https://gcc.gnu.org/g:ec43369c7e2c4b930a501252fa048df51c141970
commit r14-10870-gec43369c7e2c4b930a501252fa048df51c141970
Author: Jakub Jelinek <ja...@redhat.com>
Date: Tue Oct 29 11:14:12 2024 +0100
libstdc++: Use if consteval rather than if (std::__is_constant_evaluated())
for {,b}float16_t nextafter [PR117321]
The nextafter_c++23.cc testcase fails to link at -O0.
The problem is that eventhough std::__is_constant_evaluated() has
always_inline attribute, that at -O0 just means that we inline the
call, but its result is still assigned to a temporary which is tested
later, nothing at -O0 propagates that false into the if and optimizes
away the if body. And the __builtin_nextafterf16{,b} calls are meant
to be used solely for constant evaluation, the C libraries don't
define nextafterf16 these days.
As __STDCPP_FLOAT16_T__ and __STDCPP_BFLOAT16_T__ are predefined right
now only by GCC, not by clang which doesn't implement the extended floating
point types paper, and as they are predefined in C++23 and later modes only,
I think we can just use if consteval which is folded already during the FE
and the body isn't included even at -O0. I've added a feature test for
that just in case clang implements those and implements those in some weird
way. Note, if (__builtin_is_constant_evaluted()) would work correctly too,
that is also folded to false at gimplification time and the corresponding
if block not emitted at all. But for -O0 it can't be wrapped into a helper
inline function.
2024-10-29 Jakub Jelinek <ja...@redhat.com>
PR libstdc++/117321
* include/c_global/cmath (nextafter(_Float16, _Float16)): Use
if consteval rather than if (std::__is_constant_evaluated()) around
the __builtin_nextafterf16 call.
(nextafter(__gnu_cxx::__bfloat16_t, __gnu_cxx::__bfloat16_t)): Use
if consteval rather than if (std::__is_constant_evaluated()) around
the __builtin_nextafterf16b call.
* testsuite/26_numerics/headers/cmath/117321.cc: New test.
(cherry picked from commit 5e247ac0c28b9a2662f99c4a5420c5f7c2d0c6bd)
Diff:
---
libstdc++-v3/include/c_global/cmath | 16 ++++++++++++----
.../testsuite/26_numerics/headers/cmath/117321.cc | 5 +++++
2 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/libstdc++-v3/include/c_global/cmath
b/libstdc++-v3/include/c_global/cmath
index dd0174f1987f..c5ebebd6b861 100644
--- a/libstdc++-v3/include/c_global/cmath
+++ b/libstdc++-v3/include/c_global/cmath
@@ -2873,8 +2873,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr _Float16
nextafter(_Float16 __x, _Float16 __y)
{
- if (std::__is_constant_evaluated())
- return __builtin_nextafterf16(__x, __y);
+#if __cpp_if_consteval >= 202106L
+ // Can't use if (std::__is_constant_evaluated()) here, as it
+ // doesn't guarantee optimizing the body away at -O0 and
+ // nothing defines nextafterf16.
+ if consteval { return __builtin_nextafterf16(__x, __y); }
+#endif
#ifdef __INT16_TYPE__
using __float16_int_type = __INT16_TYPE__;
#else
@@ -3591,8 +3595,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr __gnu_cxx::__bfloat16_t
nextafter(__gnu_cxx::__bfloat16_t __x, __gnu_cxx::__bfloat16_t __y)
{
- if (std::__is_constant_evaluated())
- return __builtin_nextafterf16b(__x, __y);
+#if __cpp_if_consteval >= 202106L
+ // Can't use if (std::__is_constant_evaluated()) here, as it
+ // doesn't guarantee optimizing the body away at -O0 and
+ // nothing defines nextafterf16b.
+ if consteval { return __builtin_nextafterf16b(__x, __y); }
+#endif
#ifdef __INT16_TYPE__
using __bfloat16_int_type = __INT16_TYPE__;
#else
diff --git a/libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc
b/libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc
new file mode 100644
index 000000000000..18d108c6a3e2
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc
@@ -0,0 +1,5 @@
+// { dg-do run { target c++23 } }
+// { dg-require-cmath "" }
+// { dg-additional-options "-O0" }
+
+#include "nextafter_c++23.cc"
