[LLVMdev] llvm.sqrt intrinsic undefined behaviour

[Tom Stellard]

On Thu, Oct 31, 2013 at 01:16:20PM +0000, Nicholas Chapman wrote:
> Hi all,
> I'm working on a language in which I would like all operations to be 
> well defined. (and efficient)
> I want to define a sqrt function in my language, that will return NaN 
> for arguments < 0, and NaN for a NaN argument.
> As far as I know, these semantics map nicely to the SQRTPS SSE 
> instruction, which seems to return NaN on arguments < 0.
> However, the LLVM lang ref states "llvm.sqrt has undefined behavior for 
> negative numbers other than -0.0".
> 
> This means that, to avoid undefined behaviour, in general I will have to 
> add a runtime branch to avoid passing values less than zero to llvm.sqrt().
> This is unfortunate since I would like to avoid inefficient, unneeded 
> branching.
> 
> I propose changing the llvm.sqrt() LLVM instrinsic to be well defined on 
> all inputs, and be defined to return NaN on negative inputs.
> 
> Btw, I don't particularly care about errno or related, as my language is 
> not C.  I realise there is some kind of issue here to do with code 
> reordering and errno, but It would be a pity if these problems slowed 
> down sqrt code emission for all LLVM users.
> 
> What do people think?
> 

My suggestion is to implement sqrt() like this:

y = x >= 0.0f : llvm.sqrt(x) ? NaN;

If you are worried about performance on X86, you could have the frontend
emit the llvm.sse_sqrt_ps intrinsic for sqrt() or you could add
a pattern to the X86 backend to select this sequence to a SQRTPS instruction.

-Tom

> Thanks,
>      Nick
> 
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