On Fri, Aug 18, 2017 at 2:47 PM, Wilco Dijkstra <wilco.dijks...@arm.com> wrote: > Alexander Monakov wrote: >> >> 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. > > Yes I was assuming that unsafe_math_optimizations was enough for these > transformations to be... safe. I've added an isfinite check so that case > works fine. > It looks we need to go through the more complex transformations (especially > given pow has so many special cases) and add more finite_math checks. > Here's the new version: > > > This patch simplifies pow (C, x) into exp (x * C1) if C > 0, C1 = log (C). > Do this only for fast-math as accuracy is reduced. This is much faster > since pow is more complex than exp - with current GLIBC the speedup is > more than 7 times for this transformation. > > The worst-case ULP error of the transformation for powf (10.0, x) in SPEC > was 2.5. If we allow use of exp10 in match.pd, the ULP error would be lower.
You can use exp10/pow10 in match.pd but it will only match if the programmer already uses those functions as they are not C standard. There's also exp2 which is a C99 function. Is there any good reason to convert exp (C * x) to exp2 (C' * x)? (besides if C' becomes 1) So eventually we can match pow (10., x) -> exp10 and pow (2., x) -> exp2 (though I believe in canonicalization as well -- all exp calls could be canonicalized to pow calls to simplify the cases we need to handle...). The patch is ok. Thanks, Richard. > ChangeLog: > 2017-08-18 Wilco Dijkstra <wdijk...@arm.com> > > * match.pd: Add pow (C, x) simplification. > -- > diff --git a/gcc/match.pd b/gcc/match.pd > index > 0e36f46b914bc63c257cef47152ab1aa507963e5..a5552c5096de5100a882d52add6b620ba87c1f72 > 100644 > --- a/gcc/match.pd > +++ b/gcc/match.pd > @@ -3622,6 +3622,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) > (logs (pows @0 @1)) > (mult @1 (logs @0)))) > > + /* pow(C,x) -> exp(log(C)*x) if C > 0. */ > + (for pows (POW) > + exps (EXP) > + logs (LOG) > + (simplify > + (pows REAL_CST@0 @1) > + (if (real_compare (GT_EXPR, TREE_REAL_CST_PTR (@0), &dconst0) > + && real_isfinite (TREE_REAL_CST_PTR (@0))) > + (exps (mult (logs @0) @1))))) > + > (for sqrts (SQRT) > cbrts (CBRT) > pows (POW) > >
