[REG2.071] Issue 15984 - Interface contracts retrieve garbage instead…

[thewilsonator]

… of parameters

Rebase of #5765

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	15984	regression	[REG2.071] Interface contracts retrieve garbage instead of parameters

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + dmd#8988"

[WalterBright]

@rainers writes in #5765

"FYI: when porting the original fix to LDC, I chose to just pass the adjusted this pointer to the interface contracts. No magic required to change the actual pointer in ThisExp.
With this, your test case runs further than when built with DMD 2.071, but still asserts in C.f3.i."

I'm not sure what he means. Does he have a better fix? @rainers please clarify!

[rainers]

I'm not sure what he means. Does he have a better fix? @rainers please clarify!

I guess this referes to this commit: ldc-developers/ldc@628a560. This was 2 and a half year ago, so the following might not be accurate and also outdated:

IIRC dmd passes the object pointer to the interface contract, passes the offset as a parameter and goes to great length to correct both in the contract function when calling interface functions or accessing parameters. LDC instead passes the adjusted interface pointer as this so the contract just stays a regular unmodified function.

[WalterBright]

Does the ldc commit fix bug 15984 for LDC?

[rainers]

Does the ldc commit fix bug 15984 for LDC?

The test in this PR passes with LDC but for the checks regarding this_ in f3 (this_ is a completely different pointer, maybe the capture). IIRC the LDC commit was a fix for existing tests in testcontracts.d that worked for dmd, but not LDC.

[FeepingCreature]

This PR does not seem to actually fix the issue in 15984.

import core.stdc.stdio;
interface I {
    void foo(Object obj)
    in { printf("%p\n", obj); obj.toString; }
}
class C : I { override void foo(Object obj) in { } do { } }
void main()
{
    auto c = new C;
    c.foo(new Object);
}

edit: Which as far as I can tell, comes about because C::foo() has no idea its parameter is actually used, and so blithely reuses its register.