[libc-commits] [libc] [libc][stdfix] Implement fxdivi functions (rdivi) (PR #154914)
Shreeyash Pandey via libc-commits
libc-commits at lists.llvm.org
Sat Aug 30 12:32:11 PDT 2025
https://github.com/bojle updated https://github.com/llvm/llvm-project/pull/154914
>From f54208ee781fde878a42dcb573afa92293ffabe3 Mon Sep 17 00:00:00 2001
From: Shreeyash Pandey <shreeyash335 at gmail.com>
Date: Fri, 22 Aug 2025 14:16:57 +0530
Subject: [PATCH 1/2] [libc][stdfix] Implement fxdivi functions
Signed-off-by: Shreeyash Pandey <shreeyash335 at gmail.com>
---
libc/config/linux/riscv/entrypoints.txt | 1 +
libc/config/linux/x86_64/entrypoints.txt | 1 +
libc/include/stdfix.yaml | 8 ++++
libc/src/__support/fixed_point/fx_bits.h | 53 ++++++++++++++++++++
libc/src/stdfix/CMakeLists.txt | 14 ++++++
libc/src/stdfix/rdivi.cpp | 21 ++++++++
libc/src/stdfix/rdivi.h | 21 ++++++++
libc/test/src/stdfix/CMakeLists.txt | 16 +++++++
libc/test/src/stdfix/DivITest.h | 61 ++++++++++++++++++++++++
libc/test/src/stdfix/rdivi_test.cpp | 14 ++++++
10 files changed, 210 insertions(+)
create mode 100644 libc/src/stdfix/rdivi.cpp
create mode 100644 libc/src/stdfix/rdivi.h
create mode 100644 libc/test/src/stdfix/DivITest.h
create mode 100644 libc/test/src/stdfix/rdivi_test.cpp
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index 1a03683d72e61..b783dd4b04c74 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@@ -986,6 +986,7 @@ 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 5590f1a15ac57..f6beccc7229dd 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1019,6 +1019,7 @@ 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/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..9069031786b0a 100644
--- a/libc/src/__support/fixed_point/fx_bits.h
+++ b/libc/src/__support/fixed_point/fx_bits.h
@@ -224,6 +224,59 @@ 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();
+ }
+ bool d_is_signed = false;
+ if (d < 0) {
+ d = (d * -1);
+ d_is_signed = true;
+ }
+
+ unsigned int nv = static_cast<unsigned int>(n);
+ unsigned int dv = static_cast<unsigned int>(d);
+ unsigned int clz = cpp::countl_zero<unsigned int>(dv) - 1;
+ unsigned long int scaled_val = dv << clz;
+ /* Scale denominator to be in the range of [0.5,1] */
+ FXBits<long accum> d_scaled{scaled_val};
+ unsigned long int 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 = 2.8235lk; /* 48/17 */
+ long accum b = 1.8823lk; /* 32/17 */
+ long accum initial_approx = a - (b * d_scaled_val);
+ long accum val = nrstep(d_scaled_val, initial_approx);
+ val = nrstep(d_scaled_val, val);
+ val = nrstep(d_scaled_val, val);
+ val = nrstep(d_scaled_val, val);
+ long accum res = n_scaled_val * val;
+ if (d_is_signed) {
+ res *= static_cast<XType>(-1);
+ }
+ 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..227b412879a38
--- /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..6642a5a4c93ab
--- /dev/null
+++ b/libc/test/src/stdfix/DivITest.h
@@ -0,0 +1,61 @@
+//===-- 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() {
+ // TODO: raise error
+ return 0;
+}
+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) - 0.666666r), epsilon);
+ EXPECT_LT((func(3, 4) - 0.75r), epsilon);
