[SelectionDAG][x86] limit post-legalization store merging by type

The proposal in D62498 showed that x86 would benefit from vector
store splitting, but that may conflict with the generic DAG
combiner's store merging transforms.

Add memory type to the existing TLI hook that enables the merging
transforms, so we can limit those changes to scalars only for x86.

llvm-svn: 362507

include/llvm/CodeGen/TargetLowering.h

@@ -435,10 +435,12 @@ public:
     return false;
   }
 
-  /// Allow store merging after legalization in addition to before legalization.
-  /// This may catch stores that do not exist earlier (eg, stores created from
-  /// intrinsics).
-  virtual bool mergeStoresAfterLegalization() const { return true; }
+  /// Allow store merging for the specified type after legalization in addition
+  /// to before legalization. This may transform stores that do not exist
+  /// earlier (for example, stores created from intrinsics).
+  virtual bool mergeStoresAfterLegalization(EVT MemVT) const {
+    return true;
+  }
 
   /// Returns if it's reasonable to merge stores to MemVT size.
   virtual bool canMergeStoresTo(unsigned AS, EVT MemVT,

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -16085,7 +16085,7 @@ SDValue DAGCombiner::visitSTORE(SDNode *N) {
   // Always perform this optimization before types are legal. If the target
   // prefers, also try this after legalization to catch stores that were created
   // by intrinsics or other nodes.
-  if (!LegalTypes || (TLI.mergeStoresAfterLegalization())) {
+  if (!LegalTypes || (TLI.mergeStoresAfterLegalization(ST->getMemoryVT()))) {
     while (true) {
       // There can be multiple store sequences on the same chain.
       // Keep trying to merge store sequences until we are unable to do so

lib/Target/AMDGPU/AMDGPUISelLowering.h

@@ -234,7 +234,7 @@ public:
   // MergeConsecutiveStores() merges two stores; LegalizeStoreOps() un-merges;
   // MergeConsecutiveStores() re-merges, etc. ) to warrant turning it off for
   // now.
-  bool mergeStoresAfterLegalization() const override { return false; }
+  bool mergeStoresAfterLegalization(EVT) const override { return false; }
 
   bool isFsqrtCheap(SDValue Operand, SelectionDAG &DAG) const override {
     return true;

lib/Target/X86/X86ISelLowering.h

@@ -799,7 +799,11 @@ namespace llvm {
     /// This method returns the name of a target specific DAG node.
     const char *getTargetNodeName(unsigned Opcode) const override;
 
-    bool mergeStoresAfterLegalization() const override { return true; }
+    /// Do not merge vector stores after legalization because that may conflict
+    /// with x86-specific store splitting optimizations.
+    bool mergeStoresAfterLegalization(EVT MemVT) const override {
+      return !MemVT.isVector();
+    }
 
     bool canMergeStoresTo(unsigned AddressSpace, EVT MemVT,
                           const SelectionDAG &DAG) const override;

test/CodeGen/X86/vector-trunc-widen.ll

@@ -2076,8 +2076,8 @@ define void @store_merge_split(<8 x i32> %w1, <8 x i32> %w2, i64 %idx, <8 x i16>
 ; AVX2-NEXT:    vpshufb %ymm2, %ymm1, %ymm1
 ; AVX2-NEXT:    vpermq {{.*#+}} ymm1 = ymm1[0,2,2,3]
 ; AVX2-NEXT:    shlq $4, %rdi
-; AVX2-NEXT:    vinserti128 $1, %xmm1, %ymm0, %ymm0
-; AVX2-NEXT:    vmovdqu %ymm0, (%rsi,%rdi)
+; AVX2-NEXT:    vmovdqu %xmm0, (%rsi,%rdi)
+; AVX2-NEXT:    vmovdqu %xmm1, 16(%rsi,%rdi)
 ; AVX2-NEXT:    vzeroupper
 ; AVX2-NEXT:    retq
 ;