[llvm] [NVPTX] add an optional early copy of byval arguments (PR #113384)

[Artem Belevich via llvm-commits]

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@@ -612,29 +638,8 @@ void NVPTXLowerArgs::handleByValParam(const NVPTXTargetMachine &TM,
 
     // Do not replace Arg in the cast to param space
     CastToParam->setOperand(0, Arg);
-  } else {
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Artem-B wrote:

`NVPTXLowerArgs` does quite a bit more than just add copies. It also deals with `cvta.param`, and that requires adding `addrspacecast`, etc, and I'm not comfortable doing it that early in the pipeline. AS-specific pointers are not always handled well in LLVM. 

With the early copy, there will still be some work to do if LLVM would eliminate the inserted copy, but the code would still take a pointer to byval (i.e. using something like `escape( condition ? &byval_arg : &some_global)` )

I suspect we'll see other corner cases. That's why I want to keep it disabled by default, while I see how the change fares on a wide range of code.

https://github.com/llvm/llvm-project/pull/113384