[LLVMdev] Clarifying FMA-related TargetOptions

[Owen Anderson]

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.

--Owen