On Jun 5, 2012, at 9:24 PM, Hal Finkel wrote: > On Tue, 05 Jun 2012 20:12:00 -0700 John McCall <[email protected]> wrote: >> On Jun 5, 2012, at 3:35 PM, John McCall wrote: >>> On Jun 5, 2012, at 3:04 PM, Chandler Carruth wrote: >>>> On Tue, Jun 5, 2012 at 2:58 PM, Stephen Canon <[email protected]> >>>> wrote: On Jun 5, 2012, at 2:45 PM, John McCall >>>> <[email protected]> wrote: >>>> >>>>> On Jun 5, 2012, at 2:15 PM, Stephen Canon wrote: >>>>> >>>>>> On Jun 5, 2012, at 1:51 PM, Chandler Carruth >>>>>> <[email protected]> wrote: >>>>>> >>>>>>> That said, FP_CONTRACT doesn't apply to C++, and it's quite >>>>>>> unlikely to become a serious part of the standard given these >>>>>>> (among other) limitations. Curiously, in C++11, it may not be >>>>>>> needed to get the benefit of fused multiply-add: >>>>>> >>>>>> Perversely, a strict reading of C++11 seems (to me) to not >>>>>> allow FMA formation in C++ at all: >>>>>> >>>>>> • The values of the floating operands and the results of >>>>>> floating expressions may be represented in greater precision >>>>>> and range than that required by the type; the types are not >>>>>> changed thereby. >>>>>> >>>>>> FMA formation does not increase the precision or range of the >>>>>> result (it may or may not have smaller error, but it is not >>>>>> more precise), so this paragraph doesn't actually license FMA >>>>>> formation. I can't find anywhere else in the standard that >>>>>> could (though I am *far* less familiar with C++11 than C11, so >>>>>> I may not be looking in the right places). >>>>> >>>>> Correct me if I'm wrong, but I thought that an FMA could be >>>>> formalized as representing the result of the multiply with >>>>> greater precision than the operation's type actually provides, >>>>> and then using that as the operand of the addition. It's >>>>> understand that that can change the result of the addition in >>>>> ways that aren't just "more precise". Similarly, performing >>>>> 'float' operations using x87 long doubles can change the result >>>>> of the operation, but I'm pretty sure that the committees >>>>> explicitly had hardware limitations like that in mind when they >>>>> added this language. >>>> >>>> That's an interesting point. I'm inclined to agree with this >>>> interpretation (there are some minor details about whether or not >>>> 0*INF + NAN raises the invalid flag, but let's agree to ignore >>>> that). >>>> >>>> I'm not familiar enough with the language used in the C++ spec to >>>> know whether this makes C++ numerics equivalent to STDC >>>> FP_CONTRACT on, or equivalent to "allow greedy FMA formation". >>>> Anyone? >>>> >>>> If you agree w/ John's interpretation, and don't consider the flag >>>> case you mention, AFAICT, this allows greedy FMA formation, unless >>>> the intermediate values are round-tripped through a cast construct >>>> such as I described. >>> >>> I'm still not sure why you think this restriction *only* happens >>> when round-tripping through casts, rather than through any thing >>> which is not an operand or result, e.g. an object. >>> >>> Remember that the builtin operators are privileged in C++ — they >>> are not semantically like calls, even in the cases where they're >>> selected by overload resolution. >>> >>> I agree that my interpretation implies that a type which merely >>> wraps a double nonetheless forces stricter behavior. I also agree >>> that this sucks. >> >> To continue this thought, the most straightforward way to represent >> this in IR would be to (1) add a "contractable" bit to the LLVM >> operation (possibly as metadata) and (2) provide an explicit "value >> barrier" instruction (a unary operator preventing contraction >> "across" it). We would introduce the barrier in the appropriate >> circumstances, i.e. an explicit cast, a load from a variable, or >> whatever else we conclude requires these semantics. It would then be >> straightforward to produce FMAs from this, as well as just generally >> avoiding rounding when the doing sequences of illegal FP ops. >> -ffast-math would imply never inserting the barriers. >> >> The disadvantages I see are: >> - there might be lots of peepholes and isel patterns that would >> need to be taught to to look through a value barrier >> - the polarity of barriers is wrong, because code that lacks >> barriers is implicitly opting in to things, so e.g. LTO could pick a >> weak_odr function from an old tunit that lacks a barrier which a >> fresh compile would insist on. > > I don't like the barrier approach because it implies that the FE must > serialize each C expression as a distinct group of LLVM instructions. > While it may be true that this currently happens in practice, I don't > think we want to force it to be this way.
I think you misunderstand. By a "barrier", I mean an instruction like this: %1 = call float @llvm.fp_contract_barrier.float(float %0) readnone nounwind which states that %1 must be a representable float value and therefore blocks FP contraction "across" the intrinsic, in the sense that something using %1 can't be fused with the operation producing %0. I do not mean something like a memory barrier that divides things based on whether the instruction comes before or after the barrier; that's clearly not workable. John. _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
