[PATCH] D79175: [ARM][MVE] Tail-Predication: use @llvm.get.active.lane.mask to get the BTC

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/Target/ARM/MVETailPredication.cpp:456
+  auto *ElementCount = SE->getAddExpr(BTC, One);
+  // Tmp = ElementCount + (VW-1)
+  auto *Tmp = SE->getAddExpr(ElementCount,
----------------
SjoerdMeijer wrote:
> efriedma wrote:
> > SjoerdMeijer wrote:
> > > efriedma wrote:
> > > > Can `ElementCount + (VW-1)` overflow?  Do we need to check for that?
> > > We are not generating code for `ElementCount + (VW-1) `, so that one is fine.  We do want to know about overflow for `Ceil`, so will add a check for that.
> > Not sure I understand; even if we aren't generating code, we're using it as input to the safety check.  Does the math there work correctly even if it overflows?
> The Ceil expression doesn't have the non-wrapping flags. Therefore, my understanding is, that this 
> ceiling calculation is done in a higher bit range and so no information is lost.
> Therefore, my understanding is, that this ceiling calculation is done in a higher bit range

SCEV math is modular math; it happens in the width of SCEV::getType().  (So Add, Mul, and AddRec can overflow.)  If you want wider math, you need to explicitly zero-extend.


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  https://reviews.llvm.org/D79175/new/

https://reviews.llvm.org/D79175