Optimize 64-bit compares with zero

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 2769 matching lines @@
 TargetX86Base<Machine>::lowerIcmp64(const InstIcmp *Inst) {
   // a=icmp cond, b, c ==> cmp b,c; a=1; br cond,L1; FakeUse(a); a=0; L1:
   Operand *Src0 = legalize(Inst->getSrc(0));
   Operand *Src1 = legalize(Inst->getSrc(1));
   Variable *Dest = Inst->getDest();
   InstIcmp::ICond Condition = Inst->getCondition();
   size_t Index = static_cast<size_t>(Condition);
   assert(Index < Traits::TableIcmp64Size);
   Operand *Src0LoRM = legalize(loOperand(Src0), Legal_Reg | Legal_Mem);
   Operand *Src0HiRM = legalize(hiOperand(Src0), Legal_Reg | Legal_Mem);
+  Constant *Zero = Ctx->getConstantZero(IceType_i32);
+  Constant *One = Ctx->getConstantInt32(1);
+  Constant *SignMask = Ctx->getConstantInt32(0x80000000);
+  if (auto *Const64 = llvm::dyn_cast<ConstantInteger64>(Src1)) {
+    if (Const64->getValue() == 0) {
+      Variable *Temp = nullptr;
+      switch (Condition) {
+      default:
+        llvm_unreachable("unexpected condition");
+        break;
+      case InstIcmp::Eq:
+        _mov(Temp, Src0LoRM);
+        _or(Temp, Src0HiRM);
+        Context.insert(InstFakeUse::create(Func, Temp));
+        _setcc(Dest, Traits::Cond::Br_e);
+        return;
+      case InstIcmp::Ne:
+        _mov(Temp, Src0LoRM);
+        _or(Temp, Src0HiRM);
+        Context.insert(InstFakeUse::create(Func, Temp));
+        _setcc(Dest, Traits::Cond::Br_ne);
+        return;
+      case InstIcmp::Ugt:

[Jim Stichnoth, 2015/10/14 18:04:49]

I think you should combine Ugt with Ne, and Ule with Eq, because really, not by
coincidence, they are in fact equivalent.

[sehr, 2015/10/15 00:19:34]

Good point.  Done.

+        _mov(Temp, Src0LoRM);
+        _or(Temp, Src0HiRM);
+        Context.insert(InstFakeUse::create(Func, Temp));
+        _setcc(Dest, Traits::Cond::Br_ne);
+        return;
+      case InstIcmp::Uge:
+        _mov(Dest, One);
+        return;
+      case InstIcmp::Ult:
+        _mov(Dest, Zero);
+        return;
+      case InstIcmp::Ule:
+        _mov(Temp, Src0LoRM);
+        _or(Temp, Src0HiRM);
+        Context.insert(InstFakeUse::create(Func, Temp));
+        _setcc(Dest, Traits::Cond::Br_e);
+        return;
+      case InstIcmp::Sgt:
+        break;
+      case InstIcmp::Sge:
+        _test(Src0HiRM, SignMask);
+        _setcc(Dest, Traits::Cond::Br_e);
+        return;
+      case InstIcmp::Slt:
+        _test(Src0HiRM, SignMask);
+        _setcc(Dest, Traits::Cond::Br_ne);
+        return;
+      case InstIcmp::Sle:
+        break;
+      }
+    }
+  }
   Operand *Src1LoRI = legalize(loOperand(Src1), Legal_Reg | Legal_Imm);
   Operand *Src1HiRI = legalize(hiOperand(Src1), Legal_Reg | Legal_Imm);
-  Constant *Zero = Ctx->getConstantZero(IceType_i32);
-  Constant *One = Ctx->getConstantInt32(1);
   typename Traits::Insts::Label *LabelFalse =
       Traits::Insts::Label::create(Func, this);
   typename Traits::Insts::Label *LabelTrue =
       Traits::Insts::Label::create(Func, this);
   _mov(Dest, One);
   _cmp(Src0HiRM, Src1HiRI);
   if (Traits::TableIcmp64[Index].C1 != Traits::Cond::Br_None)
     _br(Traits::TableIcmp64[Index].C1, LabelTrue);
   if (Traits::TableIcmp64[Index].C2 != Traits::Cond::Br_None)
     _br(Traits::TableIcmp64[Index].C2, LabelFalse);
@@ skipping 2705 matching lines @@
   }
   // the offset is not eligible for blinding or pooling, return the original
   // mem operand
   return MemOperand;
 }
 
 } // end of namespace X86Internal
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX86BASEIMPL_H