[libc][stdfix] Implement fxdivi functions (rdivi) #154914

bojle · 2025-08-22T09:07:02Z

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.

@lntue @PiJoules

github-actions · 2025-08-22T09:07:19Z

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write permissions for the repository. In which case you can instead tag reviewers by name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

llvmbot · 2025-08-22T09:07:53Z

@llvm/pr-subscribers-libc

Author: Shreeyash Pandey (bojle)

Changes

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.

Full diff: https://github.com/llvm/llvm-project/pull/154914.diff

6 Files Affected:

(modified) libc/config/linux/riscv/entrypoints.txt (+1)
(modified) libc/config/linux/x86_64/entrypoints.txt (+1)
(modified) libc/include/stdfix.yaml (+8)
(modified) libc/src/__support/fixed_point/fx_bits.h (+41)
(modified) libc/src/stdfix/CMakeLists.txt (+14)
(modified) libc/test/src/stdfix/CMakeLists.txt (+16)

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..13eecba981664 100644
--- a/libc/src/__support/fixed_point/fx_bits.h
+++ b/libc/src/__support/fixed_point/fx_bits.h
@@ -224,6 +224,47 @@ idiv(T x, T y) {
   return static_cast<XType>(result);
 }
 
+inline long accum nrstep(long accum d, long accum x0) {
+  auto v = x0 * (2 - (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);
+
+  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;
+  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/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

llvmbot · 2025-08-22T09:07:53Z

@llvm/pr-subscribers-backend-risc-v

Author: Shreeyash Pandey (bojle)

Changes

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.

Full diff: https://github.com/llvm/llvm-project/pull/154914.diff

6 Files Affected:

(modified) libc/config/linux/riscv/entrypoints.txt (+1)
(modified) libc/config/linux/x86_64/entrypoints.txt (+1)
(modified) libc/include/stdfix.yaml (+8)
(modified) libc/src/__support/fixed_point/fx_bits.h (+41)
(modified) libc/src/stdfix/CMakeLists.txt (+14)
(modified) libc/test/src/stdfix/CMakeLists.txt (+16)

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..13eecba981664 100644
--- a/libc/src/__support/fixed_point/fx_bits.h
+++ b/libc/src/__support/fixed_point/fx_bits.h
@@ -224,6 +224,47 @@ idiv(T x, T y) {
   return static_cast<XType>(result);
 }
 
+inline long accum nrstep(long accum d, long accum x0) {
+  auto v = x0 * (2 - (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);
+
+  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;
+  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/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

PiJoules · 2025-08-22T18:08:04Z

Thanks. Could you add some tests for this?

bojle · 2025-08-22T18:18:01Z

Oops, forgot to add them. I've pushed the tests too. Please check now. @PiJoules

libc/test/src/stdfix/DivITest.h

libc/src/__support/fixed_point/fx_bits.h

lntue · 2025-08-26T15:42:17Z

libc/src/__support/fixed_point/fx_bits.h

+  /* 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);


Can you add error estimations for the initial approximation and after each Newton-Raphson step in the comments?

I've added error info. I've done it to the best of my understanding, though I could use a theoretical cross-check of the explanation i've provided.

github-actions · 2025-08-26T15:43:52Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp,h -- libc/src/stdfix/rdivi.cpp libc/src/stdfix/rdivi.h libc/test/src/stdfix/DivITest.h libc/test/src/stdfix/rdivi_test.cpp libc/src/__support/fixed_point/fx_bits.h

View the diff from clang-format here.

diff --git a/libc/src/stdfix/rdivi.cpp b/libc/src/stdfix/rdivi.cpp
index 227b41287..7021a8b1c 100644
--- a/libc/src/stdfix/rdivi.cpp
+++ b/libc/src/stdfix/rdivi.cpp
@@ -15,7 +15,7 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(fract, rdivi, (int a, int b)) {
-  return fixed_point::divi<fract>(a,b);
+  return fixed_point::divi<fract>(a, b);
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/src/stdfix/DivITest.h b/libc/test/src/stdfix/DivITest.h
index 7ef9b4ff7..651b4ed2a 100644
--- a/libc/test/src/stdfix/DivITest.h
+++ b/libc/test/src/stdfix/DivITest.h
@@ -21,11 +21,13 @@ public:
   typedef XType (*DivIFunc)(int, int);
 
