llvm dev - Mar 2009 - [LLVMdev] Creating Pointer Constants

Hello,

I am writing a JIT compiler for a subset of the Matlab language and as a
part of my implementation, I would like to be able to pass a constant
pointer to a native function I'm calling.

Right now, this is what I do:

llvm::Constant* constInt = llvm::ConstantInt::get(llvm::Type::Int64Ty,
(int64)thePointer);
llvm::Value* constPtr = llvm::ConstantExpr::getIntToPtr(constInt,
llvm::PointerType::getUnqual(llvm::Type::Int32Ty));
	
builder.CreateCall(my_function, constPtr);

The resulting IR call looks like this:

call void @nativeFunc(i32* inttoptr (i64 146876396 to i32*))

I'm just wondering if there is a better way to approach this. Casting a
pointer to a signed integer and back to a pointer looks both hack-ish, and
potentially risky (what happens if the address falls in the negative range
of the signed integer?). Unfortunately, I couldn't call a pointer constant
type in the LLVM doxygen documentation.

Thank you for your time,

- Maxime
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On 2009-03-08, at 12:46, Nyx wrote:
> I am writing a JIT compiler for a subset of the Matlab language and  
> as a
> part of my implementation, I would like to be able to pass a constant
> pointer to a native function I'm calling.
>
> Right now, this is what I do:
>
> llvm::Constant* constInt = llvm::ConstantInt::get(llvm::Type::Int64Ty,
> (int64)thePointer);
> llvm::Value* constPtr = llvm::ConstantExpr::getIntToPtr(constInt,
> llvm::PointerType::getUnqual(llvm::Type::Int32Ty));
> 	
> builder.CreateCall(my_function, constPtr);
>
> The resulting IR call looks like this:
>
> call void @nativeFunc(i32* inttoptr (i64 146876396 to i32*))
This is correct.
> I'm just wondering if there is a better way to approach this.  
> Casting a
> pointer to a signed integer and back to a pointer looks both hack- 
> ish, and
> potentially risky (what happens if the address falls in the negative  
> range
> of the signed integer?).
The inttoptr/ptrtoint conversion is perfectly safe so long as you  
match the pointer size and the integer size. (i.e., a 64-bit pointer  
obviously can't fit in an i32.)

— Gordon

On Mar 11, 2009, at 6:03 AM, Gordon Henriksen wrote:
>> I'm just wondering if there is a better way to approach this.
>> Casting a
>> pointer to a signed integer and back to a pointer looks both hack-
>> ish, and
>> potentially risky (what happens if the address falls in the negative
>> range
>> of the signed integer?).
If you're worried about sign extension, you could always case through  
uintptr_t instead of int64.  The LLVM target data holds the equivalent  
LLVM type; if you're writing a JIT, you can do jitEngine- 
 >getTargetData()->getIntPtrType().

John.