Optimize 64-bit compares with zero

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index 0b3a32f87c0b0c283d946e971b4f73fc0ebca0cc..e26a353a98a9e4316906b343b9cf7281d57827bf 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -2787,10 +2787,63 @@ TargetX86Base<Machine>::lowerIcmp64(const InstIcmp *Inst) {
   assert(Index < Traits::TableIcmp64Size);
   Operand *Src0LoRM = legalize(loOperand(Src0), Legal_Reg | Legal_Mem);
   Operand *Src0HiRM = legalize(hiOperand(Src0), Legal_Reg | Legal_Mem);
-  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);
+  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);
   typename Traits::Insts::Label *LabelFalse =
       Traits::Insts::Label::create(Func, this);
   typename Traits::Insts::Label *LabelTrue =