[IndVars] Ignore (s|z)exts that don't extend the induction variable

`IVVisitor::visitCast` used to have the invariant that if the
instruction it was passed was a sext or zext instruction, the result of
the instruction would be wider than the induction variable.  This is no
longer true after rL275037, so this change teaches `IndVarSimplify` s
implementation of `IVVisitor::visitCast` to work with the relaxed
invariant.

A corresponding change to SimplifyIndVar to preserve the said invariant
after rL275037 would also work, but given how `IVVisitor::visitCast` is
spelled (no indication of said invariant), I figured the current fix is
cleaner.

Fixes PR28935.

llvm-svn: 278584

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index aa722db..b014747 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -816,6 +816,14 @@ static void visitIVCast(CastInst *Cast, WideIVInfo &WI, ScalarEvolution *SE,
   if (!Cast->getModule()->getDataLayout().isLegalInteger(Width))
     return;
 
+  // Check that `Cast` actually extends the induction variable (we rely on this
+  // later).  This takes care of cases where `Cast` is extending a truncation of
+  // the narrow induction variable, and thus can end up being narrower than the
+  // "narrow" induction variable.
+  uint64_t NarrowIVWidth = SE->getTypeSizeInBits(WI.NarrowIV->getType());
+  if (NarrowIVWidth >= Width)
+    return;
+
   // Cast is either an sext or zext up to this point.
   // We should not widen an indvar if arithmetics on the wider indvar are more
   // expensive than those on the narrower indvar. We check only the cost of ADD