RE: [PATCH] Canonicalize X&-Y as X*Y in match.pd when Y is [0,1].

["Roger Sayle" <roger@nextmovesoftware.com>]

 
> > On May 25, 2022, at 7:34 AM, Richard Biener via Gcc-patches <gcc-
> patches@gcc.gnu.org> wrote:
> >
> > On Tue, May 24, 2022 at 3:55 PM Roger Sayle
> <roger@nextmovesoftware.com> wrote:
> >>
> >>
> >> "For every pessimization, there's an equal and opposite optimization".
> >>
> >> In the review of my original patch for PR middle-end/98865, Richard
> >> Biener pointed out that match.pd shouldn't be transforming X*Y into
> >> X&-Y as the former is considered cheaper by tree-ssa's cost model
> >> (operator count).  A corollary of this is that we should instead be
> >> transforming X&-Y into the cheaper X*Y as a preferred canonical form
> >> (especially as RTL expansion now intelligently selects the
> >> appropriate implementation based on the target's costs).
> >>
> >> With this patch we now generate identical code for:
> >> int foo(int x, int y) { return -(x&1) & y; } int bar(int x, int y) {
> >> return (x&1) * y; }
> 
> What, if anything, does the target description have to do for "the
appropriate
> implementation" to be selected?  For example, if the target has an "AND
with
> complement" operation, it's probably cheaper than multiply and would be
the
> preferred generated code.

RTL expansion will use an AND and NEG instruction pair if that's cheaper
than the cost of a MULT or a synth_mult sequence.  Even, without the
backend providing an rtx_costs function, GCC will default to AND and NEG
having COSTS_N_INSNS(1), and MULT having COSTS_N_INSNS(4).
But consider the case where y is cloned/inlined/CSE'd to have the
value 2, in which (on many targets) case the LSHIFT is cheaper than
a AND and a NEG.

Alas, I don't believe a existence of ANDN, such as with BMI or SSE, has
any impact on the decision, as this is NEG;AND not NOT;AND.  If you
known of any target that has an "AND with negation" instruction, I'll
probably need to tweak RTL expansion to check for that explicitly.

The correct way to think about this canonicalization, is that the
default implementation of RTL expansion of a multiply by a 0/1
value is to use NEG/AND, and it's only in the extremely rare cases
where a multiply (or synth_mult sequence) is extremely cheap,
for example a single cycle multiply, where this will be used.

Roger
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