[libc-commits] [libc] c57796d - [libc][stdfix] Implement fxdivi functions (rdivi) (#154914)
via libc-commits
libc-commits at lists.llvm.org
Wed Oct 8 12:17:35 PDT 2025
Author: Shreeyash Pandey
Date: 2025-10-08T15:17:31-04:00
New Revision: c57796dcb2690657a99bf328e0f2e3c10bc0ab4d
URL: https://github.com/llvm/llvm-project/commit/c57796dcb2690657a99bf328e0f2e3c10bc0ab4d
DIFF: https://github.com/llvm/llvm-project/commit/c57796dcb2690657a99bf328e0f2e3c10bc0ab4d.diff
LOG: [libc][stdfix] Implement fxdivi functions (rdivi) (#154914)
This PR includes only one of the fxdivi functions (rdivi). It uses a
polynomial function for initial approximation followed by 4
newton-raphson iterations to calculate the reciprocal and finally
multiplies the numerator with it to get the result.
---------
Signed-off-by: Shreeyash Pandey <shreeyash335 at gmail.com>
Added:
libc/src/stdfix/rdivi.cpp
libc/src/stdfix/rdivi.h
libc/test/src/stdfix/DivITest.h
libc/test/src/stdfix/rdivi_test.cpp
Modified:
libc/config/linux/riscv/entrypoints.txt
libc/config/linux/x86_64/entrypoints.txt
libc/docs/headers/stdfix.rst
libc/include/stdfix.yaml
libc/src/__support/fixed_point/fx_bits.h
libc/src/stdfix/CMakeLists.txt
libc/test/src/stdfix/CMakeLists.txt
Removed:
################################################################################
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index 5f407e842121e..99c9d3dba3974 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@@ -1022,6 +1022,7 @@ if(LIBC_COMPILER_HAS_FIXED_POINT)
libc.src.stdfix.idivulr
libc.src.stdfix.idivuk
libc.src.stdfix.idivulk
+ libc.src.stdfix.rdivi
)
endif()
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index 87b78a337b875..82a83a20766b6 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1058,6 +1058,7 @@ if(LIBC_COMPILER_HAS_FIXED_POINT)
libc.src.stdfix.idivulr
libc.src.stdfix.idivuk
libc.src.stdfix.idivulk
+ libc.src.stdfix.rdivi
)
endif()
diff --git a/libc/docs/headers/stdfix.rst b/libc/docs/headers/stdfix.rst
index a2f5e94774798..e75b9d8aac440 100644
--- a/libc/docs/headers/stdfix.rst
+++ b/libc/docs/headers/stdfix.rst
@@ -81,7 +81,7 @@ The following functions are included in the ISO/IEC TR 18037:2008 standard.
+---------------+----------------+-------------+---------------+------------+----------------+-------------+----------------+-------------+---------------+------------+----------------+-------------+
| muli | | | | | | | | | | | | |
+---------------+----------------+-------------+---------------+------------+----------------+-------------+----------------+-------------+---------------+------------+----------------+-------------+
-| \*divi | | | | | | | | | | | | |
+| \*divi | | | | |check| | | | | | | | | |
+---------------+----------------+-------------+---------------+------------+----------------+-------------+----------------+-------------+---------------+------------+----------------+-------------+
| round | |check| | |check| | |check| | |check| | |check| | |check| | |check| | |check| | |check| | |check| | |check| | |check| |
+---------------+----------------+-------------+---------------+------------+----------------+-------------+----------------+-------------+---------------+------------+----------------+-------------+
diff --git a/libc/include/stdfix.yaml b/libc/include/stdfix.yaml
index 5b385124eb63d..451330c3478d2 100644
--- a/libc/include/stdfix.yaml
+++ b/libc/include/stdfix.yaml
@@ -544,3 +544,11 @@ functions:
arguments:
- type: unsigned long accum
guard: LIBC_COMPILER_HAS_FIXED_POINT
+ - name: rdivi
+ standards:
+ - stdc_ext
+ return_type: fract
+ arguments:
+ - type: int
+ - type: int
+ guard: LIBC_COMPILER_HAS_FIXED_POINT
