Subzero: More asm-verbose fixes.

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 867 matching lines @@
 
 void CfgNode::emit(Cfg *Func) const {
   if (!ALLOW_DUMP)
     return;
   Func->setCurrentNode(this);
   Ostream &Str = Func->getContext()->getStrEmit();
   Liveness *Liveness = Func->getLiveness();
   bool DecorateAsm =
       Liveness && Func->getContext()->getFlags().getDecorateAsm();
   Str << getAsmName() << ":\n";
+  // LiveRegCount keeps track of the number of currently live
+  // variables that each register is assigned to.  Normally that would
+  // be only 0 or 1, but the register allocator's AllowOverlap
+  // inference allows it to be greater than 1 for short periods.
   std::vector<SizeT> LiveRegCount(Func->getTarget()->getNumRegisters());
   if (DecorateAsm) {
     const bool IsLiveIn = true;
     emitRegisterUsage(Str, Func, this, IsLiveIn, LiveRegCount);
   }
 
   for (const Inst &I : Phis) {
     if (I.isDeleted())
       continue;
     // Emitting a Phi instruction should cause an error.
     I.emit(Func);
   }
   for (const Inst &I : Insts) {
     if (I.isDeleted())
       continue;
-    if (I.isRedundantAssign())
+    if (I.isRedundantAssign()) {
+      // Usually, redundant assignments end the live range of the src
+      // variable and begin the live range of the dest variable, with
+      // no net effect on the liveness of their register.  However, if
+      // the register allocator infers the AllowOverlap condition,
+      // then this may be a redundant assignment that does not end the
+      // src variable's live range, in which case the active variable
+      // count for that register needs to be bumped.  That normally
+      // would have happened as part of emitLiveRangesEnded(), but
+      // that isn't called for redundant assignments.
+      Variable *Dest = I.getDest();
+      if (DecorateAsm && Dest->hasReg() && !I.isLastUse(I.getSrc(0)))

[jvoung (off chromium), 2015/05/27 15:58:56]

Just checking -- this checks !I.isLastUse(I.getSrc(0)), and the code used to
sort of do that too, via the && before decrementing, but now the decrement
happens in emitLiveRangesEnded regardless of Instr->isLastUse(Var); so it looks
like for assignments with Dest == Src(0), the count is bumped on the Dest and
then decremented on the Src(0) also (in emitLiveRangesEnded)?

[Jim Stichnoth, 2015/05/27 16:08:55]

Sorry - I discovered that problem yesterday and uploaded a new patch this
morning that does indeed guard the decrement with an isLastUse() check.

+        ++LiveRegCount[Dest->getRegNum()];
       continue;
+    }
     I.emit(Func);
     if (DecorateAsm)
       emitLiveRangesEnded(Str, Func, &I, LiveRegCount);
     Str << "\n";
     updateStats(Func, &I);
   }
   if (DecorateAsm) {
     const bool IsLiveIn = false;
     emitRegisterUsage(Str, Func, this, IsLiveIn, LiveRegCount);
   }
@@ skipping 300 matching lines @@
       if (!First)
         Str << ", ";
       First = false;
       Str << "%" << I->getName();
     }
     Str << "\n";
   }
 }
 
 } // end of namespace Ice