[cfe-dev] [LLVMdev] Proposal: floating point accuracy metadata (OpenCL related)

[Villmow, Micah]

Peter,
 Is there a way to make this flag globally available? Metadata can be fairly expensive to handle at each node when in many cases it is a global flag and not a per operation flag.

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Robert Quill
> Sent: Thursday, September 08, 2011 3:24 AM
> To: Peter Collingbourne
> Cc: anton.lokhmotov at arm.com; cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] [cfe-dev] Proposal: floating point accuracy
> metadata (OpenCL related)
> 
> Hi Peter,
> 
> This sounds like I really good idea. One thing that did occur to me
> though from an OpenCL point of view is that ULP accuracy requirements
> can differ for embedded and full profile so that may need to be handled
> somehow.
> 
> Thanks,
> Rob
> 
> On Wed, 2011-09-07 at 21:55 +0100, Peter Collingbourne wrote:
> > Hi,
> >
> > This is my proposal to add floating point accuracy support to LLVM.
> > The intention is that the frontend may provide metadata to signal to
> > the backend that it may select a less accurate (i.e. more efficient)
> > instruction to perform a given operation.  This is primarily a
> > requirement of OpenCL, which specifies that certain floating point
> > operations may be computed inaccurately.
> >
> > Comments appreciated.
> >
> > ---------------------------------------------------------------------
> ---
> >
> > Specification
> > -------------
> >
> > The metadata attribute is named "fpaccuracy", and contains a single
> > integer parameter which specifies the maximum relative error of the
> > operation to which it is attached, in ULPs.  For the definition of
> > ULPs we follow the definition given in the OpenCL 1.1 specification
> > (section 7.4):
> >
> > 	If x is a real number that lies between two finite
> > 	consecutive floating-point numbers a and b, without being
> > 	equal to one of them, then ulp(x) = |b - a|, otherwise
> > 	ulp(x) is the distance between the two non-equal finite
> > 	floating-point numbers nearest x. Moreover, ulp(NaN) is
> > 	NaN.
> >
> > Implementation
> > --------------
> >
> > As a start, I am attaching a Clang patch which adds this metadata
> > to single precision floating point division instructions in OpenCL
> > (which are accurate to 2.5ulp, per OpenCL 1.1 s7.4).  I would imagine
> > that in the future we may want to allow the user to control the level
> > of accuracy (one idea is that we can provide a __builtin_fpaccuracy
> > function, which could be used like this):
> >
> > __builtin_fpaccuracy(x/y, 2) // compute x/y with a maximum error of
> 2ulp
> >
> > The backend is not my area of expertise, so I won't venture to say
> > how difficult it would be to recognise and act on the attribute
> there.
> >
> > Alternatives
> > ------------
> >
> > I am proposing metadata here, since it may be removed without
> > negatively affecting accuracy.
> >
> > We could add fpaccuracy as a core LLVM attribute (a la nsw and nuw).
> > But this would be relatively intrusive (only a few clients care about
> > accuracy) and it would need to be added to a variety of unrelated
> > instruction types (most floating point operations, as well as
> > intrinsic calls).
> >
> > We could also introduce a set of intrinsics for inaccurate FP
> > operations.  The main disadvantage is that we would need to add an
> > intrinsic for each FP operation, which could add up to a lot of work.
> > Furthermore, the new intrinsics would not necessarily be recognised
> > by the existing optimisers.
> >
> > ---------------------------------------------------------------------
> ---
> >
> > Thanks,
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> 
> 
> 
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