[PATCH] D53061: [IndVars] Drop "exact" flag from lshr and udiv when substituting their args

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: etherzhhb, reames, sanjoy, greened.

There is a transform that may replace `lshr x, 1` with `lshr (x+1), 1` in case
if it can prove that the result will be the same. However the initial instruction
might have an `exact` flag set, and it now should be dropped unless we prove
that it may hold. Incorrectly set `exact` attribute may then produce poison.


https://reviews.llvm.org/D53061

Files:
  lib/Transforms/Utils/SimplifyIndVar.cpp
  test/Transforms/IndVarSimplify/drop-exact.ll


Index: test/Transforms/IndVarSimplify/drop-exact.ll
===================================================================
--- test/Transforms/IndVarSimplify/drop-exact.ll
+++ test/Transforms/IndVarSimplify/drop-exact.ll
@@ -0,0 +1,53 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -indvars -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+
+; We make a transform by getting rid of add nsw i32 %tmp17, -1; make sure that
+; we drop "exact" flag on lshr as we do it.
+define void @test(i32* %p, i64* %p1) {
+; CHECK-LABEL: @test(
+; CHECK-NOT:   lshr exact
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br label [[BB12:%.*]]
+; CHECK:       bb7:
+; CHECK-NEXT:    ret void
+; CHECK:       bb12:
+; CHECK-NEXT:    [[TMP13:%.*]] = phi i32 [ -47436, [[BB:%.*]] ], [ [[TMP15:%.*]], [[BB12]] ]
+; CHECK-NEXT:    [[TMP14:%.*]] = phi i32 [ 0, [[BB]] ], [ [[TMP42:%.*]], [[BB12]] ]
+; CHECK-NEXT:    [[TMP15]] = add nsw i32 [[TMP13]], -1
+; CHECK-NEXT:    [[TMP16:%.*]] = shl i32 [[TMP15]], 1
+; CHECK-NEXT:    [[TMP17:%.*]] = sub nsw i32 42831, [[TMP16]]
+; CHECK-NEXT:    [[TMP19:%.*]] = lshr i32 [[TMP17]], 1
+; CHECK-NEXT:    [[TMP20:%.*]] = urem i32 [[TMP19]], 250
+; CHECK-NEXT:    [[TMP22:%.*]] = lshr i32 [[TMP17]], 1
+; CHECK-NEXT:    store i32 [[TMP22]], i32* [[P:%.*]], align 4
+; CHECK-NEXT:    [[TMP26:%.*]] = zext i32 [[TMP20]] to i64
+; CHECK-NEXT:    store i64 [[TMP26]], i64* [[P1:%.*]], align 4
+; CHECK-NEXT:    [[TMP42]] = add nuw nsw i32 [[TMP14]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i32 [[TMP42]], 719
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[BB7:%.*]], label [[BB12]]
+;
+bb:
+  br label %bb12
+
+bb7:                                              ; preds = %bb12
+  ret void
+
+bb12:                                             ; preds = %bb12, %bb
+  %tmp13 = phi i32 [ -47436, %bb ], [ %tmp15, %bb12 ]
+  %tmp14 = phi i32 [ 0, %bb ], [ %tmp42, %bb12 ]
+  %tmp15 = add i32 %tmp13, -1
+  %tmp16 = shl i32 %tmp15, 1
+  %tmp17 = sub i32 42831, %tmp16
+  %tmp19 = lshr i32 %tmp17, 1
+  %tmp20 = urem i32 %tmp19, 250
+  %tmp21 = add nsw i32 %tmp17, -1
+  %tmp22 = lshr exact i32 %tmp21, 1
+  store i32 %tmp22, i32* %p, align 4
+  %tmp26 = zext i32 %tmp20 to i64
+  store i64 %tmp26, i64* %p1, align 4
+  %tmp42 = add nuw nsw i32 %tmp14, 1
+  %tmp43 = icmp ugt i32 %tmp14, 717
+  br i1 %tmp43, label %bb7, label %bb12
+}
Index: lib/Transforms/Utils/SimplifyIndVar.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyIndVar.cpp
+++ lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -108,6 +108,7 @@
   Value *IVSrc = nullptr;
   const unsigned OperIdx = 0;
   const SCEV *FoldedExpr = nullptr;
+  bool MustDropPoisonFlags = false;
   switch (UseInst->getOpcode()) {
   default:
     return nullptr;
@@ -140,6 +141,10 @@
                            APInt::getOneBitSet(BitWidth, D->getZExtValue()));
     }
     FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
+    // We might have 'exact' flag set at this point which will no longer be
+    // correct after we make the replacement.
+    if (SE->getSCEV(IVSrc) != SE->getMulExpr(FoldedExpr, SE->getSCEV(D)))
+      MustDropPoisonFlags = true;
   }
   // We have something that might fold it's operand. Compare SCEVs.
   if (!SE->isSCEVable(UseInst->getType()))
@@ -155,6 +160,9 @@
   UseInst->setOperand(OperIdx, IVSrc);
   assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
 
+  if (MustDropPoisonFlags)
+    UseInst->dropPoisonGeneratingFlags();
+
   ++NumElimOperand;
   Changed = true;
   if (IVOperand->use_empty())


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