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 69 matching lines @@
     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.

[Jim Stichnoth, 2016/04/02 01:19:52]

Update this comment to mention the cleanup now being done.

[sehr, 2016/04/04 15:38:51]

Done.

 void CfgNode::validatePhis() {

[Jim Stichnoth, 2016/04/02 01:19:52]

I'm thinking that maybe this method name should be changed because it's no
longer a read-only validation.

[sehr, 2016/04/04 15:38:51]

I changed it to enforcePhiConsistency.  WDYT?

   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 1335 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