Subzero: Make optimizations more resilient for early Target development.

src/IceCfg.cpp

 //===- subzero/src/IceCfg.cpp - Control flow graph implementation ---------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 239 matching lines @@
   TimerMarker T(TimerStack::TT_phiValidation, this);
   for (CfgNode *Node : Nodes)
     Node->validatePhis();
 }
 
 void Cfg::renumberInstructions() {
   TimerMarker T(TimerStack::TT_renumberInstructions, this);
   NextInstNumber = Inst::NumberInitial;
   for (CfgNode *Node : Nodes)
     Node->renumberInstructions();
+  // Make sure the entry node is the first node and therefore got the lowest
+  // instruction numbers, to facilitate live range computation of function
+  // arguments.  We want to model function arguments as being live on entry to
+  // the function, otherwise an argument whose only use is in the first
+  // instruction will be assigned a trivial live range and the register
+  // allocator will not recognize its live range as overlapping another
+  // variable's live range.
+  assert(Nodes.empty() || (*Nodes.begin() == getEntryNode()));
 }
 
 // placePhiLoads() must be called before placePhiStores().
 void Cfg::placePhiLoads() {
   TimerMarker T(TimerStack::TT_placePhiLoads, this);
   for (CfgNode *Node : Nodes)
     Node->placePhiLoads();
 }
 
 // placePhiStores() must be called after placePhiLoads().
@@ skipping 562 matching lines @@
       }
     }
     if (Mode == Liveness_Intervals) {
       // Special treatment for live in-args. Their liveness needs to extend
       // beyond the beginning of the function, otherwise an arg whose only use
       // is in the first instruction will end up having the trivial live range
       // [2,2) and will *not* interfere with other arguments. So if the first
       // instruction of the method is "r=arg1+arg2", both args may be assigned
       // the same register. This is accomplished by extending the entry block's
       // instruction range from [2,n) to [1,n) which will transform the
-      // problematic [2,2) live ranges into [1,2).
+      // problematic [2,2) live ranges into [1,2).  This extension works because
+      // the entry node is guaranteed to have the lowest instruction numbers.
       if (Node == getEntryNode()) {
-        // TODO(stichnot): Make it a strict requirement that the entry node
-        // gets the lowest instruction numbers, so that extending the live
-        // range for in-args is guaranteed to work.
         FirstInstNum = Inst::NumberExtended;
+        // Just in case the entry node somehow contains no instructions...
+        if (LastInstNum == Inst::NumberSentinel)
+          LastInstNum = FirstInstNum;
       }
-      Node->livenessAddIntervals(getLiveness(), FirstInstNum, LastInstNum);
+      // If this node somehow contains no instructions, don't bother trying to
+      // add liveness intervals for it, because variables that are live-in and
+      // live-out will have a bogus interval added.
+      if (FirstInstNum != Inst::NumberSentinel)
+        Node->livenessAddIntervals(getLiveness(), FirstInstNum, LastInstNum);
     }
   }
 }
 
 // Traverse every Variable of every Inst and verify that it appears within the
 // Variable's computed live range.
 bool Cfg::validateLiveness() const {
   TimerMarker T(TimerStack::TT_validateLiveness, this);
   bool Valid = true;
   OstreamLocker L(Ctx);
@@ skipping 235 matching lines @@
     }
   }
   // Print each basic block
   for (CfgNode *Node : Nodes)
     Node->dump(this);
   if (isVerbose(IceV_Instructions))
     Str << "}\n";
 }
 
 } // end of namespace Ice