[libc++] Use libm implementations in exp for complex numbers

libcxx/CMakeLists.txt

@@ -749,6 +749,7 @@ config_define(${LIBCXX_ENABLE_UNICODE} _LIBCPP_HAS_UNICODE)
 config_define(${LIBCXX_ENABLE_WIDE_CHARACTERS} _LIBCPP_HAS_WIDE_CHARACTERS)
 config_define(${LIBCXX_ENABLE_TIME_ZONE_DATABASE} _LIBCPP_HAS_TIME_ZONE_DATABASE)
 config_define(${LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS} _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS)
+config_define_if(C_SUPPORTS_C99_COMPLEX _LIBCPP_HAS_C99_COMPLEX)
 
 # TODO: Remove in LLVM 21. We're leaving an error to make this fail explicitly.
 if (LIBCXX_ENABLE_ASSERTIONS)

libcxx/cmake/config-ix.cmake

@@ -103,6 +103,31 @@ endif()
 
 check_symbol_exists(__PICOLIBC__ "string.h" PICOLIBC)
 
+# Check for the existence of <complex.h> complex math functions.
+# This is necessary even though libcxx uses the builtin versions
+# of these functions, because if the builtin cannot be used, a reference
+# to the library function is emitted. Using compiler builtins for these
+# functions requires corresponding C99 library functions to be present.
+
+cmake_push_check_state()
+list(APPEND CMAKE_REQUIRED_LIBRARIES m)
+check_c_source_compiles("
+#include <complex.h>
+typedef __complex__ float float_type;
+typedef __complex__ double double_type;
+typedef __complex__ long double ld_type;
+volatile float_type tmpf;
+volatile double_type tmpd;
+volatile ld_type tmpld;
+int main(void) {
+  tmpf = cexpf(tmpf);
+  tmpd = cexp(tmpd);
+  tmpld = cexpl(tmpld);
+  return 0;
+}
+" C_SUPPORTS_C99_COMPLEX)
+cmake_pop_check_state()
+
 # Check libraries
 if(WIN32 AND NOT MINGW)
   # TODO(compnerd) do we want to support an emulation layer that allows for the

libcxx/include/__config_site.in

@@ -32,6 +32,7 @@
 #cmakedefine01 _LIBCPP_HAS_WIDE_CHARACTERS
 #cmakedefine01 _LIBCPP_HAS_TIME_ZONE_DATABASE
 #cmakedefine01 _LIBCPP_INSTRUMENTED_WITH_ASAN
+#cmakedefine01 _LIBCPP_HAS_C99_COMPLEX
 
 // PSTL backends
 #cmakedefine _LIBCPP_PSTL_BACKEND_SERIAL

libcxx/include/complex

@@ -1074,6 +1074,16 @@ _LIBCPP_HIDE_FROM_ABI complex<_Tp> sqrt(const complex<_Tp>& __x) {
 
 // exp
 
+#  if _LIBCPP_HAS_C99_COMPLEX
+_LIBCPP_HIDE_FROM_ABI inline _Complex float __cexp(_Complex float __v) { return __builtin_cexpf(__v); }
+_LIBCPP_HIDE_FROM_ABI inline _Complex double __cexp(_Complex double __v) { return __builtin_cexp(__v); }
+_LIBCPP_HIDE_FROM_ABI inline _Complex long double __cexp(_Complex long double __v) { return __builtin_cexpl(__v); }
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI complex<_Tp> exp(const complex<_Tp>& __x) {
+  return complex<_Tp>(__from_builtin_tag(), std::__cexp(__x.__builtin()));
+}
+#  else
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI complex<_Tp> exp(const complex<_Tp>& __x) {
   _Tp __i = __x.imag();
@@ -1091,8 +1101,13 @@ _LIBCPP_HIDE_FROM_ABI complex<_Tp> exp(const complex<_Tp>& __x) {
     }
   }
   _Tp __e = std::exp(__x.real());
+  if (std::isinf(__e)) {
+    _Tp __e2 = std::exp(__x.real() * _Tp(0.5));
+    return complex<_Tp>(__e2 * std::cos(__i) * __e2, __e2 * std::sin(__i) * __e2);
+  }
   return complex<_Tp>(__e * std::cos(__i), __e * std::sin(__i));
 }
+#  endif
 
 // pow
 

libcxx/test/libcxx/numerics/complex.number/exp.pass.cpp

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <complex>
+
+// template<class T>
+//   complex<T>
+//   exp(const complex<T>& x);
+
+#include <complex>
+#include <cassert>
+#include <cmath>
+
+#include "test_macros.h"
+
+template <class T>
+void test_overflow_case() {
+  typedef std::complex<T> C;
+
+  // In this case, the overflow of exp(real_part) is compensated when
+  // sin(imag_part) is close to zero, resulting in a finite imaginary part.
+  C z(T(90.0238094), T(5.900613e-39));
+  C result = std::exp(z);
+
+  assert(std::isinf(result.real()));
+  assert(result.real() > 0);
+
+  assert(std::isfinite(result.imag()));
+  assert(std::abs(result.imag() - T(7.3746)) < T(1.0));
+}
+
+int main(int, char**) {
+  test_overflow_case<float>();
+  return 0;
+}