Replace constant conditional branches by unconditional branches

src/IceCfgNode.cpp

Index: src/IceCfgNode.cpp
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index 9fc6634af6eaff164f17b3aed4385c44fbe7b104..45979362c2fa867e54047fb4f3ca8fb2d338ae37 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -84,11 +84,13 @@ void CfgNode::computeSuccessors() {
   OutEdges = Insts.rbegin()->getTerminatorEdges();
 }
 
-// Validate each Phi instruction in the node with respect to control flow. For
-// every phi argument, its label must appear in the predecessor list. For each
-// predecessor, there must be a phi argument with that label. We don't check
-// that phi arguments with the same label have the same value.
-void CfgNode::validatePhis() {
+// Ensure each Phi instruction in the node is consistent with respect to control
+// flow.  For each predecessor, there must be a phi argument with that label.
+// For every phi argument, its label must appear in the predecessor list or the
+// value must be zero.  The latter allows us to remove some dead control flow

[Jim Stichnoth, 2016/04/04 15:50:31]

I suggest this change of the "For every phi argument..." sentence for clarity:

If a phi argument's label doesn't appear in the predecessor list (which can
happen as a result of e.g. unreachable node elimination), its value is modified
to be zero, to maintain consistency in liveness analysis.

[sehr, 2016/04/04 17:06:41]

Done.

+// without a major rework of the phi nodes.  We don't check that phi arguments

[Jim Stichnoth, 2016/04/04 15:50:31]

I prefer "phi instruction" over "phi node", but your call...

[sehr, 2016/04/04 17:06:41]

Done.

+// with the same label have the same value.
+void CfgNode::enforcePhiConsistency() {
   for (Inst &Instr : Phis) {
     auto *Phi = llvm::cast<InstPhi>(&Instr);
     // We do a simple O(N^2) algorithm to check for consistency. Even so, it
@@ -106,8 +108,11 @@ void CfgNode::validatePhis() {
           break;
         }
       }
-      if (!Found)
-        llvm::report_fatal_error("Phi error: label for bad incoming edge");
+      if (!Found) {
+        // Predecessor was unreachable, so if (impossibly) the control flow
+        // enters from that predecessor, the value should be zero.
+        Phi->clearOperandForTarget(Label);
+      }
     }
     for (CfgNode *InNode : getInEdges()) {
       bool Found = false;