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Re: [PATCH] Add missing popcount simplifications (PR90693)
On Wed, Aug 21, 2019 at 5:53 PM Jeff Law <firstname.lastname@example.org> wrote:
> On 8/21/19 7:57 AM, Wilco Dijkstra wrote:
> > Hi Richard,
> >>> I think this should be in expand stage where there could be comparison
> >>> of the cost of the RTLs.
> >> I tend to agree here, if not then for the reason the "simplified" variants
> >> have more GIMPLE stmts which means they are not "simpler". In
> >> fact I'd argue for canonicalization we'd want to have the reverse
> >> "simplifications" on GIMPLE and expansion based on target cost.
> > So how would this work? Expand works on one statement at a time, but
> > we are dealing with more complex expressions here. When we get a
> > popcount (x) > 1 in expand_gimple_cond, the popcount has already been
> > expanded. And the code in builtins.c that emits popcount doesn't see or
> > consider the comparison, so it would be difficult to change it at that point.
> > None of the infrastructure in expand seems to be set up to do complex
> > pattern matches and replacements at expand time...
> > Costing would be difficult too since rtx_cost doesn't support builtins or
> > calls, so each backend would need to be modified to add costs for these.
> > So what is the best place to do pattern matches? I thought it was all
> > moving to match.pd.
> I believe the expanders have access to more than one statement via the
> use-def chains and TER's transformations.
Either that or as repeatedly suggested elsewhere more complex
"expand time instruction selection" can happen on GIMPLE right
before RTL expansion (pass_optimize_widening_mul is a pass
doing something like that). We probably want to have a more
formalized thing at some point as part of RTL expansion itself
to also get rid of TER.
The issue with using TER for this is that TER doesn't "handle"
internal FN calls I think (which is simply an oversight):
/* Increment counter if this is a non BUILT_IN call. We allow
replacement over BUILT_IN calls since many will expand to inline
insns instead of a true call. */
if (is_gimple_call (stmt)
&& !((fndecl = gimple_call_fndecl (stmt))
&& fndecl_built_in_p (fndecl)))