Add support for undef values in ICE IR.

src/IceTargetLoweringX8632.cpp

Index: src/IceTargetLoweringX8632.cpp
diff --git a/src/IceTargetLoweringX8632.cpp b/src/IceTargetLoweringX8632.cpp
index 46a587fe1f139727438baf153e72f350b3945311..e0c020c305c5e50f9c0fcf14024bc9aa1cee9a00 100644
--- a/src/IceTargetLoweringX8632.cpp
+++ b/src/IceTargetLoweringX8632.cpp
@@ -2152,7 +2152,13 @@ Operand *TargetX8632::legalize(Operand *From, LegalMask Allowed,
     return From;
   }
   if (llvm::isa<Constant>(From)) {
-    if (!(Allowed & Legal_Imm)) {
+    if (llvm::isa<ConstantUndef>(From)) {
+      // Undef values are lowered to a register.

[Jim Stichnoth, 2014/06/17 06:28:07]

Maybe change "lowered to a register" to "lowered to an uninitialized register"
just to be clear, and that also makes it more clear why a FakeDef would be
needed.

Also, it wouldn't hurt to assert(Allowed & Legal_Reg), which I believe is true
for all current calls to legalize() and probably will always be true, but still.

[wala, 2014/06/17 20:38:02]

I agree that the comment needs to be clarified that undefs will be lowered to
uninitialized registers.

assert(Allowed & Legal_Reg) is already called at the beginning of the function.

+      Variable *Reg = makeReg(From->getType(), RegNum);
+      // Insert a FakeDef so that the live range of Reg is not overestimated.
+      Context.insert(InstFakeDef::create(Func, Reg));
+      From = Reg;
+    } else if (!(Allowed & Legal_Imm)) {
       Variable *Reg = makeReg(From->getType(), RegNum);
       _mov(Reg, From);
       From = Reg;