"Fix" bug 22175: cdstreq should always use ES:DI for 16-size structs (DO NOT MERGE- but, please, help?)

[FeepingCreature]

Do not merge! I definitely don't understand what I'm doing here! This is presented as a request for comments!

Okay.

Something very weird is going on with 22175. It needs to do a struct assignment from AX|DX, because it's trying to ?: merge a 9-byte (that is, rounded up to 16-byte) struct. But it falls into a codepath in cdstreq where I think it thinks it's trying to work with pointers? So it does some sort of weird fixup and then decides to only use DI, not DI|ES, because we're on x86_64 and we don't need segment addressing (ES:DI) there. But we're working with a streq elem, and the result of that isn't a pointer to begin with?? So then we're stuck trying to fit a 16-byte result in a 8-byte register and crash.

The PR just removes that EX_flat check, which gives it enough space to fit AX|DX into. This seems to pass the testsuite, so technically it's a fix, but the goal here is more to get some eyes on that codepath than to get this PR merged.

Help pls? 🥺

[dlang-bot]

Thanks for your pull request and interest in making D better, @FeepingCreature! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	22175	regression	Nested method access to variable formed by ternary of constructor and function call, of a struct with four shorts and a bool, crashes DMD.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#13255"

[RazvanN7]

cc @WalterBright

[RazvanN7]

ping @WalterBright , could you give us some pointers?

[FeepingCreature]

Okay, what's up here? Pushed a rebase... I'm still not really up with merging this, but it doesn't seem like anything else is going to happen here? I guess I'd rather have the commit in than not, if those are the only two options on the table?

[WalterBright]

This is the PR that caused the regression: #12409

[WalterBright]

This fix doesn't work because it needs to do a dereference of DI. Replaced with #13788