[llvm-dev] About mismatching calling conventions

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 16 10:00:48 PST 2017

This is an old FAQ that I remember running into 8 years ago:

However, I think you are right, we could make CreateCall enforce calling
convention match for direct calls, and require callers to manually set the
calling convention with setCallingConv if they want the convention to not

On Thu, Nov 16, 2017 at 12:35 AM, serge guelton via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi llvm-dev,
> Every now and then, when building a direct ``CallInst`` either from
> IRBuilder
> or through ``Create*``, I have a latent bug because the calling
> convention (CC) expected by the function is not set on the call, and I need
> to manually call ``setCallingConv``.
> Now, from LangRef#calling-conventions,
>     The calling convention of any pair of dynamic caller/callee must
>     match, or the behavior of the program is undefined.
> So the behavior is correctly defined, as emphasised by the check in
> ``InstCombineCalls`` that turns mismatching convention into an ``undef``.
> This is however very error-prone. Maybe we should either:
> - update the API to enforce the CC when building a ``CallInst``
> - update LangRef to state that mismatching CC in static call
>   is an error (note that LangRef is unclear about static call, the quote
>   only references indirect calls)
> - add an optional setting in the ``CreateCall`` that enforces CC
>   (and let it default to yes?)
> Any thoughts on that?
> Serge
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