[CodeGenPrepare] Fix crash due to an invalid CFG

[bcahoon]

Summary

The splitIndirectCriticalEdges transformation generates an invalid CFG when the 'Target' basic block is a loop to itself. When this occurs, the code that updates the predecessor terminators needs to update the terminator in the split basic block.

This occurs when there is an edge from 'D' to 'D'. Since D is split into D0 and D1, the code needs to update the terminator in D1, but D1 isn't in the OtherPreds vector so it doesn't get updated.

[mkuper]

LGTM, Thanks!

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

if (IBRPred == Target)
       IBRPred = BodyBlock;
 
     // At this point Target only has PHIs, and BodyBlock has the rest of the
     // block's body. Create a copy of Target that will be used by the "direct"
     // preds.
     ValueToValueMapTy VMap;
     BasicBlock *DirectSucc = CloneBasicBlock(Target, VMap, ".clone", &F);
 
-    for (BasicBlock *Pred : OtherPreds)
+    for (BasicBlock *Pred : OtherPreds) {
+      // If the target is a loop to itself, then the terminator of the split
+      // block needs to be updated.
+      if (Pred == Target)
+        BodyBlock->getTerminator()->replaceUsesOfWith(Target, DirectSucc);
+      else
         Pred->getTerminator()->replaceUsesOfWith(Target, DirectSucc);
+    }
 
     // Ok, now fix up the PHIs. We know the two blocks only have PHIs, and that
     // they are clones, so the number of PHIs are the same.
     // (a) Remove the edge coming from IBRPred from the "Direct" PHI
     // (b) Leave that as the only edge in the "Indirect" PHI.
     // (c) Merge the two in the body block.
     BasicBlock::iterator Indirect = Target->begin(),
                          End = Target->getFirstNonPHI()->getIterator();

llvm/trunk/test/Transforms/CodeGenPrepare/split-indirect-loop.ll

+; RUN: opt -codegenprepare -S < %s | FileCheck %s
+
+; Test that an invalid CFG is not created by splitIndirectCriticalEdges
+; transformation when the 'target' block is a loop to itself.
+
+; CHECK: .split:
+; CHECK: br label %while.body.clone
+; CHECK: if.else1:
+; CHECK: indirectbr
+; CHECK: while.body.clone:
+; CHECK: br label %.split
+
+define void @test() {
+entry:
+  br label %if.else
+
+if.else:
+  br i1 undef, label %while.body, label %preheader
+
+preheader:
+  br label %if.else1
+
+if.then:
+  unreachable
+
+while.body:
+  %dest.sroa = phi i32 [ %1, %while.body ], [ undef, %if.else1 ], [ undef, %if.else ]
+  %0 = inttoptr i32 %dest.sroa to i8*
+  %incdec.ptr = getelementptr inbounds i8, i8* %0, i32 -1
+  %1 = ptrtoint i8* %incdec.ptr to i32
+  store i8 undef, i8* %incdec.ptr, align 1
+  br label %while.body
+
+if.else1:
+  indirectbr i8* undef, [label %if.then, label %while.body, label %if.else, label %if.else1]
+}
+