casting Constant * to value *?

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1

Hi all,

extracting datafields of globals, the API code ends up in a Constant *

Constant* const_ptr_103 = ConstantExpr::getGetElementPtr(gvar_struct_foo,
const_ptr_103_indices);

it can be used to initialize e.g. a new instruction like:

StoreInst* void_119 = new StoreInst(const_float_102, const_ptr_103, false,
label_entry_113);

But how about replacing the operand of an already existing instruction using
a Constant *?

Instruction->setOperand(1,__needs_a_value*_here__);

Thx
Alex

2

Hi all,

extracting datafields of globals, the API code ends up in a Constant *

Constant* const_ptr_103 = ConstantExpr::getGetElementPtr(gvar_struct_foo,
const_ptr_103_indices);

it can be used to initialize e.g. a new instruction like:

StoreInst* void_119 = new StoreInst(const_float_102, const_ptr_103, false,
label_entry_113);

But how about replacing the operand of an already existing instruction using
a Constant *?

Instruction->setOperand(1,__needs_a_value*_here__);

If you look at the inheritance tree, you'll see that an llvm::Constant is a subclass of llvm::Value. Therefore, you can use an llvm::Constant anywhere in which an llvm::Value is needed.

To look at the inheritance tree, look at the LLVM doxygen documentation.

Regards,

John Criswell

3

John, you are right.
I 'browsed' the doxygen's inheritance diagram.
Shouldn't I then be able to cast Constant * to Value*?

Plugging the retrieved Constant* (from ConstantExpr::getGetElementPtr)
into Instruction->setOperand compiles, but gives me an assertion failure at
runtime.
I have no access to the code at the moment. I will gather more information
possibly tomorrow.

Thanks
Alex

John Criswell wrote:

4

It should work just fine WITHOUT a cast. I use this concept in several
places in my compiler. If you hit an assert, it is likely for some
other reason.

5

Alex,

Seeing your questions go by on the list, ISTM that you are focusing a lot on the CPPBackend, which tends to generate ugly code generators, and isn't particularly the best example to be following unless you're really stuck and the doxygen docs aren't helping... has anyone pointed you to the Kaleidoscope tutorial? http://llvm.org/docs/tutorial/LangImpl3.html

Cheers,

Jon

6

John, you are right.
I 'browsed' the doxygen's inheritance diagram.
Shouldn't I then be able to cast Constant * to Value*?

You should be able to, although, as others have pointed out, it is unnecessary.

Plugging the retrieved Constant* (from ConstantExpr::getGetElementPtr)
into Instruction->setOperand compiles, but gives me an assertion failure at
runtime.

What assertion did you get? It's hard to diagnose the problem without knowing what assertion you are seeing.

Regards,

John Criswell

7

Dear all,

sorry for the delay.
I found the reason for the assertin failure; I made a mistake and thought
the function pointer for the call instruction would come first, then the
argument, which is not correct. Therefore the types did not match.
Its working well now.

Thank you all for your suggestions!

Alex

Alexander Poddey wrote:

8

Hi Jon,

yes, I read through the kaleidoscope at the beginning of my project, which
is about manipulating IR code.

Kaleidoscope example is nice, but naturally can not cover every possible
situation.

What I really like about the CPPBackend is, that I can code anything in c /
c++ and see how this could be achieved in the API (e.g. accessing nested
structs). I then use these code snippets as entry points to llvm source code
browsing and seeing how I could use it for my needs.

Is there a better way / better C++Api code generator I should use instead?

Thanks
Alex

Jonathan Roelofs wrote: