[lld] [llvm] [LTO] Override TargetABI from module flags if present when creating T… (PR #126497)

[Sam Elliott via llvm-commits]

lenary wrote:

I read the description here, and was a little concerned, but on reading the code I'm not. 

> This patch changes the TargetMachine's data layout to the one specified in the module flags if present.

This sentence concerned me, but reading the implementation, it isn't quite what's happening. Instead, you're providing the `target-abi` module flag as a TMOption only if the `-target-abi` argument hasn't been specified at the command line, which in LTO is very unlikely. 

Not only is the existing datalayout logic unaffected, but i think this now brings how target-abi is handled inline with how datalayout is handled:
- first use the arguments
- then use whatever is in the LLVM IR (was skipped before for the ABI, not for the datalayout)
- then use a fallback/default

It would be good to update the message when this lands to clarify this, as my first reading did have me concerned we didn't follow this logic which could have affected a lot of tests and other users of serialised LLVM IR.

> Another possible solution is add new parameter to the TargetMachine constructor, but that would require changes in all the targets.

I looked at this a while ago (iirc for the same problem) and it's really really messy to fix. We should do it sometime, but I recognise we cannot do it quickly. This is a better step for right now.


https://github.com/llvm/llvm-project/pull/126497