[LowerSwitch] Fixed faulty PHI node update

[Ka-Ka]

Summary

When lowerswitch merge several cases into a new default block it's not
updating the PHI nodes accordingly. The code that update the PHI nodes
for the default edge only update the first entry and do not remove the
remaining ones, to make sure the number of entries match the number of
predecessors.

This is easily fixed by replacing the code that update the PHI node with
the already existing utility function for updating PHI nodes.

[arsenm]

LGTM

lib/Transforms/Utils/LowerSwitch.cpp

#endif
     // If there are no cases left, just branch.
     if (Cases.empty()) {
       BranchInst::Create(Default, CurBlock);
       SI->eraseFromParent();
       return;
     }
   }
 
+  unsigned NrOfDefaults = (SI->getDefaultDest() == Default) ? 1 : 0;
+  for (const auto &Case : SI->cases())
+    if (Case.getCaseSuccessor() == Default)
+      NrOfDefaults++;
+
   // 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);
 
-  // If there is an entry in any PHI nodes for the default edge, make sure
-  // to update them as well.
-  for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
-    PHINode *PN = cast<PHINode>(I);
-    int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
-    assert(BlockIdx != -1 && "Switch didn't go to this successor??");
-    PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
-  }
-
   BasicBlock *SwitchBlock =
       switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
                     OrigBlock, OrigBlock, NewDefault, UnreachableRanges);
 
+  // If there are entries in any PHI nodes for the default edge, make sure
+  // to update them as well.
+  fixPhis(Default, OrigBlock, NewDefault, NrOfDefaults);
+
   // Branch to our shiny new if-then stuff...
   BranchInst::Create(SwitchBlock, OrigBlock);
 
   // We are now done with the switch instruction, delete it.
   BasicBlock *OldDefault = SI->getDefaultDest();
   CurBlock->getInstList().erase(SI);
 
   // If the Default block has no more predecessors just add it to DeleteList.
   if (pred_begin(OldDefault) == pred_end(OldDefault))
     DeleteList.insert(OldDefault);
 }

test/Transforms/Util/lowerswitch.ll

 ; RUN: opt -lowerswitch -S < %s | FileCheck %s
 
 ; Test that we don't crash and have a different basic block for each incoming edge.
 define void @test0() {
 ; CHECK-LABEL: @test0
-; CHECK: %merge = phi i64 [ 1, %BB3 ], [ 0, %NewDefault ], [ 0, %NodeBlock5 ], [ 0, %LeafBlock1 ]
+; CHECK: %merge = phi i64 [ 1, %BB3 ], [ 0, %NodeBlock5 ], [ 0, %LeafBlock1 ], [ 0, %NewDefault ]
 BB1:
   switch i32 undef, label %BB2 [
     i32 3, label %BB2
     i32 5, label %BB2
     i32 0, label %BB3
     i32 2, label %BB3
     i32 4, label %BB3
   ]

 
 ; <label>:34                                      ; preds = %33, %32, %31, %30, %26, %25, %21, %17, %13, %9, %8, %7, %6, %5, %4, %3, %2, %1, %1, %1, %1, %1
   %o.0 = phi float [ undef, %33 ], [ undef, %32 ], [ undef, %31 ], [ undef, %30 ], [ undef, %26 ], [ undef, %25 ], [ undef, %21 ], [ undef, %17 ], [ undef, %13 ], [ undef, %9 ], [ undef, %8 ], [ undef, %7 ], [ undef, %6 ], [ undef, %5 ], [ undef, %4 ], [ undef, %3 ], [ undef, %2 ], [ undef, %1 ], [ undef, %1 ], [ undef, %1 ], [ undef, %1 ], [ undef, %1 ]
   br label %._crit_edge
 
 ._crit_edge:                                      ; preds = %34, %0
   ret void
 }
+
+; Test that the PHI node in for.cond should have one entry for each predecessor
+; of its parent basic block after lowerswitch merged several cases into a new
+; default block.
+define void @test3() {
+; CHECK-LABEL: @test3
+entry:
+  br label %lbl1
+
+lbl1:                                             ; preds = %cleanup, %entry
+  br label %lbl2
+
+lbl2:                                             ; preds = %cleanup, %lbl1
+  br label %for.cond
+
+for.cond:                                         ; preds = %cleanup, %cleanup, %lbl2
+; CHECK: for.cond:
+; CHECK: phi i16 [ undef, %lbl2 ], [ %b.3, %NewDefault ]{{$}}
+; CHECK: for.cond1:
+  %b.2 = phi i16 [ undef, %lbl2 ], [ %b.3, %cleanup ], [ %b.3, %cleanup ]
+  br label %for.cond1
+
+for.cond1:                                        ; preds = %for.inc, %for.cond
+  %b.3 = phi i16 [ %b.2, %for.cond ], [ undef, %for.inc ]
+  %tobool = icmp ne i16 %b.3, 0
+  br i1 %tobool, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond1
+  br i1 undef, label %if.then, label %for.inc
+
+if.then:                                          ; preds = %for.body
+  br label %cleanup
+
+for.inc:                                          ; preds = %for.body
+  br label %for.cond1
+
+for.end:                                          ; preds = %for.cond1
+  br i1 undef, label %if.then4, label %for.body7
+
+if.then4:                                         ; preds = %for.end
+  br label %cleanup
+
+for.body7:                                        ; preds = %for.end
+  br label %cleanup
+
+cleanup:                                          ; preds = %for.body7, %if.then4, %if.then
+  switch i32 undef, label %unreachable [
+    i32 0, label %for.cond
+    i32 2, label %lbl1
+    i32 5, label %for.cond
+    i32 3, label %lbl2
+  ]
+
+unreachable:                                      ; preds = %cleanup
+  unreachable
+}