llvm dev - Sep 2018 - CallSiteBase::getCalledFunction and non-trivial calls

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.

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.

Regards,
Scott

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

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

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