
HDRS
../llogb.h
DEPENDS
- libc.src.__support.FPUtil.manipulation_functions
+ libc.src.__support.math.llogb
+ libc.src.__support.macros.config
)
add_entrypoint_object(
diff --git a/libc/src/math/generic/llogb.cpp b/libc/src/math/generic/llogb.cpp
index 3850ac00d98d9..edca94e1e734c 100644
--- a/libc/src/math/generic/llogb.cpp
+++ b/libc/src/math/generic/llogb.cpp
@@ -7,12 +7,12 @@
//===----------------------------------------------------------------------===//
#include "src/math/llogb.h"
-#include "src/__support/FPUtil/ManipulationFunctions.h"
-#include "src/__support/common.h"
+#include "src/__support/math/llogb.h"
#include "src/__support/macros/config.h"
+#include "src/__support/common.h" // LLVM_LIBC_FUNCTION
namespace LIBC_NAMESPACE_DECL {
-LLVM_LIBC_FUNCTION(long, llogb, (double x)) { return fputil::intlogb<long>(x); }
+LLVM_LIBC_FUNCTION(long, llogb, (double x)) { return math::llogb(x); }
} // namespace LIBC_NAMESPACE_DECL
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
index 210c25dddd0b9..af88b034db423 100644
--- a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -3118,6 +3118,17 @@ libc_support_library(
],
)
+libc_support_library(
+ name = "__support_math_llogb",
+ hdrs = ["src/__support/math/llogb.h"],
+ deps = [
+ ":__support_cpp_bit",
+ ":__support_fputil_fp_bits",
+ ":__support_macros_config",
+ ":__support_macros_optimization",
+ ],
+)
+
libc_support_library(
name = "__support_range_reduction_double",
hdrs = [
@@ -4337,7 +4348,13 @@ libc_math_function(
],
)
-libc_math_function(name = "llogb")
+libc_math_function(
+ name = "llogb",
+ additional_deps = [
+ ":__support_math_llogb",
+ ":__support_fputil_fp_bits",
+ ],
+)
libc_math_function(name = "llogbf")
``````````
</details>
https://github.com/llvm/llvm-project/pull/175524
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