[x86] improve comments for SHRUNKBLEND node creation; NFC

llvm-svn: 294344

2 files changed, 27 insertions, 28 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 7e62c99b773..d293d6baa99 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -29520,39 +29520,38 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
     if (TLO.ShrinkDemandedConstant(Cond, DemandedMask) ||
         TLI.SimplifyDemandedBits(Cond, DemandedMask, KnownZero, KnownOne,
                                  TLO)) {
-      // If we changed the computation somewhere in the DAG, this change
-      // will affect all users of Cond.
-      // Make sure it is fine and update all the nodes so that we do not
-      // use the generic VSELECT anymore. Otherwise, we may perform
-      // wrong optimizations as we messed up with the actual expectation
+      // If we changed the computation somewhere in the DAG, this change will
+      // affect all users of Cond. Make sure it is fine and update all the nodes
+      // so that we do not use the generic VSELECT anymore. Otherwise, we may
+      // perform wrong optimizations as we messed with the actual expectation
       // for the vector boolean values.
       if (Cond != TLO.Old) {
-        // Check all uses of that condition operand to check whether it will be
-        // consumed by non-BLEND instructions, which may depend on all bits are
-        // set properly.
-        for (SDNode *U : Cond->uses())
+        // Check all uses of the condition operand to check whether it will be
+        // consumed by non-BLEND instructions. Those may require that all bits
+        // are set properly.
+        for (SDNode *U : Cond->uses()) {
+          // TODO: Add other opcodes eventually lowered into BLEND.
           if (U->getOpcode() != ISD::VSELECT)
-            // TODO: Add other opcodes eventually lowered into BLEND.
             return SDValue();
+        }
 
-        // Update all the users of the condition, before committing the change,
-        // so that the VSELECT optimizations that expect the correct vector
-        // boolean value will not be triggered.
-        for (SDNode *U : Cond->uses())
-          DAG.ReplaceAllUsesOfValueWith(
-              SDValue(U, 0),
-              DAG.getNode(X86ISD::SHRUNKBLEND, SDLoc(U), U->getValueType(0),
-                          Cond, U->getOperand(1), U->getOperand(2)));
+        // Update all users of the condition before committing the change, so
+        // that the VSELECT optimizations that expect the correct vector boolean
+        // value will not be triggered.
+        for (SDNode *U : Cond->uses()) {
+          SDValue SB = DAG.getNode(X86ISD::SHRUNKBLEND, SDLoc(U),
+                                   U->getValueType(0), Cond, U->getOperand(1),
+                                   U->getOperand(2));
+          DAG.ReplaceAllUsesOfValueWith(SDValue(U, 0), SB);
+        }
         DCI.CommitTargetLoweringOpt(TLO);
         return SDValue();
       }
-      // At this point, only Cond is changed. Change the condition
-      // just for N to keep the opportunity to optimize all other
-      // users their own way.
-      DAG.ReplaceAllUsesOfValueWith(
-          SDValue(N, 0),
-          DAG.getNode(X86ISD::SHRUNKBLEND, SDLoc(N), N->getValueType(0),
-                      TLO.New, N->getOperand(1), N->getOperand(2)));
+      // Only Cond (rather than other nodes in the computation chain) was
+      // changed. Change the condition just for N to keep the opportunity to
+      // optimize all other users their own way.
+      SDValue SB = DAG.getNode(X86ISD::SHRUNKBLEND, DL, VT, TLO.New, LHS, RHS);
+      DAG.ReplaceAllUsesOfValueWith(SDValue(N, 0), SB);
       return SDValue();
     }
   }
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index 9969c909347..4797a3dca38 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -195,9 +195,9 @@ namespace llvm {
       /// Blend where the selector is an immediate.
       BLENDI,
 
-      /// Blend where the condition has been shrunk.
-      /// This is used to emphasize that the condition mask is
-      /// no more valid for generic VSELECT optimizations.
+      /// Dynamic (non-constant condition) vector blend where only the sign bits
+      /// of the condition elements are used. This is used to enforce that the
+      /// condition mask is not valid for generic VSELECT optimizations.
       SHRUNKBLEND,
 
       /// Combined add and sub on an FP vector.