https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107745
Bug ID: 107745
Summary: long double constexprs don't work with * or /, but
work with + and - (JUST ON PPC)
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: Sebastian at SSpaeth dot de
Target Milestone: ---
TL;DR:
This looks like a compiler bug or quality-of-implementation issue, and happens
JUST ON PPC with gcc. From a bit of playing with godbolt, long double
constexprs don't work with * or /, but work with + and -.
https://godbolt.org/z/WYz5fY538
This works fine on everything but PPC that I tried. It also works with PPC
clang, but not PPC gcc. Could you report this to gcc and see what they say?
The shortest reproducible snippet is:
constexpr long double v =
(6.15348059642740421245081038903225e-15L /
5.40431955284459475358983848622456e+16L);
Details in upstream issue at https://github.com/google/s2geometry/issues/279
---------------------
The library built fine on all supported archs using Debian's gcc
(g++-12_12.2.0-7 gcc-12_12.2.0-7 libc6-dev_2.35-4 libstdc++-12-dev_12.2.0-7
libstdc++6_12.2.0-7 linux-libc-dev_6.0.3-1) EXCEPT for PPC.
The full failed build log is currently still at:
https://buildd.debian.org/status/fetch.php?pkg=s2geometry&arch=ppc64el&ver=0.10.0-2&stamp=1667081576&raw=0
-------------------
Build failure boils down to the error
```
/<<PKGBUILDDIR>>/src/s2/s2edge_crossings.cc: In instantiation of ‘bool
S2::internal::GetStableCrossProd(const Vector3<T>&, const Vector3<T>&,
Vector3<T>*) [with T = long double]’:
/<<PKGBUILDDIR>>/src/s2/s2edge_crossings.cc:127:54: required from here
/<<PKGBUILDDIR>>/src/s2/s2edge_crossings.cc:115:31: error:
‘(6.15348059642740421245081038903225e-15l /
5.40431955284459475358983848622456e+16l)’ is not a constant expression
115 | (32 * kSqrt3 * DBL_ERR) /
| ~~~~~~~~~~~~~~~~~~~~~~~~^
116 | (kRobustCrossProdError.radians() / T_ERR - (1 + 2 * kSqrt3));
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
Please let us know if you need more information.
