[PATCH] D41943: [LoopDeletion] Handle users in unreachable block

[Serguei Katkov via Phabricator via llvm-commits]

skatkov created this revision.
skatkov added reviewers: anna, davide.

This is a fix for PR35884.

When we want to delete dead loop we must clean uses in unreachable blocks
otherwise we'll get an assert during deletion of instructions from the loop.


https://reviews.llvm.org/D41943

Files:
  lib/Transforms/Utils/LoopUtils.cpp
  test/Transforms/LoopDeletion/use-in-unreachable.ll


Index: test/Transforms/LoopDeletion/use-in-unreachable.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopDeletion/use-in-unreachable.ll
@@ -0,0 +1,24 @@
+; RUN: opt < %s -loop-deletion -S | FileCheck %s
+
+; Checking that possible users of instruction from the loop in
+; unreachable blocks are handled.
+
+define i64 @foo() {
+entry:
+  br label %badloop
+; CHECK-LABEL-NOT: badloop
+badloop:
+  %indvar1 = phi i64 [ 3, %entry ], [ %indvar2, %badloop_iter ]
+  %check = icmp ult i64 %indvar1, 400
+  br i1 %check, label %badloop_iter, label %loopexit
+badloop_iter:
+  %indvar2 = add i64 %indvar1, 1
+  %baddef = add i64 0, 0
+  br label %badloop
+loopexit:
+  ret i64 0
+deadcode:
+; CHECK-LABEL: deadcode
+; CHECK: ret i64 undef
+  ret i64 %baddef
+}
\ No newline at end of file
Index: lib/Transforms/Utils/LoopUtils.cpp
===================================================================
--- lib/Transforms/Utils/LoopUtils.cpp
+++ lib/Transforms/Utils/LoopUtils.cpp
@@ -1356,6 +1356,32 @@
     DT->deleteEdge(Preheader, L->getHeader());
   }
 
+  // Strongly speaking any instruction in the loop at this moment should not
+  // have a user outside of this loop. However it is possible that there are
+  // some users in unreachable blocks and we want to remove these users here.
+  // We could do it after drop all references (in this case all users in the
+  // loop will be already eliminated and we have less work to do but according
+  // to API doc of User::dropAllReferences no one operation after dropping all
+  // references is valid except deletion. So let's substitute all usages of
+  // instruction from the loop with undef value of corresponding type first.
+  for (auto *Block : L->blocks())
+    for (Instruction &I : *Block) {
+      auto *Undef = UndefValue::get(I.getType());
+      for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E;) {
+        Use &U = *UI;
+        ++UI;
+        if (auto *Usr = dyn_cast<Instruction>(U.getUser()))
+          if (L->contains(Usr->getParent()))
+            continue;
+        // If we have a DT then we can check that uses outside a loop only in
+        // unreachable block.
+        if (DT)
+          assert(!DT->isReachableFromEntry(U) &&
+                 "Unexpected user in reachable block");
+        U.set(Undef);
+      }
+    }
+
   // Remove the block from the reference counting scheme, so that we can
   // delete it freely later.
   for (auto *Block : L->blocks())


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