Subzero: Add branch optimization.

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.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the CfgNode class, including the complexities
@@ skipping 439 matching lines @@
       continue;
     if (Begin <= FirstInstNum)
       Begin = FirstInstNum;
     if (End == Inst::NumberSentinel)
       End = LastInstNum + 1;
     Variable *Var = Liveness->getVariable(i, this);
     Liveness->addLiveRange(Var, Begin, End, 1);
   }
 }
 
+void CfgNode::doBranchOpt(const CfgNode *NextNode) {
+  TargetLowering *Target = Func->getTarget();
+  for (InstList::const_iterator I = Insts.begin(), E = Insts.end(); I != E;
+       ++I) {
+    Target->doBranchOpt(*I, NextNode);

[jvoung (off chromium), 2014/09/18 03:56:02]

I wonder if it makes sense to search backward and bail once the terminating
branch is found. I guess this is complicated by switch statements which can have
> 1 branch.

[Jim Stichnoth, 2014/09/18 11:45:51]

I thought about this a bit, but I think that would make too strong an assumption
in the target-neutral CfgNode class about the properties of the target-specific
lowering, namely that there is only one branch optimization opportunity per
block (though it happens to be true in the current x86 lowering).  I added a
comment about this potential opportunity though.  Also, I changed the
instruction-level doBranchOpt() methods to return bool based on whether an
optimization was actually done.

BTW, the current switch lowering still only leaves one opportunity for
optimization, namely the last branch for the default case.  The other cases
lower to conditional branches with no fallthrough target, which aren't
optimized.

+  }
+}
+
 // ======================== Dump routines ======================== //
 
 void CfgNode::emit(Cfg *Func) const {
   Func->setCurrentNode(this);
   Ostream &Str = Func->getContext()->getStrEmit();
   if (Func->getEntryNode() == this) {
     Str << Func->getContext()->mangleName(Func->getFunctionName()) << ":\n";
   }
   Str << getAsmName() << ":\n";
   for (PhiList::const_iterator I = Phis.begin(), E = Phis.end(); I != E; ++I) {
@@ skipping 95 matching lines @@
          I != E; ++I) {
       if (I != OutEdges.begin())
         Str << ", ";
       Str << "%" << (*I)->getName();
     }
     Str << "\n";
   }
 }
 
 } // end of namespace Ice