[llvm-commits] [llvm] r68768 - /llvm/trunk/include/llvm/Support/IRBuilder.h

[Chris Lattner]

On Apr 9, 2009, at 10:30 PM, Nick Lewycky wrote:
> URL: http://llvm.org/viewvc/llvm-project?rev=68768&view=rev
> Log:
> Add utility function to IRBuilder that takes the difference between  
> two
> pointers, taking into account the size of the pointed-to object.
> Patch by Jeffrey Yasskin!

looks good to me.  Incidentally, clang and llvm-gcc specialize the  
case when the element size is a power of two into a signed right  
shift.  This is legal because C only allows pointer subtraction  
between elements in the same array, which means the division must be  
"exact".  In this case, SRA x, c is the same as sdiv x, 2^c.  I don't  
know if you care about this case though.

-Chris

>
>
> Modified:
>    llvm/trunk/include/llvm/Support/IRBuilder.h
>
> Modified: llvm/trunk/include/llvm/Support/IRBuilder.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/IRBuilder.h?rev=68768&r1=68767&r2=68768&view=diff
>
> = 
> = 
> = 
> = 
> = 
> = 
> = 
> = 
> ======================================================================
> --- llvm/trunk/include/llvm/Support/IRBuilder.h (original)
> +++ llvm/trunk/include/llvm/Support/IRBuilder.h Fri Apr 10 00:30:48  
> 2009
> @@ -681,6 +681,20 @@
>                         Name);
>   }
>
> +  /// CreatePtrDiff - Return the i64 difference between two pointer  
> values,
> +  /// dividing out the size of the pointed-to objects.  This is  
> intended to
> +  /// implement C-style pointer subtraction.
> +  Value *CreatePtrDiff(Value *LHS, Value *RHS, const char *Name =  
> "") {
> +    assert(LHS->getType() == RHS->getType() &&
> +           "Pointer subtraction operand types must match!");
> +    const PointerType *ArgType = cast<PointerType>(LHS->getType());
> +    Value *LHS_int = CreatePtrToInt(LHS, Type::Int64Ty);
> +    Value *RHS_int = CreatePtrToInt(RHS, Type::Int64Ty);
> +    Value *Difference = CreateSub(LHS_int, RHS_int);
> +    return CreateSDiv(Difference,
> +                      ConstantExpr::getSizeOf(ArgType- 
> >getElementType()),
> +                      Name);
> +  }
> };
>
> }
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits