[Debian-med-packaging] Bug#908670: libundead: ftbfs on armhf

[peter green]

I (with my raspbian maintainer hat on) reported this to ldc upstream at https://github.com/ldc-developers/ldc/issues/2848 and some digging has been done. The short version is that we know what commit caused the build failures. However that commit was a fix for mis compilation, so I don't think we should just back it out. The proper fix is in llvm itself though a workaround may be possible in ldc.

The long version as I understand it.

ldc generates "thunks", these thunks adjust the "this" pointer before calling the original function. I understand they are related to multiple inheritance.

It was discovered that "thunks" for variadic functions were being miscompiled https://github.com/ldc-developers/ldc/issues/2613 , the variadic arguments were not being passed on correctly to the original function. It turns out that variadic functions can only be passed on correctly in a tail call (most ABIs don't pass on any meta-information about the number of variadics, so there is no real way to copy the variadics).

The fix was to make ldc specify "musttail" (which is an order) rather than "tail" (which is a hint) when generating thunks for variadic functions https://github.com/kinke/ldc/commit/1dda15a76272cdb3963754fa815d96bacc989852.

Unfortunately it seems that llvm sometimes fails to compile such thunks on arm, I did some playing around with different parameter lists and it seems that it fails to compile them if a non-variadic structure parameter overflows on to the stack.

So what are our options.

1. Back out the upstream fix, this would fix the build but would almost certainly result in miscompiled code, I don't think that is acceptable.
2. Get the issue fixed in llvm, I have filed a bug report ( https://bugs.llvm.org/show_bug.cgi?id=38931 ) but I don't know when and if it will be actioned.
3. Hack ldc to generate variadic thunks in native assembler on arm, the thunks are trivial to express in native arm asm, but i'm not sure what calls to the llvm api are needed to insert a procedure written in native asm.
4. Simply get libundead removed on armhf/raspbian. libundead seems to only have two reverse build-dependencies neither of which has ever successfully built on armhf. libbiod previously failed with a testsuite failure (probablly related to the miscompilation discussed above) and now fails with the same error that libundead does. sambamba is blocked by a dependency on libbiod