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

[Eli Friedman via Phabricator via llvm-commits]

This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG60712732ea24: [IndVars] Teach replaceCongruentIVs to avoid scrambling induction variables (authored by efriedma).
Herald added a subscriber: StephenFan.

Changed prior to commit:
  https://reviews.llvm.org/D121950?vs=416310&id=539678#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D121950/new/

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
@@ -856,7 +856,7 @@
 ; CHECK-NEXT:    [[TMP0:%.*]] = zext i32 [[START:%.*]] to i64
 ; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i64 [[TMP0]], -1
 ; CHECK-NEXT:    [[TMP2:%.*]] = zext i32 [[X:%.*]] to i64
-; CHECK-NEXT:    [[ICMP_USER_WIDE5:%.*]] = icmp ult i64 [[TMP1]], [[TMP2]]
+; CHECK-NEXT:    [[ICMP_USER_WIDE_FIRST_ITER:%.*]] = icmp ult i64 [[TMP1]], [[TMP2]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[BACKEDGE:%.*]] ], [ [[TMP0]], [[ENTRY:%.*]] ]
@@ -864,7 +864,7 @@
 ; CHECK-NEXT:    [[TMP3:%.*]] = add nsw i64 [[INDVARS_IV]], -1
 ; CHECK-NEXT:    br i1 [[COND]], label [[EXIT:%.*]], label [[GUARDED:%.*]]
 ; CHECK:       guarded:
-; CHECK-NEXT:    br i1 [[ICMP_USER_WIDE5]], label [[BACKEDGE]], label [[SIDE_EXIT:%.*]]
+; CHECK-NEXT:    br i1 [[ICMP_USER_WIDE_FIRST_ITER]], label [[BACKEDGE]], label [[SIDE_EXIT:%.*]]
 ; CHECK:       backedge:
 ; CHECK-NEXT:    [[STORE_ADDR:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[TMP3]]
 ; CHECK-NEXT:    store i32 1, ptr [[STORE_ADDR]], align 4
@@ -1196,7 +1196,7 @@
 ; CHECK-LABEL: @test16_signed_neg(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[START:%.*]], -1
-; CHECK-NEXT:    [[ICMP_USER3:%.*]] = icmp ult i32 [[TMP0]], [[X:%.*]]
+; CHECK-NEXT:    [[ICMP_USER_FIRST_ITER:%.*]] = icmp ult i32 [[TMP0]], [[X:%.*]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[START]], [[ENTRY:%.*]] ], [ [[IV_NEXT_1:%.*]], [[BACKEDGE:%.*]] ]
@@ -1204,7 +1204,7 @@
 ; CHECK-NEXT:    [[FOO:%.*]] = add i32 [[IV]], -1
 ; CHECK-NEXT:    br i1 [[COND]], label [[EXIT:%.*]], label [[GUARDED:%.*]]
 ; CHECK:       guarded:
-; CHECK-NEXT:    br i1 [[ICMP_USER3]], label [[BACKEDGE]], label [[SIDE_EXIT:%.*]]
+; CHECK-NEXT:    br i1 [[ICMP_USER_FIRST_ITER]], label [[BACKEDGE]], label [[SIDE_EXIT:%.*]]
 ; CHECK:       backedge:
 ; CHECK-NEXT:    [[INDEX:%.*]] = sext i32 [[FOO]] to i64
 ; CHECK-NEXT:    [[STORE_ADDR:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[INDEX]]
@@ -1440,15 +1440,17 @@
   ret void
 }
 
+; Don't perform replacement here; SCEV won't recognize the new PHI as an
+; induction variable.
 define void @test22(ptr %ptr) {
 ; CHECK-LABEL: @test22(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    store i16 0, ptr [[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]], ptr [[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
@@ -1652,11 +1652,17 @@
       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;
     }


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