[PATCH] D126809: [ARM64EC 8/?] Implement ARM64EC "thunk" calling conventions

[Daniel Paoliello via Phabricator via llvm-commits]

dpaoliello added inline comments.


================
Comment at: llvm/include/llvm/IR/CallingConv.h:258
+    /// ARM64 register names. The first parameter is mapped to x9.
+    ARM64EC_Thunk_X64 = 102,
+
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efriedma wrote:
> dpaoliello wrote:
> > When adding a calling convention, does it not also need to be added to the IR AsmWriter/AsmPrinter?
> If you don't add it to the asmparser/writer, there's some default handling that just uses the number, I think.  But it might be better to give them proper names in IR.
> 
> Alternatively, I was thinking about marking the calls using an attribute instead of a calling convention.  Which is basically equivalent to using a calling convention, but doesn't consume calling convention numbers.  Probably doesn't matter much in practice.
I think using a calling convention makes more sense, and should avoid these functions being accidentally treated like a normal function.


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Comment at: llvm/lib/Target/AArch64/AArch64CallingConvention.td:263
+  // convention requires "this" to be passed in RCX.
+  // FIXME: Do we need a ARM64EC_Thunk_thiscall convention?
+  // CCIfCC<"CallingConv::X86_ThisCall",
----------------
mstorsjo wrote:
> efriedma wrote:
> > dpaoliello wrote:
> > > I'll get some clarification on this...
> > I copy-pasted the X86_ThisCall thing from the x86 code.  It might be a red herring, though; I just looked at this in clang, and it looks like clang doesn't actually use the X86_ThisCall on x64 Windows, so this doesn't actually do anything, even in the original?  I'm sort of confused how it ended up that way.  Maybe can just delete it, though.
> > 
> > I probably do need some handling for C++ methods, though, which I haven't implemented yet.  (In particular, I need to ensure we emit the arguments for exit thunks for C++ methods in the correct order.)
> `thiscall` (similarly to `stdcall`, `fastcall` and `vectorcall`) are all no-ops on x86_64, they're only a thing on x86_32 windows.
`vectorcall` still exists for x64, however Arm64EC currently only supports standard x64 calling convention.

For now, I think we can drop the `thiscall` special case, but I will double check if it is required (we can always add it back later if so).


Repository:
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  https://reviews.llvm.org/D126809/new/

https://reviews.llvm.org/D126809