[llvm-dev] Showcase of the Windows/ARM64/MinGW target

[Martin Storsjö via llvm-dev]

On Mon, 21 May 2018, Hayden Livingston wrote:

> Ok. How much work was it to enable ARM64 codegen? Is it is
> significantly more work than for Linux targets? Are there any special
> relocations in ARM64 land?

It was surprisingly little work, I'd say. Yes, Windows uses COFF, and 
there's a separate set of relocations for each architecture. Probably not 
more work than for Linux targets, but just new things to implement.

The initial commits for ARM64 in COFF were done by Mandeep Singh Grang, 
and after that, there were a few details to fill in about how the 
relocations are supposed to behave. The public documentation only names 
them, but luckily link.exe supported those relocations since long ago, so 
I was able to use llvm-mc together with link.exe to figure out how e.g. 
the IMAGE_REL_ARM64_REL21/IMAGE_REL_ARM64_PAGEOFFSET_12A relocation pairs 
are supposed to handle overflow/carry.

Other than relocations, the main bits were varargs handling, hooking up 
small flags and bits here and there as you find what's missing, minor 
windows/coff specific details like linker directives and dllimport 
handling, and slightly larger things such as stack probing and TLS.

// Martin