[cfe-dev] [request] Improve debugging of ffast-math optimizations

[C Bergström via cfe-dev]

Typically this 1 by 1 process of elimination is automated and also
ideally done on a reduced test case. Maybe someone around here can
share their scripts.. (they must exist... Hal?) If you're starting
with stable code it's much easier - if you're doing active development
on the codebase as well - I'm empathetic..

On Sun, Apr 24, 2016 at 6:23 AM, Mehdi Amini via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> On Apr 22, 2016, at 2:48 AM, Gonzalo BG via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
>
> Finding which code is responsible for violating -ffast-math assumptions is a
> very tedious task that I do not wish to anybody.
>
>
> Are these "assumptions" documented anywhere?
>
> Thanks,
>
> --
> Mehdi
>
>
>
> Right now when a fast-math assumption is violated the only way I have to
> debug it is to disable fast math, and step-by-step try each of the more fine
> grained math optimizations until I find the minimal combination that
> reproduces the issue. This typically allows me to guess which kinds of
> operations I should be looking for, and start bisecting. Needles to say this
> is extremely time consuming. There must be a better way.
>
> I would like the behavior of all my programs to not change _much_ under the
> influence of fast math. The only way to achieve this is to manually perform
> the transformations that fast math is going to perform, like exploiting
> associativity, and obviously, never violate any of fast-math assumptions.
>
> It would be nice to have a warning that detects e.g. associativity
> transformations, and suggest how to rewrite them to minimize difference in
> the results between a program compiled with and without fast-math.
>
> There is a tool called Herbie [0] that is used by the rust-herbie-lint [1]
> (worht seeing in action) to improve the accuracy and stability of
> mathematical operations so maybe something similar could be done to detect
> some of the transformations that fast-math does and suggest them to the
> user.
>
> At run-time I would like some kind of fast-math sanitizer that catches all
> cases in which fast-math assumptions are violated (signaling nans, signed
> zeros, ...). Since the undefined-behavior sanitzer already covers e.g.
> division by zero, maybe a fast-math check would belong there as well.
>
> [0] http://herbie.uwplse.org/
> [1] https://github.com/mcarton/rust-herbie-lint
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