[llvm-dev] Help with bitcast instruction

[Alberto Barbaro via llvm-dev]

Hi,
sorry I forgot to mention, how can I extract the keyword bitcast? I would
like to be able to distinguish the function name.

Thanks

Il giorno mar 12 mar 2019 alle ore 18:33 Alberto Barbaro <
barbaro.alberto at gmail.com> ha scritto:

> Hi Tim,
> I'm still struggling on the instruction:
>
> 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?
>
> 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"
>
> As a clarification the function is declared like this:
>
> void PNGAPI
> png_set_mem_fn(png_structrp png_ptr, png_voidp mem_ptr, png_malloc_ptr
>   malloc_fn, png_free_ptr free_fn)
>
>
> Thanks again
>
>
> Il giorno dom 10 mar 2019 alle ore 08:59 Tim Northover <
> t.p.northover at gmail.com> ha scritto:
>
>> Hi Alberto,
>>
>> > I would like to understand where bitcast is defined because it is not
>> defined within libpng.
>>
>> It's because the function (@png_set) has been defined to take one kind
>> of argument, but is being called with another. In this case the
>> difference seems to be whether it takes pointers to
>> %struct.png_struct_def or %struct.png_struct_def.68.
>>
>> This kind of situation isn't terribly common, but can happen for
>> example in LTO where two modules are linked together, and one of them
>> only has an opaque handle for the type to hide the implementation
>> (e.g. from plain "struct png_struct_def;" in C). When that happens
>> LLVM can't merge the types, so it adds a numeric tag to one of them,
>> and then it has to insert bitcasts in the calls like this so that its
>> own type system works.
>>
>> > I also noticed that the @ is not present, what does the missing @ mean?
>> I guess it must be an LLVM function.
>>
>> The @ is only used directly before a global variable (including a
>> declared/defined function). In this case it's buried, but it is there
>> (@png_set). In other cases, like when you call a function pointer, it
>> will be missing entirely.
>>
>> Cheers.
>>
>> Tim.
>>
>
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