ltsmaster 0.17 miscompiles LDC >= 1.21

[JohanEngelen]

See #3333 (comment)

Does this mean we have to drop ltsmaster compilation support? Or is it sufficient to add a big warning when the d compiler used to build LDC is recognized to be <= 0.17, telling the user to use the just-built compiler to rebuild LDC?

[kinke]

Not related to ltsmaster, but another potential corruption with host compilers < 2.085.1: dlang/dmd#10896 (comment)

[Geod24]

Regarding the fix to the aforementioned comment, it's literally 1 line of red, so trivial to backport.

[kinke]

That's not the point, but rather that until recently, the host compiler version wasn't that crucial (except for -lowmem not working properly with older druntimes), but now we get silent corruptions wrt. @live analysis (dunno with which version that was fixed) and, potentially, this horrible vtable issue above (granted, only if compiled separately, which would need to be enabled manually - edit: we do separately compile the compiler unittests...). The nixOS packagers still ship a compiler built with ltsmaster...

[ibuclaw]

Would it be possible to use the dmd-cxx branch like gdc?

[kinke]

That wouldn't be worth the effort IMO; instead, our README.md now just recommends to use gdmd for those who want to bootstrap from plain C++. No more ltsmaster baggage incl. new-LLVM-version backports etc., and for those interested in conquering a new platform, cross-compilation (incl. LDC itself) should be way less painful than adding support in ltsmaster (compiler + libs) first, and then porting that to master.

[dnadlinger]

Would still be good to know (and note down) what the last "good" ltsmaster-bootstrapped version is so people can follow a two-step process if necessary.

[kinke]

I've added it as new 'known issue' to the release notes. The 'regular' two-step process (ltsmaster -> master -> master) should still work, as long as we don't have any @live in the compiler sources, which is unlikely to happen soon. Edit: Oh, and in druntime/Phobos neither, which might take less time to break.