Fix a crash when the default of a switch is removed

[andrew.w.kaylor]

Summary

This patch fixes a problem that occurs in LowerSwitch when a switch statement has a PHI node as its condition, and the PHI node only has two incoming blocks, and one of those incoming blocks is through an unreachable default in the switch statement. When this condition occurs, LowerSwitch holds a pointer to the condition value, but removes the switch block as a predecessor of the PHI block, causing the PHI node to be replaced. LowerSwitch then tries to use its stale pointer to the original condition value, causing a crash.

[karka]

The fix LGTM (with the minor comment about the testcase).

test/Transforms/LowerSwitch/condition-phi-unreachable-default.ll

1 ↗

(On Diff #201792)

I think that it make sense to always check that the output is sane with FileCheck even if the testcase only should demonstrate that the code don't crash. Do you mind adding "-S | FileCheck %s" (instead of -disable-output) to the command line and add a few checks?

[andrew.w.kaylor]

Updated test case

[karka]

LGTM. I set this "ready to land", but maybe you should wait a bit before landing this to see if anyone else have comments.

llvm/trunk/lib/Transforms/Utils/LowerSwitch.cpp

if (Cases.empty()) {
       BranchInst::Create(Default, OrigBlock);
       SI->eraseFromParent();
       // As all the cases have been replaced with a single branch, only keep
       // one entry in the PHI nodes.
       for (unsigned I = 0 ; I < (MaxPop - 1) ; ++I)
         PopSucc->removePredecessor(OrigBlock);
       return;
     }
+
+    // If the condition was a PHI node with the switch block as a predecessor
+    // removing predecessors may have caused the condition to be erased.
+    // Getting the condition value again here protects against that.
+    Val = SI->getCondition();
   }
 
   // Create a new, empty default block so that the new hierarchy of
   // if-then statements go to this and the PHI nodes are happy.
   BasicBlock *NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
   F->getBasicBlockList().insert(Default->getIterator(), NewDefault);
   BranchInst::Create(Default, NewDefault);
 

llvm/trunk/test/Transforms/LowerSwitch/condition-phi-unreachable-default.ll

+; RUN: opt < %s -lowerswitch -S | FileCheck %s
+
+; This test verifies -lowerswitch does not crash when an removing an
+; unreachable default branch causes a PHI node used as the switch
+; condition to be erased.
+
+define void @f() local_unnamed_addr {
+entry:
+  br label %sw.epilog
+
+sw.epilog:                                        ; preds = %sw.epilog.outer, %for.body
+  %i = phi i32 [ undef, %for.body ], [ 0, %entry ]
+  br i1 undef, label %for.body, label %for.end
+
+for.body:                                         ; preds = %sw.epilog
+  switch i32 %i, label %sw.epilog [
+    i32 0, label %sw.epilog.outer.backedge.loopexit
+    i32 1, label %sw.epilog.outer.backedge
+  ]
+
+sw.epilog.outer.backedge.loopexit:                ; preds = %for.body
+  br label %for.end
+
+sw.epilog.outer.backedge:                         ; preds = %for.body
+  unreachable
+
+for.end:                                          ; preds = %sw.epilog
+  ret void
+}
+
+; The phi and the switch should both be eliminated.
+; CHECK: @f()
+; CHECK: sw.epilog:
+; CHECK-NOT: phi
+; CHECK: for.body:
+; CHECK-NOT: switch