[DAGCombine] Slightly improve lowering of BUILD_VECTOR into a shuffle.

This handles the case of a BUILD_VECTOR being constructed out of elements extracted from a vector twice the size of the result vector. Previously this was always scalarized. Now, we try to construct a shuffle node that feeds on extract_subvectors.

This fixes PR15872 and provides a partial fix for PR21711.

Differential Revision: http://reviews.llvm.org/D6678

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224429 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetLowering.h

@@ -1531,6 +1531,15 @@ public:
                                                  Type *Ty) const {
     return false;
   }
+
+  /// Return true if EXTRACT_SUBVECTOR is cheap for this result type
+  /// with this index. This is needed because EXTRACT_SUBVECTOR usually
+  /// has custom lowering that depends on the index of the first element,
+  /// and only the target knows which lowering is cheap.
+  virtual bool isExtractSubvectorCheap(EVT ResVT, unsigned Index) const {
+    return false;
+  }
+
   //===--------------------------------------------------------------------===//
   // Runtime Library hooks
   //

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -10783,9 +10783,6 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
       SDValue ExtVal = Extract.getOperand(1);
       unsigned ExtIndex = cast<ConstantSDNode>(ExtVal)->getZExtValue();
       if (Extract.getOperand(0) == VecIn1) {
-        if (ExtIndex > VT.getVectorNumElements())
-          return SDValue();
-
         Mask.push_back(ExtIndex);
         continue;
       }
@@ -10805,20 +10802,34 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
       if (VecIn2.getNode())
         return SDValue();
 
-      // We only support widening of vectors which are half the size of the
-      // output registers. For example XMM->YMM widening on X86 with AVX.
-      if (VecIn1.getValueType().getSizeInBits()*2 != VT.getSizeInBits())
-        return SDValue();
-
       // If the input vector type has a different base type to the output
       // vector type, bail out.
       if (VecIn1.getValueType().getVectorElementType() !=
           VT.getVectorElementType())
         return SDValue();
 
-      // Widen the input vector by adding undef values.
-      VecIn1 = DAG.getNode(ISD::CONCAT_VECTORS, dl, VT,
-                           VecIn1, DAG.getUNDEF(VecIn1.getValueType()));
+      // If the input vector is too small, widen it.
+      // We only support widening of vectors which are half the size of the
+      // output registers. For example XMM->YMM widening on X86 with AVX.
+      EVT VecInT = VecIn1.getValueType();
+      if (VecInT.getSizeInBits() * 2 == VT.getSizeInBits()) {
+        // Widen the input vector by adding undef values.
+        VecIn1 = DAG.getNode(ISD::CONCAT_VECTORS, dl, VT,
+                             VecIn1, DAG.getUNDEF(VecIn1.getValueType()));
+      } else if (VecInT.getSizeInBits() == VT.getSizeInBits() * 2) {
+        // If the input vector is too large, try to split it.
+        if (!TLI.isExtractSubvectorCheap(VT, VT.getVectorNumElements()))
+          return SDValue();
+        
+        // Try to replace VecIn1 with two extract_subvectors
+        // No need to update the masks, they should still be correct.
+        VecIn2 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, VT, VecIn1, 
+          DAG.getConstant(VT.getVectorNumElements(), TLI.getVectorIdxTy()));
+        VecIn1 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, VT, VecIn1,
+          DAG.getConstant(0, TLI.getVectorIdxTy()));
+        UsesZeroVector = false;
+      } else 
+        return SDValue();
     }
 
     if (UsesZeroVector)

lib/Target/X86/X86ISelLowering.cpp

@@ -3851,6 +3851,14 @@ bool X86TargetLowering::shouldConvertConstantLoadToIntImm(const APInt &Imm,
   return true;
 }
 
