[ARM] Implement isExtractSubvectorCheap.

See https://reviews.llvm.org/D6678 for the history of
isExtractSubvectorCheap. Essentially the same considerations apply
to ARM.

This temporarily breaks the formation of vpadd/vpaddl in certain cases;
AddCombineToVPADDL essentially assumes that we won't form VUZP shuffles.
See https://reviews.llvm.org/D27779 for followup fix.

Differential Revision: https://reviews.llvm.org/D27774

llvm-svn: 290198

2 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 2c88c75d886..5895cad1922 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -12921,6 +12921,14 @@ bool ARMTargetLowering::shouldConvertConstantLoadToIntImm(const APInt &Imm,
   return true;
 }
 
+bool ARMTargetLowering::isExtractSubvectorCheap(EVT ResVT,
+                                                unsigned Index) const {
+  if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))
+    return false;
+
+  return (Index == 0 || Index == ResVT.getVectorNumElements());
+}
+
 Instruction* ARMTargetLowering::makeDMB(IRBuilder<> &Builder,
                                         ARM_MB::MemBOpt Domain) const {
   Module *M = Builder.GetInsertBlock()->getParent()->getParent();
diff --git a/llvm/lib/Target/ARM/ARMISelLowering.h b/llvm/lib/Target/ARM/ARMISelLowering.h
index 6348edbfb0d..5255d82d647 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.h
+++ b/llvm/lib/Target/ARM/ARMISelLowering.h
@@ -431,6 +431,10 @@ 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;
+
     /// \brief Returns true if an argument of type Ty needs to be passed in a
     /// contiguous block of registers in calling convention CallConv.
     bool functionArgumentNeedsConsecutiveRegisters(