On Tue, Sep 24, 2019 at 1:11 PM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Martin Liška <mli...@suse.cz> writes: > > Hi. > > > > The patch introduces couple of new TREE_CODEs that will help us to have > > a proper GIMPLE representation of current VECT_COND_EXPR. Right now, > > the first argument is typically a GENERIC tcc_expression tree with 2 > > operands > > that are visited at various places in GIMPLE code. That said, based on the > > discussion > > with Richi, I'm suggesting to come up with e.g. > > VECT_COND_LT_EXPR<COND_LHS, COND_RHS, IF_CLAUSE, ELSE_CLAUSE>. Such a > > change logically > > introduces new GIMPLE_QUATERNARY_RHS gassignments. For now, the > > VEC_COND_EXPR remains > > and is only valid in GENERIC and gimplifier will take care of the > > corresponding transition. > > > > The patch is a prototype and missing bits are: > > - folding support addition for GIMPLE_QUATERNARY_RHS is missing > > - fancy tcc_comparison expressions like LTGT_EXPR, UNORDERED_EXPR, > > ORDERED_EXPR, > > UNLT_EXPR and others are not supported right now > > - comments are missing for various functions added > > > > Apart from that I was able to bootstrap and run tests with a quite small > > fallout. > > Thoughts? > > Martin > > I think this is going in the wrong direction. There are some targets > that can only handle VEC_COND_EXPRs well if we know the associated > condition, and others where a compare-and-VEC_COND_EXPR will always be > two operations. In that situation, it seems like the native gimple > representation should be the simpler representation rather than the > more complex one. That way the comparisons can be optimised > independently of any VEC_COND_EXPRs on targets that benefit from that. > > So IMO it would be better to use three-operand VEC_COND_EXPRs with > no embedded conditions as the preferred gimple representation and > have internal functions for the fused operations that some targets > prefer. This means that using fused operations is "just" an instruction > selection decision rather than hard-coded throughout gimple. (And that > fits in well with the idea of doing more instruction selection in gimple.)
So I've been doing that before, but more generally also for COND_EXPR. We cannot rely on TER and the existing RTL expansion "magic" for the instruction selection issue you mention because TER isn't reliable. With IFNs for optabs we could do actual [vector] condition instruction selection before RTL expansion, ignoring "single-use" issues - is that what you are hinting at? How should the vectorizer deal with this? Should it directly use the optab IFNs then when facing "split" COND_EXPRs? IIRC the most fallout of a simple patch (adjusting is_gimple_condexpr) is in the vectorizer. Note I'm specifically looking for a solution that applies to both COND_EXPR and VEC_COND_EXPR since both suffer from the same issues. There was also recent work in putting back possibly trapping comparisons into [VEC_]COND_EXPR because it doesn't interfere with EH and allows better code. Also you SVE people had VN issues with cond-exprs and VN runs into the exact same issue (but would handle separate comparisons better - with the caveat of breaking TER). Richard. > > Thanks, > Richard