+bool X86TargetLowering::isExtractSubvectorCheap(EVT ResVT, 
+                                                unsigned Index) const {
+  if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))
+    return false;
+
+  return (Index == 0 || Index == ResVT.getVectorNumElements());
+}
+
 /// isUndefOrInRange - Return true if Val is undef or if its value falls within
 /// the specified range (L, H].
 static bool isUndefOrInRange(int Val, int Low, int Hi) {

lib/Target/X86/X86ISelLowering.h

@@ -791,6 +791,10 @@ namespace llvm {
     bool shouldConvertConstantLoadToIntImm(const APInt &Imm,
                                            Type *Ty) const override;
 
+    /// Return true if EXTRACT_SUBVECTOR is cheap for this result type
+    /// with this index.
+    bool isExtractSubvectorCheap(EVT ResVT, unsigned Index) const override;
+
     /// Intel processors have a unified instruction and data cache
     const char * getClearCacheBuiltinName() const override {
       return nullptr; // nothing to do, move along.

test/CodeGen/X86/vec_extract-avx.ll

@@ -0,0 +1,82 @@
+target triple = "x86_64-unknown-unknown"
+
+; RUN: llc < %s -march=x86-64 -mattr=+avx | FileCheck %s
+
+; When extracting multiple consecutive elements from a larger
+; vector into a smaller one, do it efficiently. We should use
+; an EXTRACT_SUBVECTOR node internally rather than a bunch of
+; single element extractions. 
+
+; Extracting the low elements only requires using the right kind of store.
+define void @low_v8f32_to_v4f32(<8 x float> %v, <4 x float>* %ptr) {
+  %ext0 = extractelement <8 x float> %v, i32 0
+  %ext1 = extractelement <8 x float> %v, i32 1
+  %ext2 = extractelement <8 x float> %v, i32 2
+  %ext3 = extractelement <8 x float> %v, i32 3
+  %ins0 = insertelement <4 x float> undef, float %ext0, i32 0
+  %ins1 = insertelement <4 x float> %ins0, float %ext1, i32 1
+  %ins2 = insertelement <4 x float> %ins1, float %ext2, i32 2
+  %ins3 = insertelement <4 x float> %ins2, float %ext3, i32 3
+  store <4 x float> %ins3, <4 x float>* %ptr, align 16
+  ret void
+
+; CHECK-LABEL: low_v8f32_to_v4f32
+; CHECK: vmovaps
+; CHECK-NEXT: vzeroupper
+; CHECK-NEXT: retq
+}
+
+; Extracting the high elements requires just one AVX instruction. 
+define void @high_v8f32_to_v4f32(<8 x float> %v, <4 x float>* %ptr) {
+  %ext0 = extractelement <8 x float> %v, i32 4
+  %ext1 = extractelement <8 x float> %v, i32 5
+  %ext2 = extractelement <8 x float> %v, i32 6
+  %ext3 = extractelement <8 x float> %v, i32 7
+  %ins0 = insertelement <4 x float> undef, float %ext0, i32 0
+  %ins1 = insertelement <4 x float> %ins0, float %ext1, i32 1
+  %ins2 = insertelement <4 x float> %ins1, float %ext2, i32 2
+  %ins3 = insertelement <4 x float> %ins2, float %ext3, i32 3
+  store <4 x float> %ins3, <4 x float>* %ptr, align 16
+  ret void
+
+; CHECK-LABEL: high_v8f32_to_v4f32
+; CHECK: vextractf128
+; CHECK-NEXT: vzeroupper
+; CHECK-NEXT: retq
+}
+
+; Make sure element type doesn't alter the codegen. Note that
+; if we were actually using the vector in this function and
+; have AVX2, we should generate vextracti128 (the int version).
+define void @high_v8i32_to_v4i32(<8 x i32> %v, <4 x i32>* %ptr) {
+  %ext0 = extractelement <8 x i32> %v, i32 4
+  %ext1 = extractelement <8 x i32> %v, i32 5
+  %ext2 = extractelement <8 x i32> %v, i32 6
+  %ext3 = extractelement <8 x i32> %v, i32 7
+  %ins0 = insertelement <4 x i32> undef, i32 %ext0, i32 0
+  %ins1 = insertelement <4 x i32> %ins0, i32 %ext1, i32 1
+  %ins2 = insertelement <4 x i32> %ins1, i32 %ext2, i32 2
+  %ins3 = insertelement <4 x i32> %ins2, i32 %ext3, i32 3
+  store <4 x i32> %ins3, <4 x i32>* %ptr, align 16
+  ret void
+
+; CHECK-LABEL: high_v8i32_to_v4i32
+; CHECK: vextractf128
+; CHECK-NEXT: vzeroupper
+; CHECK-NEXT: retq
+}
+
+; Make sure that element size doesn't alter the codegen.
+define void @high_v4f64_to_v2f64(<4 x double> %v, <2 x double>* %ptr) {
+  %ext0 = extractelement <4 x double> %v, i32 2
+  %ext1 = extractelement <4 x double> %v, i32 3
+  %ins0 = insertelement <2 x double> undef, double %ext0, i32 0
+  %ins1 = insertelement <2 x double> %ins0, double %ext1, i32 1
+  store <2 x double> %ins1, <2 x double>* %ptr, align 16
+  ret void
+
+; CHECK-LABEL: high_v4f64_to_v2f64
+; CHECK: vextractf128
+; CHECK-NEXT: vzeroupper
+; CHECK-NEXT: retq
+}

