[PATCH] Support __builtin_ms_va_list.

[Charles Davis]

  @rsmith wrote:
  > This seems very weird to me. Can you explain a bit about the background for this?

  Certainly.
  > Is this to support writing variadic __attribute__((ms_abi)) functions when not targeting Win64?

  Yes. Some software in the wild (Wine, for instance) has variadic `ms_abi` functions. (I wouldn't be surprised, frankly, if this were originally implemented in GCC at the behest of Wine. I know that the `ms_hook_prologue` attribute was.)
  > How is this is supposed to work when we *are* targeting Win64 directly?

  Exactly the same as a normal `va_list`. I even added a test for this.
  > (Do we get a corresponding __builtin_sysv_va_list to go in the opposite direction, for instance? Are the two types the same in that case?)

  We don't. Obviously, this is something GCC overlooked--probably because it was implemented specifically so Wine could use it (see above). (To be honest, I contemplated this myself before submitting. I wanted to add an orthogonal extension for `va_list`s in a `sysv_abi` function. But I decided that was better suited for another patch--this one's big enough as it is.)


================
Comment at: include/clang/Serialization/ASTBitCodes.h:1343
@@ -1340,1 +1342,3 @@
+      EXPR_LAMBDA,                // LambdaExpr
+      EXPR_MS_VA_ARG              // VAArgExpr (with isMicrosoftABI() true).
     };
----------------
Richard Smith wrote:
> Why are you using a separate StmtCode here, rather than just serializing the flag normally?
I think it was because I didn't want to break backwards compatibility with the existing encoding of `clang::VAArgExpr` in a PCH/module. (That's going to become important later when modules are ready for primetime.) But, if you want me to serialize the bit normally, I can go do that.


http://llvm-reviews.chandlerc.com/D1623