[LoopVersioning] Don't modify the list that we iterate over in addPHINodes

Summary:
In LoopVersioning::addPHINodes we need to iterate over all
users for a value "Inst", and if the user is outside of the
VersionedLoop we should replace the use of "Inst" by using
the value "PN" instead.

Replacing the use of "Inst" for a user of "Inst" also means
that Inst->users() is modified. So it is not safe to do the
replace while iterating over Inst->users() as we used to do.
This patch splits the task into two steps. First we iterate
over Inst->users() to find all users that should be updated.
Those users are saved into a local data structure on the stack.
And then, in the second step, we do the actual updates. This
time iterating over the local data structure.

Reviewers: mzolotukhin, anemet

Reviewed By: mzolotukhin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47134

llvm-svn: 332958

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopVersioning.cpp b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
index 95e9a186c10..abbcd5f9e3b 100644
--- a/llvm/lib/Transforms/Utils/LoopVersioning.cpp
+++ b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
@@ -140,9 +140,12 @@ void LoopVersioning::addPHINodes(
     if (!PN) {
       PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver",
                            &PHIBlock->front());
-      for (auto *User : Inst->users())
-        if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
-          User->replaceUsesOfWith(Inst, PN);
+      SmallVector<User*, 8> UsersToUpdate;
+      for (User *U : Inst->users())
+        if (!VersionedLoop->contains(cast<Instruction>(U)->getParent()))
+          UsersToUpdate.push_back(U);
+      for (User *U : UsersToUpdate)
+        U->replaceUsesOfWith(Inst, PN);
       PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
     }
   }