[LLVMdev] How to represent __attribute__((fastcall)) functions in the IL

Duncan Sands baldrick at free.fr
Fri Oct 19 09:00:45 PDT 2012

Hi Renato,

> On 19 October 2012 16:40, Chandler Carruth <chandlerc at google.com> wrote:
>> Don't get me wrong, I don't have any good ideas about how to do this, I'm
>> just hoping someone does. End result might allow something like:
>> declare void @foo(double inreg <eax,edx> %x)
> Hi Chandler,
> We were discussing about this on the Cambridge LLVM Social a while
> ago, and we felt that there were far too much target dependent stuff
> in the procedure call standards as it is.
> Our approach would be to have a PCS layer, either in the front-end or
> as a pass, that would know about both language and target to be able
> to construct calls as the target is expecting (ABI-wise).
> David Chisnall proposed a PCSBuilder (similar to IRBuilder, for
> building function calls), where you just pass the basic info (return
> type, arguments, flags, name) and it builds the function for you,
> mangling names, changing parameters and assigning things to registers
> when the ABI is less than helpful, possibly having an inreg syntax
> like you describe.
> My idea was to make it a pass, so you could delay the PCS mess up to a
> later stage, when possibly you'd already have more information about
> the target, but that's not necessarily true and might open the can of
> worms that is having a multi-layered IR. For simplicity, we might
> consider David's approach first, and move the code later, if the idea
> of a multi-layered IR gets on.
> Would that make sense in Clang? It should be a matter of code movement
> more than new code, and would provide a common place for more
> front-ends to use.
> Duncan,
> Would that make sense in dragonegg?

I was planning to go the other direction in dragonegg: be driven by GCC, which
tells you what to place on the stack, what to place in a register (and what
register to place it in).  That would fit much better with Chandler's scheme.
That said, I also don't like the idea of filling the IR with tons of target
specific stuff.

Ciao, Duncan.

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