[SelectionDAG] Compute known bits of CopyFromReg

Summary:
Teach SelectionDAG how to compute known bits of ISD::CopyFromReg if
the virtual reg used has one def only.

This can be particularly useful when calling isBaseWithConstantOffset()
with the ISD::CopyFromReg argument, as more optimizations may get enabled
in the result.

Also add a missing truncation on X86, found by testing of this patch.

Change-Id: Id1c9fceec862d118c54a5b53adf72ada5d6daefa

Reviewers: bogner, craig.topper, RKSimon

Reviewed By: RKSimon

Subscribers: lebedev.ri, nemanjai, jvesely, nhaehnle, javed.absar, jsji, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59535

llvm-svn: 357745

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 87ace6f66b0..148a3805b3b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -31,6 +31,7 @@
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/RuntimeLibcalls.h"
 #include "llvm/CodeGen/SelectionDAGAddressAnalysis.h"
 #include "llvm/CodeGen/SelectionDAGNodes.h"
@@ -3202,6 +3203,25 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
     Known.One &= Known2.One;
     break;
   }
+  case ISD::CopyFromReg: {
+    auto R = cast<RegisterSDNode>(Op.getOperand(1));
+    const unsigned Reg = R->getReg();
+
+    const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
+    if (!TRI->isVirtualRegister(Reg))
+      break;
+
+    const MachineRegisterInfo *MRI = &MF->getRegInfo();
+    if (!MRI->hasOneDef(Reg))
+      break;
+
+    const FunctionLoweringInfo::LiveOutInfo *LOI = FLI->GetLiveOutRegInfo(Reg);
+    if (!LOI || LOI->Known.getBitWidth() != BitWidth)
+      break;
+
+    Known = LOI->Known;
+    break;
+  }
   case ISD::FrameIndex:
   case ISD::TargetFrameIndex:
     TLI->computeKnownBitsForFrameIndex(Op, Known, DemandedElts, *this, Depth);