[LLVMdev] Fw: LLVMdev Digest, Vol 77, Issue 41

[Török Edwin]

You probably meant to send this to LLVMdev as well.

Begin forwarded message:

Date: Mon, 29 Nov 2010 08:26:03 +0100
From: Jonas Paulsson <jnspaulsson at hotmail.com>
To: <edwintorok at gmail.com>
Subject: RE: [LLVMdev] LLVMdev Digest, Vol 77, Issue 41



Yes, the new type is simply a static object managed by Type and
LLVMContext. This is only referred to by Values of fixed point type.
New instructions should not interfer with existing passes, as opcodes
are switched and handled and would simply continue to be unaware of the
new opcodes.

My main concern is that code evolution demands following of the LLVM
release series of the future. So, even if this got done locally, it
would be a hassle to deal with if future releases did not include it at
all. So, even if only a small part of it got committed, like say all
parts that do not concern or disturb the optimizers - like the type and
instructions themselves, this would be better then nothing at all. If
then some other party wanted to contribute, this would not lead to
enormous merge aches.

Also, the DSPTargetMachine class would not interfer at all with the
LLVMTargetMachine. Personally, I think LLVM could become a leading DSP
choice with these two features, but that's just me, or? :-)

regards,

/Jonas

> Date: Fri, 26 Nov 2010 18:08:53 +0200
> From: edwintorok at gmail.com
> To: jnspaulsson at hotmail.com
> CC: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] LLVMdev Digest, Vol 77, Issue 41
> 
> On Fri, 26 Nov 2010 16:32:42 +0100
> Jonas Paulsson <jnspaulsson at hotmail.com> wrote:
> 
> > 
> > 
> > 
> > > 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). 
> 
> Ah now it makes sense: you want the existing optimizations (GVN,
> instcombine, reassociation, etc.) applied to arithmetic ops on your
> type.
> 
> > 
> > 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?
> 
> Yes maybe introducing your own intrinsics as suggested by Duncan
> is best. You can lower the intrinsic to your hardware instruction
> directly. Optimizers won't know what your intrinsic is, so they would
> mostly skip them.
> 
> > 
> > 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.
> 
> Will existing passes keep working after you introduce this new type
> (assuming they are run on a module which doesn't make use of the new
> type)?
> 
> Best regards,
> --Edwin
 		 	   		  
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