Selectively invert ICMP operands for better address optimization

src/IceTargetLoweringX86BaseImpl.h

 //===- subzero/src/IceTargetLoweringX86BaseImpl.h - x86 lowering -*- C++ -*-==//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 5634 matching lines @@
   // it doesn't need another level of transformation.
   Variable *DestLoad = Load->getDest();
   Type Ty = DestLoad->getType();
   Operand *Src0 = formMemoryOperand(Load->getSourceAddress(), Ty);
   doMockBoundsCheck(Src0);
   auto *Assign = InstAssign::create(Func, DestLoad, Src0);
   lowerAssign(Assign);
 }
 
 template <typename TraitsType>
+void TargetX86Base<TraitsType>::doAddressOptOther() {
+  // Inverts some Icmp instructions which helps doAddressOptLoad later.
+  Inst *Instr = Context.getCur();
+  auto *VMetadata = Func->getVMetadata();
+  if (auto *Icmp = llvm::dyn_cast<InstIcmp>(Instr)) {
+    if (llvm::isa<Constant>(Icmp->getSrc(0)) ||
+        llvm::isa<Constant>(Icmp->getSrc(1)))
+      return;
+    auto *Var0 = llvm::dyn_cast<Variable>(Icmp->getSrc(0));

[Jim Stichnoth, 2016/07/14 19:10:02]

As we discussed offline, it seems like the conditions on Src0 and Src1 are (or
should be) essentially identical, so it would be nice to have a helper function
that is called twice.

We want to reverse the condition/operands when Src0 derives from a load
instruction, and Src1 does not derive from a load instruction.

+    if (Var0 == nullptr)
+      return;
+    if (!VMetadata->isTracked(Var0))
+      return;
+    auto *Op0Def = VMetadata->getFirstDefinitionSingleBlock(Var0);
+    if (Op0Def == nullptr || !llvm::isa<InstLoad>(Op0Def))
+      return;
+    if (VMetadata->getLocalUseNode(Var0) != Context.getNode())
+      return;
+
+    auto *Var1 = llvm::dyn_cast<Variable>(Icmp->getSrc(1));
+    if (Var1 != nullptr && VMetadata->isTracked(Var1)) {
+      auto *Op1Def = VMetadata->getFirstDefinitionSingleBlock(Var1);
+      if (Op1Def != nullptr && !VMetadata->isMultiBlock(Var1) &&
+          llvm::isa<InstLoad>(Op1Def)) {
+        return; // Both are loads
+      }
+    }
+    Icmp->reverseConditionAndOperands();
+  }
+}
+
+template <typename TraitsType>
 void TargetX86Base<TraitsType>::doAddressOptLoad() {
   Inst *Instr = Context.getCur();
   Operand *Addr = Instr->getSrc(0);
   Variable *Dest = Instr->getDest();
   if (auto *OptAddr = computeAddressOpt(Instr, Dest->getType(), Addr)) {
     Instr->setDeleted();
     Context.insert<InstLoad>(Dest, OptAddr);
   }
 }
 
@@ skipping 2367 matching lines @@
         emitGlobal(*Var, SectionSuffix);
       }
     }
   } break;
   }
 }
 } // end of namespace X86NAMESPACE
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX86BASEIMPL_H