diff --git a/libc/src/__support/fixed_point/fx_bits.h b/libc/src/__support/fixed_point/fx_bits.h
index 00c6119b4f353..25221d9a33161 100644
--- a/libc/src/__support/fixed_point/fx_bits.h
+++ b/libc/src/__support/fixed_point/fx_bits.h
@@ -10,9 +10,11 @@
#define LLVM_LIBC_SRC___SUPPORT_FIXED_POINT_FX_BITS_H
#include "include/llvm-libc-macros/stdfix-macros.h"
+#include "src/__support/CPP/algorithm.h"
#include "src/__support/CPP/bit.h"
#include "src/__support/CPP/limits.h" // numeric_limits
#include "src/__support/CPP/type_traits.h"
+#include "src/__support/libc_assert.h"
#include "src/__support/macros/attributes.h" // LIBC_INLINE
#include "src/__support/macros/config.h" // LIBC_NAMESPACE_DECL
#include "src/__support/macros/null_check.h" // LIBC_CRASH_ON_VALUE
@@ -21,6 +23,8 @@
#include "fx_rep.h"
+#include <stdio.h>
+
#ifdef LIBC_COMPILER_HAS_FIXED_POINT
namespace LIBC_NAMESPACE_DECL {
@@ -224,6 +228,113 @@ idiv(T x, T y) {
return static_cast<XType>(result);
}
+LIBC_INLINE long accum nrstep(long accum d, long accum x0) {
+ auto v = x0 * (2.lk - (d * x0));
+ return v;
+}
+
+// Divide the two integers and return a fixed_point value
+//
+// For reference, see:
+// https://en.wikipedia.org/wiki/Division_algorithm#Newton%E2%80%93Raphson_division
+// https://stackoverflow.com/a/9231996
+
+template <typename XType> LIBC_INLINE constexpr XType divi(int n, int d) {
+ // If the value of the second operand of the / operator is zero, the
+ // behavior is undefined. Ref: ISO/IEC TR 18037:2008(E) p.g. 16
+ LIBC_CRASH_ON_VALUE(d, 0);
+
+ if (LIBC_UNLIKELY(n == 0)) {
+ return FXRep<XType>::ZERO();
+ }
+ auto is_power_of_two = [](int n) { return (n > 0) && ((n & (n - 1)) == 0); };
+ long accum max_val = static_cast<long accum>(FXRep<XType>::MAX());
+ long accum min_val = static_cast<long accum>(FXRep<XType>::MIN());
+
+ if (is_power_of_two(cpp::abs(d))) {
+ int k = cpp::countr_zero<uint32_t>(static_cast<uint32_t>(cpp::abs(d)));
+ constexpr int F = FXRep<XType>::FRACTION_LEN;
+ int64_t scaled_n = static_cast<int64_t>(n) << F;
+ int64_t res64 = scaled_n >> k;
+ constexpr int TOTAL_BITS = sizeof(XType) * 8;
+ const int64_t max_limit = (1LL << (TOTAL_BITS - 1)) - 1;
+ const int64_t min_limit = -(1LL << (TOTAL_BITS - 1));
+ if (res64 > max_limit) {
+ return FXRep<XType>::MAX();
+ } else if (res64 < min_limit) {
+ return FXRep<XType>::MIN();
+ }
+ long accum res_accum =
+ static_cast<long accum>(res64) / static_cast<long accum>(1 << F);
+ res_accum = (d < 0) ? static_cast<long accum>(-1) * res_accum : res_accum;
+ if (res_accum > max_val) {
+ return FXRep<XType>::MAX();
+ } else if (res_accum < min_val) {
+ return FXRep<XType>::MIN();
+ }
+ return static_cast<XType>(res_accum);
+ }
+
+ bool result_is_negative = ((n < 0) != (d < 0));
+ int64_t n64 = static_cast<int64_t>(n);
+ int64_t d64 = static_cast<int64_t>(d);
+
+ uint64_t nv = static_cast<uint64_t>(n64 < 0 ? -n64 : n64);
+ uint64_t dv = static_cast<uint64_t>(d64 < 0 ? -d64 : d64);
+
+ if (d == INT_MIN) {
+ nv <<= 1;
+ dv >>= 1;
+ }
+
+ uint32_t clz = cpp::countl_zero<uint32_t>(static_cast<uint32_t>(dv)) - 1;
+ uint64_t scaled_val = dv << clz;
+ // Scale denominator to be in the range of [0.5,1]
+ FXBits<long accum> d_scaled{scaled_val};
+ uint64_t scaled_val_n = nv << clz;
+ // Scale the numerator as much as the denominator to maintain correctness of
+ // the original equation
+ FXBits<long accum> n_scaled{scaled_val_n};
+ long accum n_scaled_val = n_scaled.get_val();
+ long accum d_scaled_val = d_scaled.get_val();
+ // x0 = (48/17) - (32/17) * d_n
+ long accum a = 0x2.d89d89d8p0lk; // 48/17 = 2.8235294...
+ long accum b = 0x1.e1e1e1e1p0lk; // 32/17 = 1.8823529...
