[llvm-dev] CallSiteBase::getCalledFunction and non-trivial calls

[Friedman, Eli via llvm-dev]

On 9/12/2018 11:51 AM, via llvm-dev wrote:
> How does LLVM define "indirect call"? The documentation of 
> CallSiteBase::getCalledFunction claims it returns null for indirect 
> calls, but in practice it seems to return null for "non-trivial" 
> calls. For example, it returns null for a direct call to a bitcast'ed 
> function:
>
> %0 = call void bitcast (void (%struct.foo *)* @func to void 
> (%struct.bar *)*)(%struct.bar *qux)
>
> By some definition "direct" could mean "trivial", but here it seems 
> ambiguous at best.

An indirect call is a call to anything that isn't a Function.

It might be possible to argue for a special case for a bitcast of a 
Function, because the code generator will eventually look through the 
bitcast. But in practice transforms prefer to treat them as opaque 
anyway because they can't reason about the arguments.  And instcombine 
will transform code like your example into a direct ("trivial") call.

>
> I was able to find some discussion of this previously at 
> http://lists.llvm.org/pipermail/llvm-dev/2015-November/092396.html but 
> it feels like the docs/implementation of some of the functions in 
> CallSiteBase (isIndirectCall also does not seem quite correct, and is 
> not used in a couple places it could be) should just be updated to 
> reflect this.
>
> Is my assessment reasonable? I can update these functions, but it will 
> require updating uses throughout the codebase so I wanted to ask if 
> this makes any sense before starting the work.

Documentation updates would be fine.  Not sure what you're proposing to 
change in terms of code...

-Eli

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