[llvm-bugs] [Bug 48195] New: large codegen in clang for functions containing va_start

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48195

            Bug ID: 48195
           Summary: large codegen in clang for functions containing
                    va_start
           Product: libraries
           Version: 11.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: jeffr at radgametools.com
                CC: llvm-bugs at lists.llvm.org

Not sure what's happening here, but the code gen for va_start is really large
compared to GCC or MSVC on x64.  It looks like it's spilling all the registers
(including the sse registers) for some reason.

Are GCC and MSVC assuming something that clang cannot?  Are they doing some
optimization for intra-compliation unit? 

Here's clang11:

https://godbolt.org/z/7PrKEf

Flip to gcc and msvc and the code is what I would expect...

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