ValueTracking: ComputeNumSignBits should tolerate misshapen phi nodes

PHI nodes can have zero operands in the middle of a transform.  It is
expected that utilities in Analysis don't freak out when this happens.

Note that it is considered invalid to allow these misshapen phi nodes to
make it to another pass.

This fixes PR22086.

llvm-svn: 225126

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 6fdf4afebc2..b292af6cee7 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1821,13 +1821,16 @@ unsigned ComputeNumSignBits(Value *V, const DataLayout *TD,
 
   case Instruction::PHI: {
     PHINode *PN = cast<PHINode>(U);
+    unsigned NumIncomingValues = PN->getNumIncomingValues();
     // Don't analyze large in-degree PHIs.
-    if (PN->getNumIncomingValues() > 4) break;
+    if (NumIncomingValues > 4) break;
+    // Unreachable blocks may have zero-operand PHI nodes.
+    if (NumIncomingValues == 0) break;
 
     // Take the minimum of all incoming values.  This can't infinitely loop
     // because of our depth threshold.
     Tmp = ComputeNumSignBits(PN->getIncomingValue(0), TD, Depth+1, Q);
-    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
+    for (unsigned i = 1, e = NumIncomingValues; i != e; ++i) {
       if (Tmp == 1) return Tmp;
       Tmp = std::min(Tmp,
                      ComputeNumSignBits(PN->getIncomingValue(i), TD,