[llvm-commits] [cfe-commits] [PATCH] Add llvm.fmuladd intrinsic.

[Hal Finkel]

On Tue, 05 Jun 2012 20:12:00 -0700
John McCall <rjmccall at apple.com> 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 <scanon at apple.com>
> >> wrote: On Jun 5, 2012, at 2:45 PM, John McCall
> >> <rjmccall at apple.com> wrote:
> >> 
> >> > On Jun 5, 2012, at 2:15 PM, Stephen Canon wrote:
> >> >
> >> >> On Jun 5, 2012, at 1:51 PM, Chandler Carruth
> >> >> <chandlerc at google.com> 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.

Given the unique nature of this restriction, I think that the best way
to do this is to model it directly: add metadata, or some instruction
attribute, to each floating-point instruction indicating its
'contraction domain' (some module-unique integer will work). Only
instructions with the same contraction domain can be contracted.
Instructions without a contraction domain cannot be contracted. I
realize that this is verbose, but realistically, the only way to tell
LLVM what instructions are part of which C-language expression is to
tag each relevant instruction.

 -Hal

> 
> John.


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