[DAGCombiner] Slightly improve readability of matchRotateSub

Slightly change the wording in the function comment. Originally, it can be
misunderstood as we turned the input into two subsequent rotates.

Better connect the comment which talks about Mask and the code which used
LoBits.  Renamed variable to MaskLoBits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203314 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index aa35f6d3e77156c6f2a2af33d0d01c5d1dcf28e0..97885d4f3f84122518db446abe379e6e868848a3 100644 (file)
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3414,9 +3414,9 @@ static bool MatchRotateHalf(SDValue Op, SDValue &Shift, SDValue &Mask) {
 //
 //     (or (shift1 X, Neg), (shift2 X, Pos))
 //
-// reduces to a rotate in direction shift2 by Pos and a rotate in direction
-// shift1 by Neg.  The range [0, OpSize) means that we only need to consider
-// shift amounts with defined behavior.
+// reduces to a rotate in direction shift2 by Pos or (equivalently) a rotate
+// in direction shift1 by Neg.  The range [0, OpSize) means that we only need
+// to consider shift amounts with defined behavior.
 static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {
   // If OpSize is a power of 2 then:
   //
@@ -3437,7 +3437,7 @@ static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {
   //
   // for all Neg and Pos.  Note that the (or ...) then invokes undefined
   // behavior if Pos == 0 (and consequently Neg == OpSize).
-  // 
+  //
   // We could actually use [A] whenever OpSize is a power of 2, but the
   // only extra cases that it would match are those uninteresting ones
   // where Neg and Pos are never in range at the same time.  E.g. for
@@ -3449,13 +3449,13 @@ static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {
   // always invokes undefined behavior for 32-bit X.
   //
   // Below, Mask == OpSize - 1 when using [A] and is all-ones otherwise.
-  unsigned LoBits = 0;
+  unsigned MaskLoBits = 0;
   if (Neg.getOpcode() == ISD::AND &&
       isPowerOf2_64(OpSize) &&
       Neg.getOperand(1).getOpcode() == ISD::Constant &&
       cast<ConstantSDNode>(Neg.getOperand(1))->getAPIntValue() == OpSize - 1) {
     Neg = Neg.getOperand(0);
-    LoBits = Log2_64(OpSize);
+    MaskLoBits = Log2_64(OpSize);
   }
 
   // Check whether Neg has the form (sub NegC, NegOp1) for some NegC and NegOp1.
@@ -3496,8 +3496,9 @@ static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {
     return false;
 
   // Now we just need to check that OpSize & Mask == Width & Mask.
-  if (LoBits)
-    return Width.getLoBits(LoBits) == 0;
+  if (MaskLoBits)
+    // Opsize & Mask is 0 since Mask is Opsize - 1.
+    return Width.getLoBits(MaskLoBits) == 0;
   return Width == OpSize;
 }