[llvm-dev] Trouble when suppressing a portion of fast-math-transformations

[Sanjay Patel via llvm-dev]

Hi Warren -

IIUC, we need to audit/fix the current transforms that use FMF to respond
to the individual flags, and those fixes can occur in parallel. They might
cosmetically conflict with the redefining/replumbing work on the flags
struct, but that shouldn't be too hard to fix?

Let me know what I can do to help without interfering with any patches you
might have in progress. Also, if you'll be at the conference next week, we
could find some time for interested parties to meet to discuss/hack this
more if needed.

On Wed, Oct 4, 2017 at 3:32 AM, Ristow, Warren via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> > It might be clearer, instead of using 'libm', to use something like
> 'trans' (for transcendental functions).
>
>
>
> That does seem clearer.  ‘trans’ is definitely good with me.
>
>
>
> -Warren
>
>
>
> *From:* Hal Finkel [mailto:hfinkel at anl.gov]
> *Sent:* Tuesday, October 3, 2017 5:13 PM
> *To:* Ristow, Warren; Bruce Hoult
> *Cc:* llvm-dev at lists.llvm.org
>
> *Subject:* Re: [llvm-dev] Trouble when suppressing a portion of
> fast-math-transformations
>
>
>
>
>
> On 10/03/2017 05:02 AM, Ristow, Warren wrote:
>
> >>> I'd like to emphasise in the latter one: "This option also relaxes the
> precision of
>
> >>> commonly used math functions."
>
> >>
>
> >> Isn't this the "libm" flag that is proposed in this thread?
>
> >
>
> > I don't know. I didn't see any definition of it.
>
> >
>
> > In my case I'm talking about allowing the use of lower precision but
> very fast machine
>
> > instructions instead of a slow sequence of inline instructions. But I
> guess instruction
>
> > vs library is equivalent.
>
>
>
> I haven't defined "libm" explicitly.  The concept of "reassoc" and "libm"
> are a result of the discussion last November.  The "libm" aspect in
> particular, came from:
>
>
>
> http://lists.llvm.org/pipermail/llvm-dev/2016-November/107114.html
>
>
>
> It was intended to mean actual library functions, which is what I thought
> you were referring to when you said "This option also relaxes the precision
> of commonly used math functions."  From your elaboration describing it as a
> sequence of inline instructions, I think whether "libm" is the right flag
> to control it would depend on what that sequence of instructions were
> doing.  If they were implementing 'pow()' or 'sqrt()' or 'sin()' etc., then
> yes, I think "libm" would be the right flag.  If something else (unrelated
> to functions generally in libm), then probably some other flag (or set of
> flags) would control whether a transformation was done.
>
>
>
> More generally, my intended approach of doing this change (of removing the
> "fast" umbrella flag, and adding the two new flags "reassoc" and "libm"),
> is to audit all the places that currently enable an optimization based on
> whether ‘hasUnsafeAlgebra()’ is true (which essentially is asking whether
> _all_ the existing FastMathFlags are enabled), and see which of them
> can/should be controlled by one (or a subset of) the full set.  So it's
> possible that a particular slow sequence of inline instructions you are
> transforming would be controlled by "libm", and a different slow sequence
> of inline instructions would be controlled by some other flag (or flags).
>
>
> I agree. We should give the flags semantic meanings. Whether or not
> something is generally a function call is irrelevant. libm, if we use that
> name, refers to the functions in libm, and perhaps close extensions,
> regardless of how the target generally generates code for them.
>
> It might be clearer, instead of using 'libm', to use something like
> 'trans' (for transcendental functions).
>
>  -Hal
>
>
>
>
> -Warren
>
>
>
> *From:* bruce.hoult at gmail.com [mailto:bruce.hoult at gmail.com
> <bruce.hoult at gmail.com>] *On Behalf Of *Bruce Hoult
> *Sent:* Tuesday, October 3, 2017 10:05 AM
> *To:* Hal Finkel
> *Cc:* Ristow, Warren; llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Trouble when suppressing a portion of
> fast-math-transformations
>
>
>
> On Tue, Oct 3, 2017 at 4:03 AM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> On 10/02/2017 11:10 AM, Bruce Hoult via llvm-dev wrote:
>
> -cl-fast-relaxed-math
>
> Sets the optimization options -cl-finite-math-only and
> -cl-unsafe-math-optimizations. This allows optimizations for floating-point
> arithmetic that may violate the IEEE 754 standard and the OpenCL numerical
> compliance requirements for single precision and double precision
> floating-point, as well as floating point edge case behavior. This option
> also relaxes the precision of commonly used math functions. This option
> causes the preprocessor macro __FAST_RELAXED_MATH__ to be defined in the
> OpenCL program. The original and modified values are defined in the SPIR-V
> OpenCL environment specification
>
>
>
> I'd like to emphasise in the latter one: "This option also relaxes the
> precision of commonly used math functions."
>
> Isn't this the "libm" flag that is proposed in this thread?
>
>
>
> I don't know. I didn't see any definition of it.
>
>
>
> In my case I'm talking about allowing the use of lower precision but very
> fast machine instructions instead of a slow sequence of inline
> instructions. But I guess instruction vs library is equivalent.
>
>
>
>
>
> --
>
> Hal Finkel
>
> Lead, Compiler Technology and Programming Languages
>
> Leadership Computing Facility
>
> Argonne National Laboratory
>
>
> _______________________________________________
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>
>
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