On Jun 21, 2012, at 3:27 AM, Chandler Carruth <[email protected]> wrote:
> The flag semantics '-fexcess-precision' (as spec'ed for GCC) don't seem to
> map clearly onto this use case, especially as John argued so effectively in
> the previous email thread. I feel like this flag should remain very
> specifically attached to FP operations with more bits, but none of the
> asymmetrical properties of FMAs, so that there is no confusion between
> Clang's semantics and GCC's.
A minor point: GCC "excess precision" introduces exactly the same asymmetries
as does FMA in the presence of spill-fill or common subexpression elimination.
Consider an expression like a*b - c*d with a = c and b = d. If computed
entirely in excess precision, there is no asymmetry. However, if a*b is either
eliminated or computed earlier and spilled in native precision, then it is
rounded to native precision before the subtraction, whereas the product c*d may
not be; this is exactly the same asymmetry that occurs from using FMA.
That said, I don't love overloading excess-precision for this, even if fma is
formally a special case of excess precision, mostly because excess precision
has even more bizarre corner cases -- it leads to such monstrosities as cos(x)
!= cos(x).
> I'd suggest a new flag without any baggage, and one that perhaps ties more
> closely into the idea of contracted expressions, or some other concept
> intrinsic to fusing operations. Maybe
> '-fcontract-expressions={fast,standard,strict}'? Dunno. Naming things is hard.
Relating the flag to contracted expressions is probably wrong; contracted
expressions has a specific meaning in the C standard, which is not the same as
liberal (cross-expression) FMA formation. As you say, naming things is hard.
If we're not going to use excess-precision, I would suggest a name that is
completely explicit; something along the lines of -fallow-fma-anywhere or
-fallow-fma-across-expressions (those might be too wordy).
- Steve
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