On Mon, 20 Nov 2023, Jakub Jelinek wrote: > On Mon, Nov 20, 2023 at 07:54:54AM +0000, Richard Biener wrote: > > On Fri, 17 Nov 2023, Jakub Jelinek wrote: > > > Per the earlier discussions on this PR, the following patch folds > > > popcount (x) == 1 (and != 1) into (x ^ (x - 1)) > x - 1 (or <=) > > > if the corresponding popcount optab isn't implemented (I think any > > > double-word popcount or call will be necessarily slower than the > > > above cheap 3 op check and even for -Os larger or same size). > > > > > > I've noticed e.g. C++ aligned new starts with std::has_single_bit > > > which does popcount (x) == 1. > > > > > > As a follow-up, I'm considering changing in this routine the popcount > > > call to IFN_POPCOUNT with 2 arguments and during expansion test costs. > > > > > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? > > > > Classically this would have been an RTL expansion alternative, given > > we want to do less of those the next place would have been the ISEL > > pass. Any particular reason you chose widening-mul for this (guess > > that pass just has a bad name and it's the effective "optimize" ISEL pass > > we have). > > I think the ssa-math-opts pass does far more of this staff than the isel > pass which only deals with vector stuff right now and you've even mentioned > that pass for that in the PR90693 thread. > That said, I can move it into the isel pass as well if you prefer that.
I think it's fine as you posted (and Jeff approved), I'm just wondering if we should rename that pass somehow ;) Note ISEL is more for required pre-expansion stuff and widen-mul is for expansion related but optimization parts. Richard.
