[IndVars] Teach replaceCongruentIVs to avoid scrambling induction variables

replaceCongruentIVs analysis is based on ScalarEvolution; this makes
comparing different PHIs and performing the replacement straightforward.
However, it can have some side-effects: it isn't aware whether an
induction variable is in canonical form, so it can perform replacements
which obscure the meaning of the IR.

In test22 in widen-loop-comp.ll, the resulting loop can't be analyzed by
ScalarEvolution at all.

My attempted solution is to restrict the transform: don't try to replace
induction variables using PHI nodes that don't represent simple
induction variables.

I'm not sure if this is the best solution; suggestions welcome.

Differential Revision: https://reviews.llvm.org/D121950

1 files changed, 11 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index dc094cf..2084427 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1652,11 +1652,17 @@ SCEVExpander::replaceCongruentIVs(Loop *L, const DominatorTree *DT,
       OrigPhiRef = Phi;
       if (Phi->getType()->isIntegerTy() && TTI &&
           TTI->isTruncateFree(Phi->getType(), Phis.back()->getType())) {
-        // This phi can be freely truncated to the narrowest phi type. Map the
-        // truncated expression to it so it will be reused for narrow types.
-        const SCEV *TruncExpr =
-          SE.getTruncateExpr(SE.getSCEV(Phi), Phis.back()->getType());
-        ExprToIVMap[TruncExpr] = Phi;
+        // Make sure we only rewrite using simple induction variables;
+        // otherwise, we can make the trip count of a loop unanalyzable
+        // to SCEV.
+        const SCEV *PhiExpr = SE.getSCEV(Phi);
+        if (isa<SCEVAddRecExpr>(PhiExpr)) {
+          // This phi can be freely truncated to the narrowest phi type. Map the
+          // truncated expression to it so it will be reused for narrow types.
+          const SCEV *TruncExpr =
+              SE.getTruncateExpr(PhiExpr, Phis.back()->getType());
+          ExprToIVMap[TruncExpr] = Phi;
+        }
       }
       continue;
     }