[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

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

test/Transforms/IndVarSimplify/pr28935.ll

@@ -0,0 +1,20 @@
+; RUN: opt -S -indvars < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare i16 @fn1(i16 returned, i64)
+
+define void @fn2() {
+; CHECK-LABEL: @fn2(
+entry:
+  br label %for.cond
+
+for.cond:
+  %f.0 = phi i64 [ undef, %entry ], [ %inc, %for.cond ]
+  %conv = trunc i64 %f.0 to i16
+  %call = tail call i16 @fn1(i16 %conv, i64 %f.0)
+  %conv2 = zext i16 %call to i32
+  %inc = add nsw i64 %f.0, 1
+  br label %for.cond
+}