Index: llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -2372,6 +2372,27 @@
     CmpIndVar = IndVar->getIncomingValueForBlock(L->getExitingBlock());
   }
 
+  // It may be necessary to drop nowrap flags on the incrementing instruction
+  // if either LFTR moves from a pre-inc check to a post-inc check (in which
+  // case the increment might have previously been poison on the last iteration
+  // only) or if LFTR switches to a different IV that was previously dynamically
+  // dead (and as such may be arbitrarily poison). We remove any nowrap flags
+  // that SCEV didn't infer for the post-inc addrec (even if we use a pre-inc
+  // check), because the pre-inc addrec flags may be adopted from the original
+  // instruction, while SCEV has to explicitly prove the post-inc nowrap flags.
+  // TODO: This handling is inaccurate for one case: If we switch to a
+  // dynamically dead IV that wraps on the first loop iteration only, which is
+  // not covered by the post-inc addrec. (If the new IV was not dynamically
+  // dead, it could not be poison on the first iteration in the first place.)
+  Value *IncVar = IndVar->getIncomingValueForBlock(L->getLoopLatch());
+  if (auto *BO = dyn_cast<BinaryOperator>(IncVar)) {
+    const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(IncVar));
+    if (BO->hasNoUnsignedWrap())
+      BO->setHasNoUnsignedWrap(AR->hasNoUnsignedWrap());
+    if (BO->hasNoSignedWrap())
+      BO->setHasNoSignedWrap(AR->hasNoSignedWrap());
+  }
+
   Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE);
   assert(ExitCnt->getType()->isPointerTy() ==
              IndVar->getType()->isPointerTy() &&
Index: llvm/trunk/test/Transforms/IndVarSimplify/lftr-pr31181.ll
===================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/lftr-pr31181.ll
+++ llvm/trunk/test/Transforms/IndVarSimplify/lftr-pr31181.ll
@@ -15,7 +15,7 @@
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ -2, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
-; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[STOREMERGE]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], 0
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
@@ -42,7 +42,7 @@
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
-; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[INC]] = add nuw i32 [[STOREMERGE]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], -2147483648
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
@@ -155,7 +155,7 @@
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[STOREMERGE:%.*]] = phi i32 [ [[INC:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
 ; CHECK-NEXT:    store i32 [[STOREMERGE]], i32* @a
-; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[STOREMERGE]], 1
+; CHECK-NEXT:    [[INC]] = add nuw i32 [[STOREMERGE]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[INC]], [[TMP0]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
 ; CHECK:       exit.loopexit:
@@ -194,7 +194,7 @@
 ; CHECK-NEXT:    br label [[ALWAYS_TAKEN]]
 ; CHECK:       always_taken:
 ; CHECK-NEXT:    [[IV_INC]] = add nsw i32 [[IV]], 1
-; CHECK-NEXT:    [[IV2_INC]] = add nuw nsw i32 [[IV2]], 1
+; CHECK-NEXT:    [[IV2_INC]] = add nuw i32 [[IV2]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[IV2_INC]], -2147483627
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_COND]], label [[FOR_END:%.*]]
 ; CHECK:       for.end:
@@ -242,7 +242,7 @@
 ; CHECK-NEXT:    br label [[ALWAYS_TAKEN]]
 ; CHECK:       always_taken:
 ; CHECK-NEXT:    [[IV_INC]] = add nsw i32 [[IV]], 1
-; CHECK-NEXT:    [[IV2_INC]] = add nuw nsw i32 [[IV2]], 1
+; CHECK-NEXT:    [[IV2_INC]] = add nuw i32 [[IV2]], 1
 ; CHECK-NEXT:    br label [[FOR_COND]]
 ; CHECK:       for.end:
 ; CHECK-NEXT:    ret i32 0
@@ -329,7 +329,7 @@
 ; CHECK-NEXT:    store volatile i32 [[IV2]], i32* [[PTR]]
 ; CHECK-NEXT:    br label [[ALWAYS_TAKEN]]
 ; CHECK:       always_taken:
-; CHECK-NEXT:    [[IV2_INC]] = add nuw nsw i32 [[IV2]], 1
+; CHECK-NEXT:    [[IV2_INC]] = add nsw i32 [[IV2]], 1
 ; CHECK-NEXT:    [[IV_INC]] = add nsw i32 [[IV]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i32 [[IV2_INC]], -2147483629
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_COND]], label [[FOR_END:%.*]]