[PATCH] match.pd: undistribute (a << s) & C, when C = (M << s) and exact_log2(M - 1)

[Philipp Tomsich]

On Wed, 11 Nov 2020 at 19:17, Jeff Law <law@redhat.com> wrote:
>
>
> On 11/11/20 3:55 AM, Jakub Jelinek via Gcc-patches wrote:
> > On Wed, Nov 11, 2020 at 11:43:34AM +0100, Philipp Tomsich wrote:
> >> The patch addresses this by disallowing that rule, if an exact power-of-2 is
> >> seen as C1.  The reason why I would prefer to have this canonicalised the
> >> same way the (X & C1) * C2 is canonicalised, is that cleaning this up during
> >> combine is more difficult on some architectures that require multiple insns
> >> to represent the shifted constant (i.e. C1 << C2).
> > It is bad to have many exceptions for the canonicalization
> > and it is unclear why exactly these were chosen, and it doesn't really deal
> > with say:
> > (x & 0xabcdef12ULL) << 13
> > being less expensive on some targets than
> > (x << 13) & (0xabcdef12ULL << 13).
> > (x & 0x7ffff) << 3 vs. (x << 3) & 0x3ffff8 on the other side is a wash on
> > many targets.
> > As I said, it is better to decide which one is better before or during
> > expansion based on target costs, sure, combine can't catch everything.
>
> I think Jakub is hitting a key point here.  Gimple should canonicalize
> on what is simpler from a gimple standpoint, not what is better for some
> set of targets.   Target dependencies like this shouldn't be introduced
> until expansion time.

The simplification that distributes the shift (i.e. the one that Jakub referred
to as fighting the new rule) is also run after GIMPLE has been expanded to
RTX.  In my understanding, this still implies that even if we have a cost-aware
expansion, this existing rule will nonetheless distribute the shift.

Philipp.