[LLVMdev] Presenting Unsafe Math Flag to Optimizer

[Chris Lattner]

On Jan 14, 2010, at 2:01 AM, Duncan Sands wrote:

> Hi Bill,
>
>> The current implementation of the "allow unsafe math" option is to  
>> specify it via the TargetOptions object. However, this prevents the  
>> target-independent optimizer from using it. Are there any opinions  
>> (ha!) on how this could be achieved in a nice clean manner which  
>> doesn't require using the TargetOptions object in the optimizer?
>
> a flag on each floating point operation, saying whether it does  
> "exact" math or
> not?

Yes, the right approach for this is to add flags to each fp operations  
just like the NUW/NSW bits on integer ops.  We want the ability to  
represent the C99 pragmas which are scoped more tightly than a  
function body.

This is actually really easy to do, the big issue is defining the  
'bits' that we want to carry on each operation.  For example, I think  
it would be reasonable to have an "assume finite" bit (saying no  
nan's / inf), it would also be useful to know you can do reassociation  
etc, useful to know that you don't care about signed zero, etc.

I don't have enough expertise to propose exactly how this should work.

-Chris