Hi!
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
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.
--- libstdc++-v3/include/c_global/cmath.jj 2024-09-25 17:25:07.227360607
+0200
+++ libstdc++-v3/include/c_global/cmath 2024-10-28 13:10:46.348655046 +0100
@@ -2880,8 +2880,12 @@ _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
@@ -3598,8 +3602,12 @@ _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
--- libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc.jj
2024-10-28 12:56:10.699876240 +0100
+++ libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc 2024-10-28
12:56:59.875189173 +0100
@@ -0,0 +1,5 @@
+// { dg-do run { target c++23 } }
+// { dg-require-cmath "" }
+// { dg-additional-options "-O0" }
+
+#include "nextafter_c++23.cc"
Jakub