+ EXPECT_LT((func(1043, 2764) - 0.37735r), epsilon);
+ EXPECT_LT((func(60000, 720293) - 0.083299r), 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) {
+ EXPECT_EQ(func(0,10), 0.r);
+ EXPECT_DEATH([func] { func(10, 0); }, WITH_SIGNAL(-1));
+ EXPECT_EQ(func(-32768,32768), FRACT_MIN);
+ EXPECT_EQ(func(32767,32768), FRACT_MAX);
+ }
+};
+
+#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);
>From 040e83342ab4bd051b584e7cdbad91158ce06426 Mon Sep 17 00:00:00 2001
From: Shreeyash Pandey <shreeyash335 at gmail.com>
Date: Sun, 31 Aug 2025 01:01:23 +0530
Subject: [PATCH 2/2] [libc][stdfix] add a couple more tests and error info for
divi
Signed-off-by: Shreeyash Pandey <shreeyash335 at gmail.com>
---
libc/src/__support/fixed_point/fx_bits.h | 11 +++++++++++
libc/test/src/stdfix/DivITest.h | 20 ++++++++++++--------
2 files changed, 23 insertions(+), 8 deletions(-)
diff --git a/libc/src/__support/fixed_point/fx_bits.h b/libc/src/__support/fixed_point/fx_bits.h
index 9069031786b0a..b6c88998d2bd4 100644
--- a/libc/src/__support/fixed_point/fx_bits.h
+++ b/libc/src/__support/fixed_point/fx_bits.h
@@ -265,10 +265,21 @@ template <typename XType> LIBC_INLINE constexpr XType divi(int n, int d) {
/* x0 = (48/17) - (32/17) * d_n */
long accum a = 2.8235lk; /* 48/17 */
long accum b = 1.8823lk; /* 32/17 */
+ /* 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);
+ /* 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);
+ /* E2 = 0.0000121 */
val = nrstep(d_scaled_val, val);
+ /* E3 = 1.468e−10 */
val = nrstep(d_scaled_val, val);
+ /* E4 = 2.155e−20 */
val = nrstep(d_scaled_val, val);
long accum res = n_scaled_val * val;
if (d_is_signed) {
diff --git a/libc/test/src/stdfix/DivITest.h b/libc/test/src/stdfix/DivITest.h
index 6642a5a4c93ab..f3ac145ad0af8 100644
--- a/libc/test/src/stdfix/DivITest.h
+++ b/libc/test/src/stdfix/DivITest.h
@@ -11,10 +11,7 @@
#include "src/__support/fixed_point/fx_rep.h"
#include "test/UnitTest/Test.h"
-template <typename XType> XType get_epsilon() {
- // TODO: raise error
- return 0;
-}
+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; }
@@ -29,10 +26,10 @@ class DivITest : public LIBC_NAMESPACE::testing::Test {
void testBasic(DivIFunc func) {
XType epsilon = get_epsilon<XType>();
- EXPECT_LT((func(2, 3) - 0.666666r), epsilon);
+ EXPECT_LT((func(2, 3) - 0.666656494140625r), epsilon);
EXPECT_LT((func(3, 4) - 0.75r), epsilon);
- EXPECT_LT((func(1043, 2764) - 0.37735r), epsilon);
- EXPECT_LT((func(60000, 720293) - 0.083299r), 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);
@@ -48,9 +45,16 @@ class DivITest : public LIBC_NAMESPACE::testing::Test {
void testSpecial(DivIFunc func) {
EXPECT_EQ(func(0,10), 0.r);
- EXPECT_DEATH([func] { func(10, 0); }, WITH_SIGNAL(-1));
+ EXPECT_EQ(func(0,-10), 0.r);
EXPECT_EQ(func(-32768,32768), FRACT_MIN);
EXPECT_EQ(func(32767,32768), FRACT_MAX);
+ EXPECT_EQ(func(INT_MAX,INT_MAX), 1.0r);
+ EXPECT_EQ(func(INT_MAX-1,INT_MAX), 0.99999999r);
+ EXPECT_EQ(func(INT_MIN,INT_MAX), FRACT_MIN);
+ /* Expecting 0 here as fract is not precise enough to
+ * handle 1/INT_MAX
+ */
+ EXPECT_EQ(func(1, INT_MAX), 0.r);
}
};
More information about the libc-commits
mailing list