On Wed, May 30, 2018 at 03:15:21PM +0200, Jakub Jelinek wrote: > On Wed, May 30, 2018 at 12:58:49PM +0000, Segher Boessenkool wrote: > > This patch changes the (C++) mangling of the 128-bit float types: > > > > __ieee128 becomes u9__ieee128 > > __ibm128 becomes u8__ieee128 > > ^^^^^^ what is the advantage/reason for the above, rather than mangling it > as g? > > > __float128 is not a type anymore > > IEEE long double becomes u9__ieee128 > > IBM long double stays g > > I mean, the above change will mean a significant burden e.g. on libstdc++, > when we have to export all symbols that refer to the > long double/__ieee128/__ibm128 types 4 times, once as aliases to symbols > with double instead (with the exception when there is no such double > symbol) using mangling e, then make sure libstdc++ files are all compiled > with long double equal to IBM to get the g mangling, then add aliases to > those for u8__ieee128 and finally build with __ieee128 or long double equal > to IEEE754 quad to get the u9__ieee128 mangling. > And besides libstdc++ on everything else that wants to achieve ABI > compatibility with both formats. > > The above doesn't make long double distinct type from __ieee128 when it > is the same binary type anyway, so why should long double be distinct from > __ibm128 when long double is the same binary type as __ibm128? > > If you need to keep g for compatibility (you do), then why not just have > e (long double is double) > g (long double when matching __ibm128, or explicit __ibm128) > u9__ieee128 (long double when matching __ieee128, or explicit __ieee128)
"g" means __float128. Which is __ieee128. And it has to be, because so much code expects that already, and it will only become more. But "g" is demangled as __float128. Confusion galore. We need to keep "g" mangling for compatibility, but over time everything will default to quad precision long double so people will only see the explicit __ibm128 anymore (if they use __ibm128 at all). The plan is to have the compiler generate the aliases (g vs. u8__ibm128) by itself, btw. Segher