   void testBasic(DivIFunc func) {
-    EXPECT_EQ(static_cast<XType>(func(128,256)), static_cast<XType>(0.5r));
-    EXPECT_EQ(static_cast<XType>(func(2,3)), static_cast<XType>(0.666666r));
-    EXPECT_EQ(static_cast<XType>(func(3,4)), static_cast<XType>(0.75r));
-    EXPECT_EQ(static_cast<XType>(func(1043, 2764)), static_cast<XType>(0.37735r));
-    EXPECT_EQ(static_cast<XType>(func(60000, 720293)), static_cast<XType>(0.083299r));
+    EXPECT_EQ(static_cast<XType>(func(128, 256)), static_cast<XType>(0.5r));
+    EXPECT_EQ(static_cast<XType>(func(2, 3)), static_cast<XType>(0.666666r));
+    EXPECT_EQ(static_cast<XType>(func(3, 4)), static_cast<XType>(0.75r));
+    EXPECT_EQ(static_cast<XType>(func(1043, 2764)),
+              static_cast<XType>(0.37735r));
+    EXPECT_EQ(static_cast<XType>(func(60000, 720293)),
+              static_cast<XType>(0.083299r));
   }
 };

libc/src/__support/fixed_point/fx_bits.h

Signed-off-by: Shreeyash Pandey <[email protected]>

libc/test/src/stdfix/DivITest.h

Signed-off-by: Shreeyash Pandey <[email protected]>

llvmbot added backend:RISC-V libc labels Aug 22, 2025

bojle mentioned this pull request Aug 22, 2025

Implement fixed point fxdivi functions in llvm-libc #129126

Open

PiJoules requested review from PiJoules and lntue August 22, 2025 18:01

bojle force-pushed the main branch from 7c517bf to 019fc9b Compare August 22, 2025 18:16

PiJoules reviewed Aug 25, 2025

View reviewed changes

libc/test/src/stdfix/DivITest.h Outdated Show resolved Hide resolved

libc/test/src/stdfix/DivITest.h Outdated Show resolved Hide resolved

libc/test/src/stdfix/DivITest.h Outdated Show resolved Hide resolved

libc/test/src/stdfix/DivITest.h Show resolved Hide resolved

lntue reviewed Aug 26, 2025

View reviewed changes

libc/src/__support/fixed_point/fx_bits.h Outdated Show resolved Hide resolved

lntue reviewed Aug 26, 2025

View reviewed changes

libc/src/__support/fixed_point/fx_bits.h Outdated Show resolved Hide resolved

bojle force-pushed the main branch from 4d1cb9b to 5d02bd8 Compare August 26, 2025 12:51

lntue reviewed Aug 26, 2025

View reviewed changes

PiJoules reviewed Aug 26, 2025

View reviewed changes

libc/src/__support/fixed_point/fx_bits.h Show resolved Hide resolved

[libc][stdfix] Implement fxdivi functions

f54208e

Signed-off-by: Shreeyash Pandey <[email protected]>

bojle force-pushed the main branch from 5d02bd8 to f54208e Compare August 29, 2025 13:29

PiJoules reviewed Aug 29, 2025

View reviewed changes

libc/test/src/stdfix/DivITest.h Outdated Show resolved Hide resolved

PiJoules reviewed Aug 29, 2025

View reviewed changes

libc/test/src/stdfix/DivITest.h Outdated Show resolved Hide resolved

PiJoules reviewed Aug 29, 2025

View reviewed changes

libc/test/src/stdfix/DivITest.h Show resolved Hide resolved

PiJoules reviewed Aug 29, 2025

View reviewed changes

libc/test/src/stdfix/DivITest.h Outdated Show resolved Hide resolved

[libc][stdfix] add a couple more tests and error info for divi

040e833

Signed-off-by: Shreeyash Pandey <[email protected]>

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

[libc][stdfix] Implement fxdivi functions (rdivi) #154914

[libc][stdfix] Implement fxdivi functions (rdivi) #154914

Uh oh!

bojle commented Aug 22, 2025 •

edited

Loading

Uh oh!

github-actions bot commented Aug 22, 2025

Uh oh!

llvmbot commented Aug 22, 2025

Uh oh!

llvmbot commented Aug 22, 2025

Uh oh!

PiJoules commented Aug 22, 2025

Uh oh!

bojle commented Aug 22, 2025 •

edited

Loading

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

lntue Aug 26, 2025

Uh oh!

bojle Aug 30, 2025 •

edited

Loading

Uh oh!

github-actions bot commented Aug 26, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

[libc][stdfix] Implement fxdivi functions (rdivi) #154914

Are you sure you want to change the base?

[libc][stdfix] Implement fxdivi functions (rdivi) #154914

Uh oh!

Conversation

bojle commented Aug 22, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

github-actions bot commented Aug 22, 2025

Uh oh!

llvmbot commented Aug 22, 2025

Uh oh!

llvmbot commented Aug 22, 2025

Uh oh!

PiJoules commented Aug 22, 2025

Uh oh!

bojle commented Aug 22, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

lntue Aug 26, 2025

Choose a reason for hiding this comment

Uh oh!

bojle Aug 30, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

github-actions bot commented Aug 26, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

bojle commented Aug 22, 2025 •

edited

Loading

bojle commented Aug 22, 2025 •

edited

Loading

bojle Aug 30, 2025 •

edited

Loading