[LLVMdev] LLVMdev Digest, Vol 77, Issue 41

[Jonas Paulsson]

> On Thu, 25 Nov 2010 11:06:48 +0100
> Duncan Sands <baldrick at free.fr> wrote:
> 
> > Hi Jonas,
> > 
> > > I am investigating the possibilities of incorporating fixed point
> > > support into the LLVM I/R.
> > 
> > I think you should write a rationale explaining why you want to
> > introduce new types etc rather than using the existing integer types,
> > with intrinsic functions for the operations, or some other such
> > scheme.  Introducing new types is hard work and creates a maintenance
> > burden for everyone, since they will need to be properly supported by
> > all parts of the compiler forever more.  It is therefore important to
> > give a cogent explanation of why this is the best approach, why the
> > benefits outweigh the costs, and so forth.
> 
> Also can't fixed point be handled entirely by the frontend?
> You store the scaling factor as an attribute on the type.
> 
> When you perform operations that involve the same fixed point types you
> can perform them with integers, and when you need to perform
> conversions you emit the appropriate code to perform the conversions.
> The emitted LLVM IR needs to know nothing about the scaling factors
> involved.
> For saturation, etc. you can use the SSE intrinsics.
> 
> Best regards,
> --Edwin

Hi again,

thanks for the answers.

Well, the reason DSP compilers would benefit from the new type, is that fixed point numbers must not be optimized as integers - eg if two saturated fixed point constants would overflow in an addition operation, the result should as well be saturated. Doing this in a series of steps with intrinsics would be quite ridiculous as far as performance goes, because this is _supported by the DSP in hardware_ in a single instruction :-) 

What about perhaps introducing a new type, and allowing just minor extensions, while barring it from more heavy scattered code? I mean, the GVN pass works by just adding an enum entry and a switch case for each new instruction. But in other cases, there are more LOCs involved in lifting in new instructions. Then, these passes would be left untouched, for the sake of maintainability. These passes could be copied for fixed point versions. This would mean a just a little slower compiler (dual passes in some cases), for the case of a separation of the code that not everywone is interested in. Then, these passes could be run just by the DSP targets that need them, and the integer developers would not have to worry at all (perhaps by explicitly excluding fixed point instructions here and there, though). 

Another idea is to introduce a scheme of intrinsics and perhaps handle optimizations in the backend as much as possible, as a first step. If this works well, and interest grows, maybe then the rest will follow?

My point is that there does not seem to be a great open source DSP compiler available, yet there is certainly some interest. GCC has passes that can handle DSP targets, but it is still not a DSP compiler as such, really. I think you have a chance to fill a need here which would invite even more people to work on the project. With fixed point support, a new DSPTargetMachine could be made which could really make a difference in the DSP domain as a _DSP compiler_! I have made extensions already - very simple - that seems to work great for VLIW scheduling and RA. It would be a pity not to give all this serious thought, I'd say.

regards,

Jonas



 
 		 	   		  
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