[CGObjC] Use llvm::isKnownNonZero to determine if receiver can be null.

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@ellishg @kyulee-com @plotfi Did you know why we didn't do this earlier? Is it because there's some issue with isKnownNonZero or simply because we forgot?

This patch is necessary if we want to erase some nil check thunk.

[kyulee-com]

To me, using isKnownNonZero seems reasonable.
Although logically it's same, I slightly prefer this way:

  if (llvm::isKnownNonZero(receiver, CGM.getDataLayout())
    return false;
  // Otherwise, assume it can be null.
  return true;

[ellishg]

Yeah it seems good to me, but we should have a test case too

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I just came up with this on my way home. Will add tests tmr.

…

On Mon, Nov 4, 2024, 8:58 PM Ellis Hoag ***@***.***> wrote: Yeah it seems good to me, but we should have a test case too — Reply to this email directly, view it on GitHub <#114914 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AFQQFGHYCEJRNHUI7GB5T4TZ7A7ABAVCNFSM6AAAAABRFQSW36VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDINJWGIZTMNZYGI> . You are receiving this because you were assigned.Message ID: ***@***.***>

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The quest to write some tests has failed.

I realized that this call has little to no effect. isKnownNonZero requires some function analysis (e.g. Dominator Tree), which doesn't exist yet during frontend CodeGen.