On Mon, Jul 20, 2020 at 3:06 PM Roger Sayle <ro...@nextmovesoftware.com> wrote: > > > This patch complements one from June 12th which is still awaiting > review: https://gcc.gnu.org/pipermail/gcc-patches/2020-June/547937.html > > This patch optimizes popcount and parity of an argument known to have > at most a single bit set, to be that single bit. Hence, popcount(x&8) > is simplified to (x>>3)&1. This generalizes the existing optimization > of popcount(x&1) being simplified to x&1, which is moved with this > patch to avoid a duplicate pattern warning in match.pd. > > This patch has been tested on x86_64-pc-linux-gnu with a "make bootstrap" > and "make -k check" with no new failures. If this is approved after > (or at the same time) as the patch above, I'm happy to resolve the > conflicts and retest before committing.
Given you know the constant bit position of the possibly nonzero bit you can elide the conversion to unsigned for all but the case of a possibly negative input (IIRC GCC doesn't yet take advantage of negative right shift undefinedness - but maybe sanitizers complain). Also the shift amount doesn't need to be in the same type as the shifted amount so using either size_int() or integer_type_node for that argument should reduce INTEGER_CST waste. Any reason you are not tackling IFN_POPCOUNT/PARITY? You could use (for pfun (POPCOUNT PARITY) ... and automagically get all builtins and the IFN. Thanks, Richard. > > 2020-07-20 Roger Sayle <ro...@nextmovesoftware.com> > > gcc/ChangeLog > * match.pd (popcount(x) -> x>>C): New simplification. > > gcc/testsuite > * gcc.dg/fold-popcount-5.c: New test. > * gcc.dg/fold-parity-5.c: Likewise. > > > Ok for mainline? > Thanks in advance, > Roger > -- > Roger Sayle > NextMove Software > Cambridge, UK >
