> On 9 Jan 2019, at 10:14, Jakub Jelinek <ja...@redhat.com> wrote: > > On Wed, Jan 09, 2019 at 09:37:40AM +0000, Jonathan Wakely wrote: >> The IBM128 long double format isn't foldable in constant expressions, so >> conditionally skip the std::complex<long double> cases when they'll >> fail. >> >> PR libstdc++/88204 >> * testsuite/26_numerics/complex/operators/more_constexpr.cc: Do not >> test std::complex<long double> if long double format is IBM128. >> * testsuite/26_numerics/complex/requirements/more_constexpr.cc: >> Likewise. >> >> Tested powerpc64le-linux, committed to trunk. > > Thanks for doing that. Due to the variable number of bits in the mantissa > unless we represent the double-double constants in the compiler as a pair of > doubles there will be always many cases that can't be folded. That said, > I've been wondering if we couldn't fold at least the basic arithmetics on > some constants, essentially by emulating what will happen at runtime. I.e. > if we want to constant fold PLUS_EXPR (or MINUS_EXPR, MULT_EXPR, *DIV_EXPR, > and a few others) of two double-double REAL_CSTs, split them into two > double values by rounding to a double, subtracting that double and rounding > the remainder to double, perform whatever we emit inline or the runtime > library does for the basic operations and in the end check if the result of > that operation fits in the canonical format we pretend for the double-double > (i.e. the 106 bits mantissa etc.) and if it does, allow constant folding > that. E.g. in Red Hat bugzilla I think we have several spots where people > complain about this lack of folding, it breaks a couple of real-world apps > etc. Or is double-double considered dead format that will hopefully soon go > away, not worth spending more time on?
It’s also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19779 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374 in the latter, Andrew Pinski suggests using MPFR, but there’s no patch. (there might be others in our BZ) Iain