+ // Error of the initial approximation, as derived
+ // from the wikipedia article is
+ // E0 = 1/17 = 0.059 (5.9%)
+ long accum initial_approx = a - (b * d_scaled_val);
+ // Since, 0.5 <= d_scaled_val <= 1.0, 0.9412 <= initial_approx <= 1.88235
+ LIBC_ASSERT((initial_approx >= 0x0.78793dd9p0lk) &&
+ (initial_approx <= 0x1.f0f0d845p0lk));
+ // Each newton-raphson iteration will square the error, due
+ // to quadratic convergence. So,
+ // E1 = (0.059)^2 = 0.0034
+ long accum val = nrstep(d_scaled_val, initial_approx);
+ if constexpr (FXRep<XType>::FRACTION_LEN > 8) {
+ // E2 = 0.0000121
+ val = nrstep(d_scaled_val, val);
+ if constexpr (FXRep<XType>::FRACTION_LEN > 16) {
+ // E3 = 1.468e−10
+ val = nrstep(d_scaled_val, val);
+ }
+ }
+ long accum res = n_scaled_val * val;
+
+ if (result_is_negative) {
+ res *= static_cast<long accum>(-1);
+ }
+
+ // Per clause 7.18a.6.1, saturate values on overflow
+ if (res > max_val) {
+ return FXRep<XType>::MAX();
+ } else if (res < min_val) {
+ return FXRep<XType>::MIN();
+ } else {
+ return static_cast<XType>(res);
+ }
+}
+
} // namespace fixed_point
} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/stdfix/CMakeLists.txt b/libc/src/stdfix/CMakeLists.txt
index 843111e3f80b1..3cbabd17ad34c 100644
--- a/libc/src/stdfix/CMakeLists.txt
+++ b/libc/src/stdfix/CMakeLists.txt
@@ -89,6 +89,20 @@ foreach(suffix IN ITEMS r lr k lk ur ulr uk ulk)
)
endforeach()
+foreach(suffix IN ITEMS r)
+ add_entrypoint_object(
+ ${suffix}divi
+ HDRS
+ ${suffix}divi.h
+ SRCS
+ ${suffix}divi.cpp
+ COMPILE_OPTIONS
+ ${libc_opt_high_flag}
+ DEPENDS
+ libc.src.__support.fixed_point.fx_bits
+ )
+endforeach()
+
add_entrypoint_object(
uhksqrtus
HDRS
diff --git a/libc/src/stdfix/rdivi.cpp b/libc/src/stdfix/rdivi.cpp
new file mode 100644
index 0000000000000..7021a8b1c8b2a
--- /dev/null
+++ b/libc/src/stdfix/rdivi.cpp
@@ -0,0 +1,21 @@
+//===-- Implementation of rdivi 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "rdivi.h"
+#include "include/llvm-libc-macros/stdfix-macros.h" // fract
+#include "src/__support/common.h" // LLVM_LIBC_FUNCTION
+#include "src/__support/fixed_point/fx_bits.h" // fixed_point
+#include "src/__support/macros/config.h" // LIBC_NAMESPACE_DECL
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(fract, rdivi, (int a, int b)) {
+ return fixed_point::divi<fract>(a, b);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/stdfix/rdivi.h b/libc/src/stdfix/rdivi.h
new file mode 100644
index 0000000000000..aeda1ee9d40f0
--- /dev/null
+++ b/libc/src/stdfix/rdivi.h
@@ -0,0 +1,21 @@
+//===-- Implementation header for rdivi ------------------------*- 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_STDFIX_RDIVI_H
+#define LLVM_LIBC_SRC_STDFIX_RDIVI_H
+
+#include "include/llvm-libc-macros/stdfix-macros.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+fract rdivi(int a, int b);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_STDFIX_RDIVI_H
diff --git a/libc/test/src/stdfix/CMakeLists.txt b/libc/test/src/stdfix/CMakeLists.txt
index e2b4bc1805f7c..741522276feaa 100644
--- a/libc/test/src/stdfix/CMakeLists.txt
+++ b/libc/test/src/stdfix/CMakeLists.txt
@@ -120,6 +120,22 @@ foreach(suffix IN ITEMS r lr k lk ur ulr uk ulk)
)
endforeach()
+foreach(suffix IN ITEMS r)
+ add_libc_test(
+ ${suffix}divi_test
+ SUITE
+ libc-stdfix-tests
+ HDRS
+ DivITest.h
+ SRCS
+ ${suffix}divi_test.cpp
+ DEPENDS
+ libc.src.stdfix.${suffix}divi
