[LLVMdev] Clarifying FMA-related TargetOptions

[Hal Finkel]

On Wed, 2012-02-08 at 10:46 -0800, Owen Anderson wrote:
> On Feb 8, 2012, at 10:42 AM, Hal Finkel wrote:
> > In my experience, users of numerical codes expect that the compiler will
> > use FMA instructions where it can, unless specifically asked to avoid
> > doing so by the user. Even though this can sometimes produce a different
> > result (*almost* always a better one), the performance gain is too large
> > to be ignored by default. I highly recommend that we continue to enable
> > FMA instruction-generation by default (as is the current practice, not
> > only here, but in most vendor compilers with which I am familiar). We
> > should also implement the FP_CONTRACT pragma, but that is another
> > matter.
> 
> The caveat I would add to this is that, when I tried enabling FMA-by-default on an ARM target, I saw a large number of testcases in the LLVM test suite that either failed their output comparisons, crashed, or failed to terminate (!!!).  That seems pretty scary to me.
> 

That is quite scary. I could obviously be wrong, but I would suspect
that something else is going on here. Either there is a bug in the FMA
patch, or the patch is triggering a bug elsewhere. Although I've
certainly seen cases where enabling FMA changes some numerical results
(slightly), crashing and failing to terminate in a large number of cases
seems like another issue. Most code just does not depend on exact
floating-point cancelation in a way that impacts loop termination
conditions, memory indexing, etc.

 -Hal

> --Owen

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory