[libc][math][c23] Add rsqrtf() function (llvm#159615)

amemov · akadutta · commit f96b791c9526 · 2025-10-14T09:48:25.000-05:00
Closes llvm#159614 **Changes:** - Initial implementation of rsqrt for single precision float **Some small unrelated style changes to this PR (that I missed in my rsqrtf16 PR):** - Added extra - to the top comments to make it look nicer in libc/shared/math/rsqrtf16.h - Put rsqrtf16 inside of libc/src/__support/math/CMakeLists.txt in sorted order - Rearanged libc_math_function rsqrtf16 in Bazel to match alphabetical order
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
@@ -735,6 +735,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.rintf16
     libc.src.math.roundevenf16
     libc.src.math.roundf16
+    libc.src.math.rsqrtf
     libc.src.math.rsqrtf16
     libc.src.math.scalblnf16
     libc.src.math.scalbnf16
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
@@ -750,6 +750,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.rintf16
     libc.src.math.roundevenf16
     libc.src.math.roundf16
+    libc.src.math.rsqrtf
     libc.src.math.rsqrtf16
     libc.src.math.scalblnf16
     libc.src.math.scalbnf16
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
@@ -786,6 +786,7 @@ if(LIBC_TYPES_HAS_FLOAT16)
     libc.src.math.rintf16
     libc.src.math.roundevenf16
     libc.src.math.roundf16
+    libc.src.math.rsqrtf
     libc.src.math.rsqrtf16
     libc.src.math.scalblnf16
     libc.src.math.scalbnf16
diff --git a/libc/docs/headers/math/index.rst b/libc/docs/headers/math/index.rst
@@ -343,7 +343,7 @@ Higher Math Functions
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+------------------------+----------------------------+
 | rootn     |                  |                 |                        |                      |                        |                        | 7.12.7.8               | F.10.4.8                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+------------------------+----------------------------+
-| rsqrt     |                  |                 |                        | |check|              |                        |                        | 7.12.7.9               | F.10.4.9                   |
+| rsqrt     | |check|          |                 |                        | |check|              |                        |                        | 7.12.7.9               | F.10.4.9                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+------------------------+----------------------------+
 | sin       | |check|          | |check|         |                        | |check|              |                        |                        | 7.12.4.6               | F.10.1.6                   |
 +-----------+------------------+-----------------+------------------------+----------------------+------------------------+------------------------+------------------------+----------------------------+
diff --git a/libc/include/math.yaml b/libc/include/math.yaml
@@ -2349,6 +2349,12 @@ functions:
     return_type: long double
     arguments:
       - type: long double
+  - name: rsqrtf
+    standards:
+      - stdc
+    return_type: float
+    arguments:
+      - type: float
   - name: rsqrtf16
     standards:
       - stdc
diff --git a/libc/shared/math.h b/libc/shared/math.h
@@ -57,7 +57,7 @@
 #include "math/ldexpf.h"
 #include "math/ldexpf128.h"
 #include "math/ldexpf16.h"
-
+#include "math/rsqrtf.h"
 #include "math/rsqrtf16.h"
 
 #endif // LLVM_LIBC_SHARED_MATH_H
diff --git a/libc/shared/math/rsqrtf.h b/libc/shared/math/rsqrtf.h
@@ -0,0 +1,23 @@
+//===-- Shared rsqrtf 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_RSQRTF_H
+#define LLVM_LIBC_SHARED_MATH_RSQRTF_H
+
+#include "shared/libc_common.h"
+#include "src/__support/math/rsqrtf.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::rsqrtf;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_MATH_RSQRTF_H
diff --git a/libc/shared/math/rsqrtf16.h b/libc/shared/math/rsqrtf16.h
@@ -1,4 +1,4 @@
-//===-- Shared rsqrtf16 function -------------------------------*- C++ -*-===//
+//===-- Shared rsqrtf16 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.
diff --git a/libc/src/__support/math/CMakeLists.txt b/libc/src/__support/math/CMakeLists.txt
@@ -109,22 +109,6 @@ add_header_library(
     libc.src.__support.macros.properties.types
 )
 
