[clang] [llvm] [transforms] Inline simple variadic functions (PR #81058)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

> I don't really like the whole "sufficiently simple function" thing. It seems fragile. You should be able to just take a arbitrary internal varargs function, rewrite its signature to take a va_list argument, rewrite calls to va_start to make a copy of that va_list, and rewrite the callers to construct that va_list. If that function turns out to be inlinable, great; if not, you haven't really lost anything.

Yes, you can and I do. That's patch 2 of the series, numbered 1. in the list

> (Rewriting the signature of a function is complicated in its own way because you need to allocate a new Function, then transplant the original function's body into it. But it's not uncharted territory: we should be able to refactor code out of llvm/lib/Transforms/IPO/ArgumentPromotion.cpp .)
> 
> Do we have a testing plan for this? Messing up calling convention stuff tends to lead to extremely subtle bugs.

And this is why it's a separate patch. 

The rewrite-call-instruction is the target dependent bit, in this patch for an initial target.

The rewrite-function is (almost) target agnostic and involves a surprisingly large amount of book keeping. We have duplicated code in ArgumentPromotion, dead argument removal, the function cloning in attributor and probably elsewhere.

https://github.com/llvm/llvm-project/pull/81058