LLVM error compiling core and builtin functions using 16-bit pointers in data layout

[Serentty]

I'm trying to compile Rust code to run in 386 real mode, where pointers larger than 2^16 aren't meaningful, even though there are 32-bit registers to work with. It works if I use full 32-bit pointers, but this is quite wasteful, so I've written my target JSON file to specify 16-bit pointers with 16-bit alignment in the LLVM data layout, and set target-pointer-width to 16. However, I get errors from LLVM itself trying to compiler certain functions in core and builtins.

LLVM ERROR: Cannot select: 0x7f962aae09c0: i16 = GlobalAddress<<{ [8 x i8], i8*, [2 x i8] }>* @anon.d1d52a4f9fbe2e9a9c47ed4caed11e39.2.llvm.4753579327427470583> 0
In function: _ZN4core3num7dec2flt3num21compare_with_half_ulp17h4eb4cc321c32da3aE
error: could not compile `core`.
warning: build failed, waiting for other jobs to finish...
LLVM ERROR: Cannot select: 0x7f75cd47f2d8: i16 = FrameIndex<-1>
In function: _ZN51_$LT$u64$u20$as$u20$compiler_builtins..int..Int$GT$12extract_sign17hef80dd72940759b7E
error: could not compile `compiler_builtins`.

[jonas-schievink]

Does LLVM support this?

[Serentty]

@jonas-schievink LLVM supports code16, which despite the confusing name is the LLVM mode for generating 32-bit real mode code. This is what I'm using. This isn't a separate mode from 16-bit real mode, but just means real mode using 386 instructions including 32-bit math. LLVM supports this because it's necessary for writing non-UEFI bootloaders, and it's used in projects like GRUB.

[Enselic]

Triage: this seems like an LLVM bug rather than a rustc bug? So probably best to report to LLVM with a minimal reproducer in order to make progress. Thanks!

[Enselic]

The reproducer is maybe rustc specific though, so let's reopen.

However, can this still be reproduced? It would be useful to have a way for others to reproduce. Labelling accordingly.