-
-add_header_library(
-  rsqrtf16
-  HDRS
-    rsqrtf16.h
-  DEPENDS
-    libc.src.__support.FPUtil.cast
-    libc.src.__support.FPUtil.fenv_impl
-    libc.src.__support.FPUtil.fp_bits
-    libc.src.__support.FPUtil.multiply_add
-    libc.src.__support.FPUtil.polyeval
-    libc.src.__support.FPUtil.manipulation_functions
-    libc.src.__support.macros.optimization
-    libc.src.__support.macros.properties.types
-)
-
 add_header_library(
   asin_utils
   HDRS
@@ -866,6 +850,34 @@ add_header_library(
     libc.src.__support.common
 )
 
+add_header_library(
+  rsqrtf
+  HDRS
+    rsqrtf.h
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.hdr.fenv_macros
+    libc.src.__support.FPUtil.cast
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.sqrt
+    libc.src.__support.macros.optimization
+)
+
+add_header_library(
+  rsqrtf16
+  HDRS
+    rsqrtf16.h
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.hdr.fenv_macros
+    libc.src.__support.FPUtil.cast
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.sqrt
+    libc.src.__support.macros.optimization
+)
+
 add_header_library(
   sincos_eval
   HDRS
diff --git a/libc/src/__support/math/rsqrtf.h b/libc/src/__support/math/rsqrtf.h
@@ -0,0 +1,71 @@
+//===-- Implementation header for rsqrtf ------------------------*- 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_RSQRTF_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_RSQRTF_H
+
+#include "hdr/errno_macros.h"
+#include "hdr/fenv_macros.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/cast.h"
+#include "src/__support/FPUtil/sqrt.h"
+#include "src/__support/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace math {
+
+LIBC_INLINE static constexpr float rsqrtf(float x) {
+  using FPBits = fputil::FPBits<float>;
+  FPBits xbits(x);
+
+  uint32_t x_u = xbits.uintval();
+  uint32_t x_abs = x_u & 0x7fff'ffffU;
+
+  constexpr uint32_t INF_BITS = FPBits::inf().uintval();
+
+  // x is 0, inf/nan, or negative.
+  if (LIBC_UNLIKELY(x_u == 0 || x_u >= INF_BITS)) {
+    // x is NaN
+    if (x_abs > INF_BITS) {
+      if (xbits.is_signaling_nan()) {
+        fputil::raise_except_if_required(FE_INVALID);
+        return FPBits::quiet_nan().get_val();
+      }
+      return x;
+    }
+
+    // |x| = 0
+    if (x_abs == 0) {
+      fputil::raise_except_if_required(FE_DIVBYZERO);
+      fputil::set_errno_if_required(ERANGE);
+      return FPBits::inf(xbits.sign()).get_val();
+    }
+
+    // -inf <= x < 0
+    if (x_u > 0x7fff'ffffU) {
+      fputil::raise_except_if_required(FE_INVALID);
+      fputil::set_errno_if_required(EDOM);
+      return FPBits::quiet_nan().get_val();
+    }
+
+    // x = +inf => rsqrt(x) = 0
+    return FPBits::zero(xbits.sign()).get_val();
+  }
+
+  // TODO: add float based approximation when
+  // LIBC_TARGET_CPU_HAS_FPU_DOUBLE is not defined
+  double result = 1.0 / fputil::sqrt<double>(fputil::cast<double>(x));
+
+  return fputil::cast<float>(result);
+}
+
+} // namespace math
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_MATH_RSQRTF_H
diff --git a/libc/src/math/CMakeLists.txt b/libc/src/math/CMakeLists.txt
@@ -517,6 +517,7 @@ add_math_entrypoint_object(roundevenf16)
 add_math_entrypoint_object(roundevenf128)
 add_math_entrypoint_object(roundevenbf16)
 
