[clang] [clang][CodeGen] `used` globals && the payloads for global ctors & dtors are globals (PR #93601)

[Matt Arsenault via cfe-commits]

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@@ -2928,12 +2928,13 @@ static void emitUsed(CodeGenModule &CGM, StringRef Name,
   for (unsigned i = 0, e = List.size(); i != e; ++i) {
     UsedArray[i] =
         llvm::ConstantExpr::getPointerBitCastOrAddrSpaceCast(
-            cast<llvm::Constant>(&*List[i]), CGM.Int8PtrTy);
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arsenm wrote:

> Ok, I'm still struggling to see why it is best create (possibly) broken IR

It's not broken. It just needs to provide a use, and whatever casts are there do not matter. We can write whatever rules we want and have the verifier enforce it. Currently we don't have any generic concept of "illegal addrspacecasts" (relatedly we should probably stop throwing codegen errors on the cases we don't handle, and just lower them to poison) 

> Why not make it a special global that uses AS 42

Behavior of address spaces are target defined. We don't want to just grab random numbers for generic purposes. We've gradually been migrating away from 0 being special in more contexts, but nothing has been done for used (and I don't see a particularly compelling reason to do, used just needs to be a box that is a use)


> IR-level compat between LLVM versions (yes it frequently works but it's not guaranteed to).

We do guarantee forward compatible bitcode

https://github.com/llvm/llvm-project/pull/93601