[clang] [llvm] [CodeGen][i386] Move -mregparm storage earlier and fix Runtime calls (PR #89707)

[Eli Friedman via cfe-commits]

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@@ -62,6 +62,13 @@ namespace llvm {
                      LibFunc TheLibFunc, AttributeList AttributeList,
                      FunctionType *Invalid, ArgsTy... Args) = delete;
 
+  // Handle -mregparm for the given function.
+  // FIXME: This should likely be implemented in
+  // CodeGenModule::SetLLVMFunctionAttributes() since callers of
+  // markRegisterParameterAttributes() will not have gotten appropriate
+  // attributes for things like sign/zero-extension, etc.
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efriedma-quic wrote:

This doesn't really properly capture the divide between clang, the frontend, and LLVM the optimizer.

What we want here is basically a warning: this interface shouldn't be used unless you know exactly what you're doing.  Then a FIXME on the clang side to indicate we plan to revise the CreateRuntimeFunction() interface at some point.

Suggested text on the LLVM API:

Note that this function is a rough approximation that only works for simple function signatures.  Note this does not apply other relevant attributes for function signatures, including sign/zero-extension for arguments and return values.

https://github.com/llvm/llvm-project/pull/89707