[PATCH] D132571: [RLEV] Pick a correct insert point when incoming instruction is itself a phi node

[Philip Reames via Phabricator via llvm-commits]

reames added inline comments.


================
Comment at: llvm/lib/Transforms/Utils/LoopUtils.cpp:1400
+          (isa<PHINode>(Inst) || isa<LandingPadInst>(Inst)) ?
+          &*Inst->getParent()->getFirstInsertionPt() : Inst;
+        RewritePhiSet.emplace_back(PN, i, ExitValue, InsertPt, HighCost);
----------------
nikic wrote:
> reames wrote:
> > nikic wrote:
> > > Any reason we don't just use `PN->getIncomingBlock(i)->getTerminator()` as the insertion point? That seems like a more obvious place to materialize an edge value.
> > I'd been trying for a minimal fix.  Do you mind if we land this, and then do a follow up change as you suggest?  It'll cause significant test diffs which I think obscure the (rather narrow) impact of the fix.  
> Sure
Went to do this in a follow up, and realized it's more complicated than it sounds.

Since a loop can have multiple exiting blocks which share a single exit block, there is no single source for the exiting edge.  Instead, we'd have to either a) duplicate the expansion or b) find a common dominating point.  The current code effectively does the later, and leaves the decision on whether to duplicate in order to sink to later code.

(Remember: SCEV expander reuses expressions, so expanding the same expression at the same point multiple times only results in one expansion.)

It's not entirely clear to me we *should* do running RLEV when we can't sink, but it's definitely not a straight forward NFC any more.

Unless you feel strongly about this, I'm not going to follow up further.  


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https://reviews.llvm.org/D132571