[LLVMdev] folding x * 0 = 0

[Chris Lattner]

On Mar 3, 2010, at 10:23 AM, Jochen Wilhelmy wrote:

> 
>> You should check out the -enable-finite-only-fp-math and -enable-unsafe-fp-math options.
> 
> Good hint, but
> 
>     llvm::UnsafeFPMath = true;
>     llvm::FiniteOnlyFPMathOption = true;
> 
> at the beginning of my code does not help.
> I found llvm::Reassociate::OptimizeExpression in 
> llvm\lib\Transforms\Scalar\Reassociate.cpp
> which looks like it does X * 0 = 0 for int, but it does not get called 
> for int,
> but it works for int.

These flags only affect the code generator. If you want to add this optimization to your copy of llvm, you can do so by adding it to the instcombine pass.  We'd need IR enhancements to do this sort of thing for real in the mid-level optimizer, some thoughts are here:
http://nondot.org/sabre/LLVMNotes/FloatingPointChanges.txt

However, this is far from being a concrete proposal.

-Chris