Enhance ComputeMaskedBits to know that aligned frameindexes

have their low bits set to zero.  This allows us to optimize
out explicit stack alignment code like in stack-align.ll:test4 when
it is redundant.

Doing this causes the code generator to start turning FI+cst into
FI|cst all over the place, which is general goodness (that is the
canonical form) except that various pieces of the code generator
don't handle OR aggressively.  Fix this by introducing a new
SelectionDAG::isBaseWithConstantOffset predicate, and using it
in places that are looking for ADD(X,CST).  The ARM backend in
particular was missing a lot of addressing mode folding opportunities
around OR.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125470 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 35 insertions, 8 deletions

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 51c743ca5d..501264f23d 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2040,6 +2040,15 @@ void SelectionDAG::ComputeMaskedBits(SDValue Op, const APInt &Mask,
     KnownZero = APInt::getHighBitsSet(BitWidth, Leaders) & Mask;
     return;
   }
+  case ISD::FrameIndex:
+  case ISD::TargetFrameIndex:
+    if (unsigned Align = InferPtrAlignment(Op)) {
+      // The low bits are known zero if the pointer is aligned.
+      KnownZero = APInt::getLowBitsSet(BitWidth, Log2_32(Align));
+      return;
+    }
+    break;
+      
   default:
     // Allow the target to implement this method for its nodes.
     if (Op.getOpcode() >= ISD::BUILTIN_OP_END) {
@@ -2279,6 +2288,25 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, unsigned Depth) const{
   return std::max(FirstAnswer, std::min(VTBits, Mask.countLeadingZeros()));
 }
 
+/// isBaseWithConstantOffset - Return true if the specified operand is an
+/// ISD::ADD with a ConstantSDNode on the right-hand side, or if it is an
+/// ISD::OR with a ConstantSDNode that is guaranteed to have the same
+/// semantics as an ADD.  This handles the equivalence:
+///     X|Cst == X+Cst iff X&~Cst = 0.
+bool SelectionDAG::isBaseWithConstantOffset(SDValue Op) const {
+  if ((Op.getOpcode() != ISD::ADD && Op.getOpcode() != ISD::OR) ||
+      !isa<ConstantSDNode>(Op.getOperand(1)))
+    return false;
+  
+  if (Op.getOpcode() == ISD::OR && 
+      !MaskedValueIsZero(Op.getOperand(0),
+                     cast<ConstantSDNode>(Op.getOperand(1))->getAPIntValue()))
+    return false;
+  
+  return true;
+}
+
+
 bool SelectionDAG::isKnownNeverNaN(SDValue Op) const {
   // If we're told that NaNs won't happen, assume they won't.
   if (NoNaNsFPMath)
@@ -6327,11 +6355,11 @@ bool SelectionDAG::isConsecutiveLoad(LoadSDNode *LD, LoadSDNode *Base,
     if (FS != BFS || FS != (int)Bytes) return false;
     return MFI->getObjectOffset(FI) == (MFI->getObjectOffset(BFI) + Dist*Bytes);
   }
-  if (Loc.getOpcode() == ISD::ADD && Loc.getOperand(0) == BaseLoc) {
-    ConstantSDNode *V = dyn_cast<ConstantSDNode>(Loc.getOperand(1));
-    if (V && (V->getSExtValue() == Dist*Bytes))
-      return true;
-  }
+
+  // Handle X+C
+  if (isBaseWithConstantOffset(Loc) && Loc.getOperand(0) == BaseLoc &&
+      cast<ConstantSDNode>(Loc.getOperand(1))->getSExtValue() == Dist*Bytes)
+    return true;
 
   const GlobalValue *GV1 = NULL;
   const GlobalValue *GV2 = NULL;
@@ -6372,15 +6400,14 @@ unsigned SelectionDAG::InferPtrAlignment(SDValue Ptr) const {
   int64_t FrameOffset = 0;
   if (FrameIndexSDNode *FI = dyn_cast<FrameIndexSDNode>(Ptr)) {
     FrameIdx = FI->getIndex();
-  } else if (Ptr.getOpcode() == ISD::ADD &&
-             isa<ConstantSDNode>(Ptr.getOperand(1)) &&
+  } else if (isBaseWithConstantOffset(Ptr) &&
              isa<FrameIndexSDNode>(Ptr.getOperand(0))) {
+    // Handle FI+Cst
     FrameIdx = cast<FrameIndexSDNode>(Ptr.getOperand(0))->getIndex();
     FrameOffset = Ptr.getConstantOperandVal(1);
   }
 
   if (FrameIdx != (1 << 31)) {
-    // FIXME: Handle FI+CST.
     const MachineFrameInfo &MFI = *getMachineFunction().getFrameInfo();
     unsigned FIInfoAlign = MinAlign(MFI.getObjectAlignment(FrameIdx),
                                     FrameOffset);