[LLVMdev] Functions with unnamed parameters in LLVM IR

[Dan Liew]

On 13 August 2014 15:48, Tim Northover <t.p.northover at gmail.com> wrote:
>> The above LLVM IR was confusing to me because I usually see that
>> unnamed registers start counting from 1 (i.e. %1 = add ...).
>
> There's a (usually hidden) %0 representing the entry basic block
> there. The general rule is "start from 0 and keep counting; skip named
> values".

Okay that's good to know. So unnamed arguments get assigned temporary
registers before the entry block? If so the entry block isn't always
%0.

>> I'm slightly surprised that unnamed function arguments are allowed at
>> all. Maybe there is a use case but I can't think of a good one off the
>> top of my head.
>
> They're useful for ensuring ABI conformance: padding out (hardware)
> registers that you don't want to use for a particular call.
>
> You might map void foo(int32_t a, int64_t b) to "declare void @foo(i32
> %a, i32, i64 %b)" for example, if your 64-bit value had to start at an
> even (32-bit) register number, as is the case on ARM. (Actually, it's
> not necessary there, but is in more complicated cases).

Thanks for the concrete use case.

>> I think it should be documented in the LLVM reference manual what the
>> register names are for unnamed arguments.
>
> It does sound reasonable to add a note that function arguments get
> included too, since they're not obviously value computations.

Included in what?

Anyway I've attached a patch that tries to clear this up. Is this good
enough to commit?
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