+ libc.src.__support.fixed_point.fx_bits
+ libc.hdr.signal_macros
+ )
+endforeach()
+
add_libc_test(
uhksqrtus_test
SUITE
diff --git a/libc/test/src/stdfix/DivITest.h b/libc/test/src/stdfix/DivITest.h
new file mode 100644
index 0000000000000..ad15637a3455f
--- /dev/null
+++ b/libc/test/src/stdfix/DivITest.h
@@ -0,0 +1,74 @@
+//===-- Utility class to test fxdivi functions ------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/CPP/type_traits.h"
+#include "src/__support/fixed_point/fx_bits.h"
+#include "src/__support/fixed_point/fx_rep.h"
+#include "test/UnitTest/Test.h"
+
+template <typename XType> XType get_epsilon() = delete;
+template <> fract get_epsilon() { return FRACT_EPSILON; }
+template <> unsigned fract get_epsilon() { return UFRACT_EPSILON; }
+template <> long fract get_epsilon() { return LFRACT_EPSILON; }
+
+template <typename XType>
+class DivITest : public LIBC_NAMESPACE::testing::Test {
+ using FXRep = LIBC_NAMESPACE::fixed_point::FXRep<XType>;
+ using FXBits = LIBC_NAMESPACE::fixed_point::FXBits<XType>;
+
+public:
+ typedef XType (*DivIFunc)(int, int);
+
+ void testBasic(DivIFunc func) {
+ XType epsilon = get_epsilon<XType>();
+ EXPECT_LT((func(2, 3) - 0.666656494140625r), epsilon);
+ EXPECT_LT((func(3, 4) - 0.75r), epsilon);
+ EXPECT_LT((func(1043, 2764) - 0.3773516643r), epsilon);
+ EXPECT_LT((func(60000, 720293) - 0.08329943509r), epsilon);
+
+ EXPECT_EQ(func(128, 256), 0.5r);
+ EXPECT_EQ(func(1, 2), 0.5r);
+ EXPECT_EQ(func(1, 4), 0.25r);
+ EXPECT_EQ(func(1, 8), 0.125r);
+ EXPECT_EQ(func(1, 16), 0.0625r);
+
+ EXPECT_EQ(func(-1, 2), -0.5r);
+ EXPECT_EQ(func(1, -4), -0.25r);
+ EXPECT_EQ(func(-1, 8), -0.125r);
+ EXPECT_EQ(func(1, -16), -0.0625r);
+ }
+
+ void testSpecial(DivIFunc func) {
+ XType epsilon = get_epsilon<XType>();
+ EXPECT_EQ(func(0, 10), 0.r);
+ EXPECT_EQ(func(0, -10), 0.r);
+ EXPECT_EQ(func(-(1 << FRACT_FBIT), 1 << FRACT_FBIT), FRACT_MIN);
+ EXPECT_EQ(func((1 << FRACT_FBIT) - 1, 1 << FRACT_FBIT), FRACT_MAX);
+ // From Section 7.18a.6.1, functions returning a fixed-point value, the
+ // return value is saturated on overflow.
+ EXPECT_EQ(func(INT_MAX, INT_MAX), FRACT_MAX);
+ EXPECT_LT(func(INT_MAX - 1, INT_MAX) - 0.99999999r, epsilon);
+ EXPECT_EQ(func(INT_MIN, INT_MAX), FRACT_MIN);
+ // Expecting 0 here as fract is not precise enough to
+ // handle 1/INT_MAX
+ EXPECT_LT(func(1, INT_MAX) - 0.r, epsilon);
+ // This results in 1.1739, which should be saturated to FRACT_MAX
+ EXPECT_EQ(func(27, 23), FRACT_MAX);
+
+ EXPECT_EQ(func(INT_MIN, 1), FRACT_MIN);
+ EXPECT_LT(func(1, INT_MIN) - 0.r, epsilon);
+
+ EXPECT_EQ(func(INT_MIN, INT_MIN), 1.r);
+ }
+};
+
+#define LIST_DIVI_TESTS(Name, XType, func) \
+ using LlvmLibc##Name##diviTest = DivITest<XType>; \
+ TEST_F(LlvmLibc##Name##diviTest, Basic) { testBasic(&func); } \
+ TEST_F(LlvmLibc##Name##diviTest, Special) { testSpecial(&func); } \
+ static_assert(true, "Require semicolon.")
diff --git a/libc/test/src/stdfix/rdivi_test.cpp b/libc/test/src/stdfix/rdivi_test.cpp
new file mode 100644
index 0000000000000..10ab366679a36
--- /dev/null
+++ b/libc/test/src/stdfix/rdivi_test.cpp
@@ -0,0 +1,14 @@
+//===-- Unittests for rdivi -----------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "DivITest.h"
+
+#include "llvm-libc-macros/stdfix-macros.h" // fract
+#include "src/stdfix/rdivi.h"
+
+LIST_DIVI_TESTS(r, fract, LIBC_NAMESPACE::rdivi);
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