[x86] Generalize BuildVectorSDNode::getConstantSplatValue to work for

any constant, constant FP, or undef splat and to tolerate any undef
lanes in a splat, then replace all uses of isSplatVector in X86's
lowering with it.

This fixes issues where undef lanes in an otherwise splat vector would
prevent the splat logic from firing. It is a touch more awkward to use
this interface, but it is much more accurate. Suggestions for better
interface structuring welcome.

With this fix, the code generated with the widening legalization
strategy for widen_cast-4.ll is *dramatically* improved as the special
lowering strategies for a v16i8 SRA kick in even though the high lanes
are undef.

We also get a slightly different choice for broadcasting an aligned
memory location, and use vpshufd instead of vbroadcastss. This looks
like a minor win for pipelining and domain crossing, but a minor loss
for the number of micro-ops. I suspect its a wash, but folks can easily
tweak the lowering if they want.

llvm-svn: 212324

3 files changed, 44 insertions, 31 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7198203036b..9a91dcc341b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -654,13 +654,12 @@ static ConstantSDNode *isConstOrConstSplat(SDValue N) {
   if (ConstantSDNode *CN = dyn_cast<ConstantSDNode>(N))
     return CN;
 
-  if (BuildVectorSDNode *BV = dyn_cast<BuildVectorSDNode>(N)) {
-    ConstantSDNode *CN = BV->getConstantSplatValue();
-
-    // BuildVectors can truncate their operands. Ignore that case here.
-    if (CN && CN->getValueType(0) == N.getValueType().getScalarType())
-      return CN;
-  }
+  if (BuildVectorSDNode *BV = dyn_cast<BuildVectorSDNode>(N))
+    if (SDValue Splat = BV->getConstantSplatValue())
+      if (auto *CN = dyn_cast<ConstantSDNode>(Splat))
+        // BuildVectors can truncate their operands. Ignore that case here.
+        if (CN->getValueType(0) == N.getValueType().getScalarType())
+          return CN;
 
   return nullptr;
 }
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 3a8a5f9601f..fb7d1b18d8b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -6603,16 +6603,28 @@ bool BuildVectorSDNode::isConstantSplat(APInt &SplatValue,
   return true;
 }
 
-ConstantSDNode *BuildVectorSDNode::getConstantSplatValue() const {
-  SDValue Op0 = getOperand(0);
-  if (Op0.getOpcode() != ISD::Constant)
-    return nullptr;
+SDValue BuildVectorSDNode::getConstantSplatValue() const {
+  SDValue Splatted;
+  for (unsigned i = 0, e = getNumOperands(); i != e; ++i) {
+    SDValue Op = getOperand(i);
+    if (Op.getOpcode() == ISD::UNDEF)
+      continue;
+    if (Op.getOpcode() != ISD::Constant && Op.getOpcode() != ISD::ConstantFP)
+      return SDValue();
 
-  for (unsigned i = 1, e = getNumOperands(); i != e; ++i)
-    if (getOperand(i) != Op0)
-      return nullptr;
+    if (!Splatted)
+      Splatted = Op;
+    else if (Splatted != Op)
+      return SDValue();
+  }
+
+  if (!Splatted) {
+    assert(getOperand(0).getOpcode() == ISD::UNDEF &&
+           "Can only have a splat without a constant for all undefs.");
+    return getOperand(0);
+  }
 
-  return cast<ConstantSDNode>(Op0);
+  return Splatted;
 }
 
 bool BuildVectorSDNode::isConstant() const {
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index ad91d4a87c1..1b3e42848bf 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1152,14 +1152,15 @@ bool TargetLowering::isConstTrueVal(const SDNode *N) const {
 
   bool IsVec = false;
   const ConstantSDNode *CN = dyn_cast<ConstantSDNode>(N);
-  if (!CN) {
-    const BuildVectorSDNode *BV = dyn_cast<BuildVectorSDNode>(N);
-    if (!BV)
-      return false;
-
-    IsVec = true;
-    CN = BV->getConstantSplatValue();
-  }
+  if (!CN)
+    if (auto *BV = dyn_cast<BuildVectorSDNode>(N))
+      if (SDValue Splat = BV->getConstantSplatValue())
+        if (auto *SplatCN = dyn_cast<ConstantSDNode>(Splat)) {
+          IsVec = true;
+          CN = SplatCN;
+        }
+  if (!CN)
+    return false;
 
   switch (getBooleanContents(IsVec)) {
   case UndefinedBooleanContent:
@@ -1179,14 +1180,15 @@ bool TargetLowering::isConstFalseVal(const SDNode *N) const {
 
   bool IsVec = false;
   const ConstantSDNode *CN = dyn_cast<ConstantSDNode>(N);
-  if (!CN) {
-    const BuildVectorSDNode *BV = dyn_cast<BuildVectorSDNode>(N);
-    if (!BV)
-      return false;
-
-    IsVec = true;
-    CN = BV->getConstantSplatValue();
-  }
+  if (!CN)
+    if (auto *BV = dyn_cast<BuildVectorSDNode>(N))
+      if (SDValue Splat = BV->getConstantSplatValue())
+        if (auto *SplatCN = dyn_cast<ConstantSDNode>(Splat)) {
+          IsVec = true;
+          CN = SplatCN;
+        }
+  if (!CN)
+    return false;
 
   if (getBooleanContents(IsVec) == UndefinedBooleanContent)
     return !CN->getAPIntValue()[0];