[llvm-dev] Help with bitcast instruction

[Alberto Barbaro via llvm-dev]

Thanks you Tim,
that solved my problem!

Il giorno mar 12 mar 2019 alle ore 18:50 Tim Northover <
t.p.northover at gmail.com> ha scritto:

> Hi Alberto,
>
> On Tue, 12 Mar 2019 at 18:33, Alberto Barbaro <barbaro.alberto at gmail.com>
> wrote:
> > call void bitcast (void (%struct.png_struct_def.68*, i8*, i8*
> (%struct.png_struct_def.68*, i64)*, void (%struct.png_struct_def.68*,
> i8*)*)* @png_set_mem_fn to void (%struct.png_struct_def*, i8*, i8*
> (%struct.png_struct_def*, i64)*, void (%struct.png_struct_def*,
> i8*)*)*)(%struct.png_struct_def* %create_struct, i8* %mem_ptr, i8*
> (%struct.png_struct_def*, i64)* %malloc_fn, void (%struct.png_struct_def*,
> i8*)* %free_fn) #15
> >
> > I'm pretty sure that this is a CallInst but when I try to call
> getCalledFunction() I receive a null pointer and that really surprise me.
> >
> > I would like to understand how to get the function that is called and
> its parameters. Can you tell me how please?
>
> The problem is that getCalledFunction is pretty naive, it just takes
> the getCalleeValue and dyn_cast<Function>s it. Because you've got a
> cast in the IR itself your situation is a bit more complicated.
>
> In this case, you could write a "findCalledFunction" something like:
>
>     Function *findCalledFunction(CallBase &CI) {
>       Value *Callee = CI.getCalledValue()->stripPointerCasts();
>       if (auto F = dyn_cast<Function>(Callee))
>         return F;
>       return nullptr;
>     }
>
> Where the "stripPointerCast" is the key extra ingredient that looks
> through the bitcast. (Someone could certainly argue that's what
> getCalledFunction should be doing anyway, but there are subtleties on
> weird targets like GPUs so it's not 100%).
>
> It won't catch all callsites (e.g. virtual function calls could still
> be plain %X values), but pretty much any algorithm has to be robust
> against an unknown callee anyway.
>
> > Am i right saying that:
> >
> > (void (%struct.png_struct_def.68*, i8*, i8* (%struct.png_struct_def.68*,
> i64)*, void (%struct.png_struct_def.68*, i8*)*)* -> "old parameters type"
> > (%struct.png_struct_def*, i8*, i8* (%struct.png_struct_def*, i64)*, void
> (%struct.png_struct_def*, i8*)*)*) ->  "new parameters type"
> > (%struct.png_struct_def* %create_struct, i8* %mem_ptr, i8*
> (%struct.png_struct_def*, i64)* %malloc_fn, void (%struct.png_struct_def*,
> i8*)* %free_fn) "the parameters passed to the png_set_mem_fn"
>
> I think so, from what I understand. The essence of that cast is just
> changing one pointer type to another, so it doesn't actually affect
> how parameters are passed at all, and that is definitely the function
> called.
>
> As an aside, we're working towards a world where there's just one
> "ptr" type in LLVM. I think this whole situation would disappear
> completely there.
>
> > sorry I forgot to mention, how can I extract the keyword bitcast? I
> would like to be able to distinguish the function name.
>
> I think the answer to this question is the "stripPointerCasts"
> function I mentioned above, though I'm not 100% sure.
>
> Cheers.
>
> Tim.
>
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