[LLVMdev] Representing -ffast-math at the IR level

[Harris, Kevin]

Duncan,
	Thanks for the thoughtful response.  Some follow up:

> -----Original Message-----
> From: Duncan Sands [mailto:baldrick at free.fr] 
> Sent: Tuesday, April 17, 2012 11:53 AM
> To: Harris, Kevin
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Representing -ffast-math at the IR level
>
> Hi Kevin,
>
>>  1. Most compiler and back-end control of floating point behavior appears to be
>>     motivated by controlling the loss or gain of a few low bits of precision on
>>     a whole module scale. In fact, these concerns are usually insignificant for
>>     programmers of floating-point intensive applications. The input to most
>>     floating point computations have far lower significance than the
>>     computations themselves, and therefore they have precision to burn. So the
>>     vast majority of such app developers would happily trade precision for
>>     performance, even as the default behavior. However, the place where trouble
>>     DOES occur is with overflow and underflow behavior at critical points.
>>     Changing the order of operations, or combining operations, can cause
>>     overflows or underflows to occur that wouldn't otherwise occur, and vice
>>     versa.
>
> for the moment I'm distinguishing (mentally) between transformations that
> introduce a uniformly bounded relative error, for example x+0 -> x, or
> x/constant -> x * (1/constant) if constant and 1/constant are normal (and
> not denormal), and those that can introduce an unbounded relative error.
> Reassociation is an example of a transformation that can introduce unbounded
> relative error, for example (1 + epsilon) - 1 -> 0 if epsilon is small enough,
> while (1 - 1) + epsilon -> epsilon.  I'm basically assuming that everyone is
> happy with the transforms that introduce a bounded relative error - it sounds
> to me like this is the distinction that you are making too.  Transforms that
> introduce unbounded relative error (like reassocation) are a can of worms, and
> I'm not sure how best to handle them.  So for the moment I'm not planning to
> handle them, just gather ideas and discuss.

This is a reasonable distinction.  How you could enforce it across the various
optimization passes is not obvious.  Loss of precision problems are difficult
to diagnose even when strong fp correctness goals and methods are in place.

>>  Sometimes this is beneficial, but it is almost always unexpected.
>>     Underflows may sound less important in this regard, but they can be worse
>>     than overflows, because they can mostly or completely eliminate the
>>     significant bits, in complete silence, leaving the entire computation
>>     worthless. Much of numerical analysis, especially in writing floating point
>>     library functions, concerns the precise control of overflow and loss of
>>     significance in specific operations. To the extent that optimizations which
>>     make such control difficult or impossible, can render the use of a compiler
>>     or backend unusable for that purpose.
>>  2. While the use of metadata for control of LLVM behavior is attractive for its
>>     simplicity and power, the philosophy that it can be safely ignored or even
>>     removed in some optimization passes would seem to doom its effectiveness for
>>     controlling floating point optimizations. For anyone trying to use source
>>     language and compiler option mechanisms to control for fp overflow and
>>     underflow, this approach would seem ill conceived.
>
> I think there may be a misunderstanding here.  True, the design of metadata is
> that it is not wrong to drop it.  However the compiler isn't trying to drop it,
> it tries hard not to drop it: any cases of pointlessly dropped metadata are a
> bug.  In this fpmath metadata is analogous to tbaa (type based alias analysis
> metadata): if it is dropped you get conservatively correct results, but some
> optimizations are missed.  Compiler writers don't like missing optimizations!
> If you see any cases of fpmath metadata being dropped then please report it.

Yes, I (and others, obviously) have been confused before about the extent to
which metadata can be ignored or dropped.  I think its use for providing
additional information that allows optimizations that would otherwise be
invalid is well motivated and reasonably straightforward.  And your proposal
doesn't change that usage.  Any attempt to provide tighter restrictions for fp
optimizations, however, would seem to muddy the situation, since it
would violate the basic assumption that the undecorated IR is the "most 
conservative".

>>  For the purpose of
>>     providing a Front-End developer with a powerful platform for supporting
>>     fp-intensive programming,
>
> Let me just say up front that it is not clear to me that this is a goal of LLVM.

I realize that good fp precision and control is a fairly specialized niche, esp. 
for an open source compiler.  This is the main reason why I hesitated to comment
initially.  I didn't even necessarily mean to inject any additional goals in this
space.  But since you had made an effort in this direction, and generated some 
thoughtful discussion already, I wanted to alert the community to some practical 
issues they might not have considered.

>>  the primary requirement is that the Front-end
>>     should be able to precisely control optimizations that can change the fp
>>     intermediate results under all optimization levels for each individual fp
>>     operation specified in the IR. The vast majority of such usage can and
>>     should chosen to default to high performance behavior. But it should be
>>     possible for the front-end to precisely control IR re-ordering, operation
>>     combining (including exploitation of mul-add hardware support), and
>>     reactions to overflow and underflow conditions (using the exception handling
>>     conditions and underlying the hardware support). By providing this power in
>>     the IR, it allows a Front-end developer to reliably support source language
>>     mechanisms (e.g. use of parentheses) and front-end recognized compiler
>>     options (e.g. for fp exception handling) to respond to the needs of the
>>     source language programmer for fp-intensive applications.
>
> Given that LLVM doesn't even properly support rounding modes, I think you are
> going to have to wait a few years at least before we are anywhere near something
> like this.  That said, we'd get there sooner (assuming we actually want to go
> there) if you help - patches welcome!

Point taken.  I definitely have fantasies in this area, but won't likely have
extra cycles to devote to this area in the near future.  :-(  

Regarding rounding modes specifically, in spite of the hardware support for these, 
I think they are an even more specialized area than controlling for overflow / 
underflow.  They are almost never useful outside the context of fp library function 
authorship, and there are several commercial compilers that support library
development adequately.

>> It should be possible to define one or more attribute flags for FP operations in
>> the IR with semantics that guarantee allowance or suppression of optimizations
>> that might create or eliminate overflow, underflow, or significant precision
>> loss. The implementation of such semantics in the existing optimization passes
>> might take a fair amount of work, I admit. But that is exactly what Front-End
>> developers and their source language programmers would most benefit from.
>
> I'm pretty sure that building lots of flags into floating point operations is
> not going to fly at this stage.  Metadata allows us to grow lots of flags if we
> want without much impact on the compiler.  Once the metadata approach has
> matured and shown its usefulness or limitations then we can consider baking
> things into the IR or other such approaches.  But that's a long way off.

The role of metadata as a prototyping vehicle is clear, and may indeed continue
to be useful in this space.  Clarifying the role of metadata in cases where it
would restrict optimizations rather than permit them would seem to be a step in
the right direction.
	-Kevin