[PATCH] D116692: [SimplifyCFG] Tail-merging all blocks with `unreachable` terminator, final take

Thu Jan 6 14:50:33 PST 2022

lebedev.ri added inline comments.

================
Comment at: llvm/test/Transforms/PhaseOrdering/AArch64/peel-multiple-unreachable-exits-for-vectorization.ll:39
-; CHECK-NEXT:    br i1 [[MIN_ITERS_CHECK]], label [[LOOP_PREHEADER22:%.*]], label [[VECTOR_PH:%.*]]
-; CHECK:       vector.ph:
-; CHECK-NEXT:    [[N_MOD_VF:%.*]] = and i64 [[TMP1]], 3
----------------
nikic wrote:
> lebedev.ri wrote:
> > lebedev.ri wrote:
> > > rnk wrote:
> > > > It looks to me like tail merging unreachable blocks is preventing vectorization in this test case and the next, which seems like a blocking issue. The test was added here, if that helps understand why it no longer works:
> > > > https://reviews.llvm.org/rG39cc0b8c68b8d316954ecfac0d1f8498ea42866c
> > > > @fhahn 
> > > Filed https://github.com/llvm/llvm-project/issues/53020
> > CC @aqjune @nikic
> > 
> > I'm not actually sure that we can solve this within LV itself,
> > but i would love to be proven wrong here.
> > 
> > I'm pretty sure LV does expand the backedge taken count,
> > so i suppose only not being allowed to expand BTC wouldn't help here.
> > 
> > As i see it, the options are:
> > * ignore this failure
> > * adjust the test to mask the failure (i would hope adding `noundef`'s should help?), potentially coupled with:
> > * are there some missing reasoning bits in `impliesPoison()` and friends that could prevent this regression?
> > * Introduce UB-safe mode for SCEVExpander, lift backedge taken count poison-safety restriction
> > * Prevent simplifycfg from merging conditions like that (as in, iff plain `and`/`or` isn't going to be used)
> > 
> I think the proper way to address this (and other poison-safety issues in SCEV) is to add umin variants in both IR and SCEV that don't propagate op2 poison if op1 is zero.
Posted D116766

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  https://reviews.llvm.org/D116692/new/

https://reviews.llvm.org/D116692