Replace constant conditional branches by unconditional branches

src/IceCfgNode.cpp

 //===- subzero/src/IceCfgNode.cpp - Basic block (node) implementation -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 66 matching lines @@
 // computePredecessors() pass finalizes it by creating the InEdges list.
 void CfgNode::computePredecessors() {
   for (CfgNode *Succ : OutEdges)
     Succ->InEdges.push_back(this);
 }
 
 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
     // shows up as only about 0.2% of the total translation time. But if
     // necessary, we could improve the complexity by using a hash table to
     // count how many times each node is referenced in the Phi instruction, and
     // how many times each node is referenced in the incoming edge list, and
     // compare the two for equality.
     for (SizeT i = 0; i < Phi->getSrcSize(); ++i) {
       CfgNode *Label = Phi->getLabel(i);
       bool Found = false;
       for (CfgNode *InNode : getInEdges()) {
         if (InNode == Label) {
           Found = true;
           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;
       for (SizeT i = 0; i < Phi->getSrcSize(); ++i) {
         CfgNode *Label = Phi->getLabel(i);
         if (InNode == Label) {
           Found = true;
           break;
         }
       }
@@ skipping 1334 matching lines @@
   auto *Instr = InstIntrinsicCall::create(
       Func, 5, Func->makeVariable(IceType_i64), RMWI64Name, Info->Info);
   Instr->addArg(AtomicRMWOp);
   Instr->addArg(Counter);
   Instr->addArg(One);
   Instr->addArg(OrderAcquireRelease);
   Insts.push_front(Instr);
 }
 
 } // end of namespace Ice