+add_math_entrypoint_object(rsqrtf)
 add_math_entrypoint_object(rsqrtf16)
 
 add_math_entrypoint_object(scalbln)
diff --git a/libc/src/math/generic/CMakeLists.txt b/libc/src/math/generic/CMakeLists.txt
@@ -988,6 +988,16 @@ add_entrypoint_object(
     ROUND_OPT
 )
 
+add_entrypoint_object(
+  rsqrtf
+  SRCS
+    rsqrtf.cpp
+  HDRS
+    ../rsqrtf.h
+  DEPENDS
+    libc.src.__support.math.rsqrtf
+)
+
 add_entrypoint_object(
   rsqrtf16
   SRCS
diff --git a/libc/src/math/generic/rsqrtf.cpp b/libc/src/math/generic/rsqrtf.cpp
@@ -0,0 +1,16 @@
+//===-- Single-precision rsqrt 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 "src/math/rsqrtf.h"
+#include "src/__support/math/rsqrtf.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(float, rsqrtf, (float x)) { return math::rsqrtf(x); }
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/math/rsqrtf.h b/libc/src/math/rsqrtf.h
@@ -0,0 +1,20 @@
+//===-- Implementation header for rsqrtf ------------------------*- 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_MATH_RSQRTF_H
+#define LLVM_LIBC_SRC_MATH_RSQRTF_H
+
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+float rsqrtf(float x);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_MATH_RSQRTF_H
diff --git a/libc/test/shared/CMakeLists.txt b/libc/test/shared/CMakeLists.txt
@@ -53,6 +53,6 @@ add_fp_unittest(
     libc.src.__support.math.ldexpf
     libc.src.__support.math.ldexpf128
     libc.src.__support.math.ldexpf16
+    libc.src.__support.math.rsqrtf
     libc.src.__support.math.rsqrtf16
-
 )
diff --git a/libc/test/shared/shared_math_test.cpp b/libc/test/shared/shared_math_test.cpp
@@ -70,6 +70,8 @@ TEST(LlvmLibcSharedMathTest, AllFloat) {
 
   ASSERT_FP_EQ(float(8 << 5), LIBC_NAMESPACE::shared::ldexpf(8.0f, 5));
   ASSERT_FP_EQ(float(-1 * (8 << 5)), LIBC_NAMESPACE::shared::ldexpf(-8.0f, 5));
+
+  EXPECT_FP_EQ(0x1p+0f, LIBC_NAMESPACE::shared::rsqrtf(1.0f));
 }
 
 TEST(LlvmLibcSharedMathTest, AllDouble) {
diff --git a/libc/test/src/math/CMakeLists.txt b/libc/test/src/math/CMakeLists.txt
@@ -1678,6 +1678,17 @@ add_fp_unittest(
     libc.src.math.sqrtl
 )
 
