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

[Sameer Sahasrabuddhe via llvm-dev]

It would be nice to have the new function that Scott suggests ... this
little detail has bitten us before, too. The current getCalledFunction()
creates an impression that we will always see the callee for a direct call,
but in reality it doesn't actually look through constant expressions. A new
function will make this more obvious to someone expecting to always find a
direct callee. Any use of this function will also make it apparent that the
analysis or pass is prepared to handle the arguments.

Sameer.

On Tue, Sep 18, 2018 at 1:54 AM, Friedman, Eli via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> The current CallGraphSCC passes aren't prepared to deal with edges like
> that, I think, but that could be fixed; it's only a few passes.  Maybe
> simpler for your purposes to pull the call transform code out of
> instcombine so you can call it at -O0, though.
>
> -Eli
>
> On 9/12/2018 12:47 PM, scott at scottlinder.com wrote:
>
>> The immediate change I have in mind is in CallGraph; our implementation
>> of LowerCall in AMDGPU currently relies on the the callee arguments being
>> lowered before the call is lowered, and we simply do not support indirect
>> calls. However, we should be able to support these bitcast calls, as they
>> are effectively direct for our purposes, but the CallGraph does not seem to
>> consider them (it uses .getCalledFunction()). Maybe a new function for
>> `dyn_cast<Function>(CS.getCalledValue()->stripPointerCasts())` would
>> make sense? I am not sure if this is valid to use in CallGraph, but it does
>> not care about arguments as far as I can tell.
>>
>> At the very least I can try to clarify the docs, because until you
>> explained it I had a different definition of "indirect call" in mind.
>>
>> Scott
>>
>> On 2018-09-12 15:21, Friedman, Eli wrote:
>>
>>> 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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