Teach the X86 backend about unreachable and undef. Among other things, we

now compile:

'foo() {}' into "ret" instead of "mov EAX, 0; ret"

llvm-svn: 17049

1 files changed, 15 insertions, 1 deletions

diff --git a/llvm/lib/Target/X86/X86ISelSimple.cpp b/llvm/lib/Target/X86/X86ISelSimple.cpp
index a938bae3fc7..926492e31f5 100644
--- a/llvm/lib/Target/X86/X86ISelSimple.cpp
+++ b/llvm/lib/Target/X86/X86ISelSimple.cpp
@@ -175,6 +175,7 @@ namespace {
     // Control flow operators
     void visitReturnInst(ReturnInst &RI);
     void visitBranchInst(BranchInst &BI);
+    void visitUnreachableInst(UnreachableInst &UI) {}
 
     struct ValueRecord {
       Value *Val;
@@ -447,7 +448,20 @@ unsigned X86ISel::getFixedSizedAllocaFI(AllocaInst *AI) {
 void X86ISel::copyConstantToRegister(MachineBasicBlock *MBB,
                                      MachineBasicBlock::iterator IP,
                                      Constant *C, unsigned R) {
-  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
+  if (isa<UndefValue>(C)) {
+    switch (getClassB(C->getType())) {
+    case cFP:
+      // FIXME: SHOULD TEACH STACKIFIER ABOUT UNDEF VALUES!
+      BuildMI(*MBB, IP, X86::FLD0, 0, R);
+      return;
+    case cLong:
+      BuildMI(*MBB, IP, X86::IMPLICIT_DEF, 0, R+1);
+      // FALL THROUGH
+    default:
+      BuildMI(*MBB, IP, X86::IMPLICIT_DEF, 0, R);
+      return;
+    }
+  } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
     unsigned Class = 0;
     switch (CE->getOpcode()) {
     case Instruction::GetElementPtr: