llvm dev - Mar 2019 - Help with bitcast instruction

Hi,
I'm playing with the IR of pngpixel and I encountered this 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 would like to understand where bitcast is defined because it is not
defined within libpng. I also noticed that the @ is not present, what does
the missing @ mean? I guess it must be an LLVM function.

The bitcast operation seems to represent the following C code:

if (png_ptr != NULL)
   {
      png_ptr->mem_ptr = mem_ptr;
      png_ptr->malloc_fn = malloc_fn;
      png_ptr->free_fn = free_fn;
   }
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bitcast is an LLVM IR instruction. You can find its description here
https://llvm.org/docs/LangRef.html#bitcast-to-instruction

~Craig


On Sun, Mar 10, 2019 at 12:18 AM Alberto Barbaro via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
> I'm playing with the IR of pngpixel and I encountered this 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 would like to understand where bitcast is defined because it is not
> defined within libpng. I also noticed that the @ is not present, what does
> the missing @ mean? I guess it must be an LLVM function.
>
> The bitcast operation seems to represent the following C code:
>
> if (png_ptr != NULL)
>    {
>       png_ptr->mem_ptr = mem_ptr;
>       png_ptr->malloc_fn = malloc_fn;
>       png_ptr->free_fn = free_fn;
>    }
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Thanks I missed that

Alberto

On Sun, Mar 10, 2019, 08:36 Craig Topper <craig.topper at gmail.com>
wrote:
> bitcast is an LLVM IR instruction. You can find its description here
> https://llvm.org/docs/LangRef.html#bitcast-to-instruction
>
> ~Craig
>
>
> On Sun, Mar 10, 2019 at 12:18 AM Alberto Barbaro via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>> I'm playing with the IR of pngpixel and I encountered this
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 would like to understand where bitcast is defined because it is not
>> defined within libpng. I also noticed that the @ is not present, what
does
>> the missing @ mean? I guess it must be an LLVM function.
>>
>> The bitcast operation seems to represent the following C code:
>>
>> if (png_ptr != NULL)
>>    {
>>       png_ptr->mem_ptr = mem_ptr;
>>       png_ptr->malloc_fn = malloc_fn;
>>       png_ptr->free_fn = free_fn;
>>    }
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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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.

Hi Tim,
Thanks again for your help. All clear now

Alberto

On Sun, Mar 10, 2019, 08:59 Tim Northover <t.p.northover at gmail.com>
wrote:
> 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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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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