Subzero: Ignore variables with no actual uses.

src/IceInst.cpp

 //===- subzero/src/IceInst.cpp - High-level instruction implementation ----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 188 matching lines @@
     setLastUse(IndexOfVarInInst(Var));
   }
 }
 
 bool Inst::liveness(InstNumberT InstNumber, LivenessBV &Live,
                     Liveness *Liveness, LiveBeginEndMap *LiveBegin,
                     LiveBeginEndMap *LiveEnd) {
   assert(!isDeleted());
 
   Dead = false;
-  if (Dest) {
+  if (Dest && !Dest->isRematerializable()) {
     SizeT VarNum = Liveness->getLiveIndex(Dest->getIndex());
     if (Live[VarNum]) {
       if (!isDestRedefined()) {
         Live[VarNum] = false;
         if (LiveBegin && Liveness->getRangeMask(Dest->getIndex())) {
           LiveBegin->push_back(std::make_pair(VarNum, InstNumber));
         }
       }
     } else {
       if (!hasSideEffects())
         Dead = true;
     }
   }
   if (Dead)
     return false;
   // Phi arguments only get added to Live in the predecessor node, but we still
   // need to update LiveRangesEnded.
   bool IsPhi = llvm::isa<InstPhi>(this);
   resetLastUses();
   FOREACH_VAR_IN_INST(Var, *this) {
+    if (Var->isRematerializable())
+      continue;
     SizeT VarNum = Liveness->getLiveIndex(Var->getIndex());
     if (!Live[VarNum]) {
       setLastUse(IndexOfVarInInst(Var));
       if (!IsPhi) {
         Live[VarNum] = true;
         // For a variable in SSA form, its live range can end at most once in a
         // basic block. However, after lowering to two-address instructions, we
         // end up with sequences like "t=b;t+=c;a=t" where t's live range
         // begins and ends twice. ICE only allows a variable to have a single
         // liveness interval in a basic block (except for blocks where a
@@ skipping 164 matching lines @@
 // Updates liveness for a particular operand based on the given predecessor
 // edge. Doesn't mark the operand as live if the Phi instruction is dead or
 // deleted.
 void InstPhi::livenessPhiOperand(LivenessBV &Live, CfgNode *Target,
                                  Liveness *Liveness) {
   if (isDeleted() || Dead)
     return;
   for (SizeT I = 0; I < getSrcSize(); ++I) {
     if (Labels[I] == Target) {
       if (auto *Var = llvm::dyn_cast<Variable>(getSrc(I))) {
-        SizeT SrcIndex = Liveness->getLiveIndex(Var->getIndex());
-        if (!Live[SrcIndex]) {
-          setLastUse(I);
-          Live[SrcIndex] = true;
+        if (!Var->isRematerializable()) {
+          SizeT SrcIndex = Liveness->getLiveIndex(Var->getIndex());
+          if (!Live[SrcIndex]) {
+            setLastUse(I);
+            Live[SrcIndex] = true;
+          }
         }
       }
       return;
     }
   }
   llvm_unreachable("Phi operand not found for specified target node");
 }
 
 // Change "a=phi(...)" to "a_phi=phi(...)" and return a new instruction
 // "a=a_phi".
@@ skipping 583 matching lines @@
     // upper 32 bits of rax. We need to recognize and preserve these.
     return true;
   }
   if (!Dest->hasReg() && !SrcVar->hasReg() &&
       Dest->getStackOffset() == SrcVar->getStackOffset())
     return true;
   return false;
 }
 
 } // end of namespace Ice