[PATCH] [AArch64] Fix a build_vector pattern fail caused by defect in isBuildVectorAllZeros().

[Kevin Qin]

Hi t.p.northover,

Comments inside patch show the details.  Please review, thanks.

http://reviews.llvm.org/D4228

Files:
  lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  test/CodeGen/AArch64/arm64-build-vector.ll

Index: lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -148,33 +148,34 @@
 
   if (N->getOpcode() != ISD::BUILD_VECTOR) return false;
 
-  unsigned i = 0, e = N->getNumOperands();
-
-  // Skip over all of the undef values.
-  while (i != e && N->getOperand(i).getOpcode() == ISD::UNDEF)
-    ++i;
+  bool IsAllUndef = true;
+  for (unsigned i = 0, e = N->getNumOperands(); i < e; ++i) {
+    if (N->getOperand(i).getOpcode() == ISD::UNDEF)
+      continue;
+    IsAllUndef = false;
+    // Do not accept build_vectors that aren't all constants or which have non-0
+    // elements. We have to be a bit careful here, as the type of the constant
+    // may not be the same as the type of the vector elements due to type
+    // legalization (the elements are promoted to a legal type for the target
+    // and a vector of a type may be legal when the base element type is not).
+    // We only want to check enough bits to cover the vector elements, because
+    // we care if the resultant vector is all zeros, not whether the individual
+    // constants are.
+    SDValue Zero = N->getOperand(i);
+    unsigned EltSize = N->getValueType(0).getVectorElementType().getSizeInBits();
+    if (ConstantSDNode *CN = dyn_cast<ConstantSDNode>(Zero)) {
+      if (CN->getAPIntValue().countTrailingZeros() < EltSize)
+        return false;
+    } else if (ConstantFPSDNode *CFPN = dyn_cast<ConstantFPSDNode>(Zero)) {
+      if (CFPN->getValueAPF().bitcastToAPInt().countTrailingZeros() < EltSize)
+        return false;
+    } else
+      return false;
+  }
 
   // Do not accept an all-undef vector.
-  if (i == e) return false;
-
-  // Do not accept build_vectors that aren't all constants or which have non-0
-  // elements.
-  SDValue Zero = N->getOperand(i);
-  if (ConstantSDNode *CN = dyn_cast<ConstantSDNode>(Zero)) {
-    if (!CN->isNullValue())
-      return false;
-  } else if (ConstantFPSDNode *CFPN = dyn_cast<ConstantFPSDNode>(Zero)) {
-    if (!CFPN->getValueAPF().isPosZero())
-      return false;
-  } else
+  if (IsAllUndef)
     return false;
-
-  // Okay, we have at least one 0 value, check to see if the rest match or are
-  // undefs.
-  for (++i; i != e; ++i)
-    if (N->getOperand(i) != Zero &&
-        N->getOperand(i).getOpcode() != ISD::UNDEF)
-      return false;
   return true;
 }
 
Index: test/CodeGen/AArch64/arm64-build-vector.ll
===================================================================
--- test/CodeGen/AArch64/arm64-build-vector.ll
+++ test/CodeGen/AArch64/arm64-build-vector.ll
@@ -33,3 +33,13 @@
   %4 = insertelement <4 x float> %3, float %d, i32 3
   ret <4 x float> %4
 }
+
+define <8 x i16> @build_all_zero(<8 x i16> %a) #1 {
+; CHECK-LABEL: build_all_zero:
+; CHECK: movn	w[[GREG:[0-9]+]], #0x517f
+; CHECK-NEXT:	fmov	s[[FREG:[0-9]+]], w[[GREG]]
+; CHECK-NEXT:	mul.8h	v0, v0, v[[FREG]]
+  %b = add <8 x i16> %a, <i16 -32768, i16 undef, i16 undef, i16 undef, i16 undef, i16 undef, i16 undef, i16 undef>
+  %c = mul <8 x i16> %b, <i16 -20864, i16 undef, i16 undef, i16 undef, i16 undef, i16 undef, i16 undef, i16 undef>
+  ret <8 x i16> %c
+  }
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