[PATCH] D121950: [IndVars] Teach replaceCongruentIVs to avoid scrambling induction variables

[Eli Friedman via Phabricator via llvm-commits]

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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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121950

Files:
  llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
  llvm/test/Transforms/IndVarSimplify/AArch64/widen-loop-comp.ll


Index: llvm/test/Transforms/IndVarSimplify/AArch64/widen-loop-comp.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/AArch64/widen-loop-comp.ll
+++ llvm/test/Transforms/IndVarSimplify/AArch64/widen-loop-comp.ll
@@ -1446,9 +1446,9 @@
 ; CHECK-NEXT:    store i16 0, i16* [[PTR:%.*]], align 4
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[INDVARS:%.*]] = trunc i32 [[IV]] to i16
-; CHECK-NEXT:    [[VAL_INC:%.*]] = add i16 [[INDVARS]], 1
+; CHECK-NEXT:    [[VAL:%.*]] = phi i16 [ [[VAL_INC:%.*]], [[LOOP]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[ENTRY]] ]
+; CHECK-NEXT:    [[VAL_INC]] = add i16 [[VAL]], 1
 ; CHECK-NEXT:    store i16 [[VAL_INC]], i16* [[PTR]], align 4
 ; CHECK-NEXT:    [[IV_WIDE:%.*]] = zext i32 [[IV]] to i64
 ; CHECK-NEXT:    call void @foo(i64 [[IV_WIDE]])
Index: llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
===================================================================
--- llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -2070,11 +2070,14 @@
       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;
+        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;
     }


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