test/CodeGen/X86/vector-shuffle-combining.ll

@@ -1552,6 +1552,37 @@ define <4 x i32> @combine_test20(<4 x i32> %a, <4 x i32> %b) {
   ret <4 x i32> %2
 }
 
+define <4 x i32> @combine_test21(<8 x i32> %a, <4 x i32>* %ptr) {
+; SSE-LABEL: combine_test21:
+; SSE:       # BB#0:
+; SSE-NEXT:    movdqa %xmm0, %xmm2
+; SSE-NEXT:    punpcklqdq  {{.*#+}} xmm2 = xmm2[0],xmm1[0]
+; SSE-NEXT:    punpckhqdq  {{.*#+}} xmm0 = xmm0[1],xmm1[1]
+; SSE-NEXT:    movdqa %xmm2,
+; SSE-NEXT:    retq
+;
+; AVX1-LABEL: combine_test21:
+; AVX1:       # BB#0:
+; AVX1-NEXT:    vextractf128 $1, %ymm0, %xmm1
+; AVX1-NEXT:    vpunpcklqdq  {{.*#+}} xmm2 = xmm0[0],xmm1[0]
+; AVX1-NEXT:    vpunpckhqdq  {{.*#+}} xmm0 = xmm0[1],xmm1[1]
+; AVX1-NEXT:    movdqa %xmm2,
+; AVX1-NEXT:    vzeroupper
+; AVX1-NEXT:    retq
+;
+; AVX2-LABEL: combine_test21:
+; AVX2:       # BB#0:
+; AVX2-NEXT:    vextracti128 $1, %ymm0, %xmm1
+; AVX2-NEXT:    vpunpcklqdq  {{.*#+}} xmm2 = xmm0[0],xmm1[0]
+; AVX2-NEXT:    vpunpckhqdq  {{.*#+}} xmm0 = xmm0[1],xmm1[1]
+; AVX2-NEXT:    movdqa %xmm2,
+; AVX2-NEXT:    vzeroupper
+; AVX2-NEXT:    retq
+  %1 = shufflevector <8 x i32> %a, <8 x i32> %a, <4 x i32> <i32 0, i32 1, i32 4, i32 5>
+  %2 = shufflevector <8 x i32> %a, <8 x i32> %a, <4 x i32> <i32 2, i32 3, i32 6, i32 7>
+  store <4 x i32> %1, <4 x i32>* %ptr, align 16
+  ret <4 x i32> %2
+}
 
 ; Check some negative cases.
 ; FIXME: Do any of these really make sense? Are they redundant with the above tests?