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Re: [PATCH v2] Simplify pow with constant
On Thu, Aug 17, 2017 at 5:43 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
> On Thu, 17 Aug 2017, Wilco Dijkstra wrote:
>
>> This patch simplifies pow (C, x) into exp (x * C1) if C > 0, C1 = log (C).
>
> Note this changes the outcome for C == +Inf, x == 0 (pow is specified to
> return 1.0 in that case, but x * C1 == NaN). There's another existing
> transform with the same issue, 'pow(expN(x), y) -> expN(x*y)', so this is
> not a new problem.
>
> The whole set of these match.pd transforms is guarded by
> flag_unsafe_math_optimizations, which is a bit strange, on the one hand
> it does not include -ffinite-math-only, but on the other hand it's
> defined broadly enough to imply that.
No, flag_unsafe_math_optimization doesn't imply -ffinite-math-only.
-funsafe-math-optimization these days should guard transforms that
affect the value of the result (but not its classification). There are
more specific variants like -fassociative-math which also affects
results but in a more specific way.
> (to be clear, I'm not objecting to the patch, just pointing out an
> existing inconsistency in case someone can offer clarifications)
We shouldn't add such issue knowingly, I guess for this case it's easy
to check that C is not +Inf.
For the existing transforms with the same issue guarding with
-ffinite-math-only is an appropriate fix.
Richard.
> Alexander