+add_fp_unittest(
+  rsqrtf_test
+  NEED_MPFR
+  SUITE
+    libc-math-unittests
+  SRCS
+    rsqrtf_test.cpp
+  DEPENDS
+    libc.src.math.rsqrtf
+)
+
 add_fp_unittest(
   rsqrtf16_test
   NEED_MPFR
diff --git a/libc/test/src/math/RsqrtTest.h b/libc/test/src/math/RsqrtTest.h
@@ -0,0 +1,74 @@
+//===-- Utility class to test rsqrt[f|l] ------------------------*- 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_TEST_SRC_MATH_RSQRTTEST_H
+#define LLVM_LIBC_TEST_SRC_MATH_RSQRTTEST_H
+
+#include "test/UnitTest/FEnvSafeTest.h"
+#include "test/UnitTest/FPMatcher.h"
+#include "test/UnitTest/Test.h"
+#include "utils/MPFRWrapper/MPFRUtils.h"
+
+namespace mpfr = LIBC_NAMESPACE::testing::mpfr;
+
+template <typename OutType, typename InType = OutType>
+class RsqrtTest : public LIBC_NAMESPACE::testing::FEnvSafeTest {
+
+  DECLARE_SPECIAL_CONSTANTS(InType)
+
+  static constexpr StorageType HIDDEN_BIT =
+      StorageType(1) << LIBC_NAMESPACE::fputil::FPBits<InType>::FRACTION_LEN;
+
+public:
+  using RsqrtFunc = OutType (*)(InType);
+
+  // Subnormal inputs: probe both power-of-two mantissas and an even sampling
+  // across the subnormal range.
+  void test_denormal_values(RsqrtFunc func) {
+    // Powers of two in the subnormal mantissa space.
+    for (StorageType mant = 1; mant < HIDDEN_BIT; mant <<= 1) {
+      FPBits denormal(zero);
+      denormal.set_mantissa(mant);
+      InType x = denormal.get_val();
+      ASSERT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Rsqrt, x, func(x), 0.5);
+    }
+
+    // Even sampling across all subnormals.
+    constexpr StorageType COUNT = 200'001;
+    constexpr StorageType STEP = HIDDEN_BIT / COUNT;
+    for (StorageType i = 0, v = 0; i <= COUNT; ++i, v += STEP) {
+      InType x = FPBits(i).get_val();
+      ASSERT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Rsqrt, x, func(x), 0.5);
+    }
+  }
+
+  // Positive normal range sampling: skip NaNs and negative values.
+  void test_normal_range(RsqrtFunc func) {
+    constexpr StorageType COUNT = 200'001;
+    constexpr StorageType STEP = STORAGE_MAX / COUNT;
+    for (StorageType i = 0, v = 0; i <= COUNT; ++i, v += STEP) {
+      FPBits x_bits(v);
+      InType x = x_bits.get_val();
+      if (x_bits.is_nan() || x_bits.is_neg())
+        continue;
+      ASSERT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Rsqrt, x, func(x), 0.5);
+    }
+  }
+};
+
+#define LIST_RSQRT_TESTS(T, func)                                              \
+  using LlvmLibcRsqrtTest = RsqrtTest<T, T>;                                   \
+  TEST_F(LlvmLibcRsqrtTest, DenormalValues) { test_denormal_values(&func); }   \
+  TEST_F(LlvmLibcRsqrtTest, NormalRange) { test_normal_range(&func); }
+
+#define LIST_NARROWING_RSQRT_TESTS(OutType, InType, func)                      \
+  using LlvmLibcRsqrtTest = RsqrtTest<OutType, InType>;                        \
+  TEST_F(LlvmLibcRsqrtTest, DenormalValues) { test_denormal_values(&func); }   \
+  TEST_F(LlvmLibcRsqrtTest, NormalRange) { test_normal_range(&func); }
+
+#endif // LLVM_LIBC_TEST_SRC_MATH_RSQRTTEST_H
diff --git a/libc/test/src/math/exhaustive/CMakeLists.txt b/libc/test/src/math/exhaustive/CMakeLists.txt
diff --git a/libc/test/src/math/exhaustive/rsqrtf_test.cpp b/libc/test/src/math/exhaustive/rsqrtf_test.cpp
diff --git a/libc/test/src/math/rsqrtf_test.cpp b/libc/test/src/math/rsqrtf_test.cpp
diff --git a/libc/test/src/math/smoke/CMakeLists.txt b/libc/test/src/math/smoke/CMakeLists.txt
diff --git a/libc/test/src/math/smoke/rsqrtf16_test.cpp b/libc/test/src/math/smoke/rsqrtf16_test.cpp
diff --git a/libc/test/src/math/smoke/rsqrtf_test.cpp b/libc/test/src/math/smoke/rsqrtf_test.cpp
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-//===-- Shared rsqrtf16 function -------------------------------- C++ --===//`
	`1`	`+//===-- Shared rsqrtf16 function --------------------------------- C++ --===//`
`2`	`2`	`//`
`3`	`3`	`// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.`
`4`	`4`	`// See https://llvm.org/LICENSE.txt for license information.`