[globalisel][legalizer] G_LOAD/G_STORE NarrowScalar should not emit G_GEP x, 0.

Summary:
When legalizing G_LOAD/G_STORE using NarrowScalar, we should avoid emitting
	%0 = G_CONSTANT ty 0
	%1 = G_GEP %x, %0
since it's cheaper to not emit the redundant instructions than it is to fold them
away later.

Reviewers: qcolombet, t.p.northover, ab, rovka, aditya_nandakumar, kristof.beyls

Reviewed By: qcolombet

Subscribers: javed.absar, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D32746

llvm-svn: 305340

2 files changed, 33 insertions, 12 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index ef5818dabe2..e05b355254c 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -237,17 +237,18 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
     unsigned NarrowSize = NarrowTy.getSizeInBits();
     int NumParts =
         MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() / NarrowSize;
-    LLT NarrowPtrTy = LLT::pointer(
-        MRI.getType(MI.getOperand(1).getReg()).getAddressSpace(), NarrowSize);
+    LLT OffsetTy = LLT::scalar(
+        MRI.getType(MI.getOperand(1).getReg()).getScalarSizeInBits());
 
     SmallVector<unsigned, 2> DstRegs;
     for (int i = 0; i < NumParts; ++i) {
       unsigned DstReg = MRI.createGenericVirtualRegister(NarrowTy);
-      unsigned SrcReg = MRI.createGenericVirtualRegister(NarrowPtrTy);
-      unsigned Offset = MRI.createGenericVirtualRegister(LLT::scalar(64));
+      unsigned SrcReg = 0;
+      unsigned Adjustment = i * NarrowSize / 8;
+
+      MIRBuilder.materializeGEP(SrcReg, MI.getOperand(1).getReg(), OffsetTy,
+                                Adjustment);
 
-      MIRBuilder.buildConstant(Offset, i * NarrowSize / 8);
-      MIRBuilder.buildGEP(SrcReg, MI.getOperand(1).getReg(), Offset);
       // TODO: This is conservatively correct, but we probably want to split the
       // memory operands in the future.
       MIRBuilder.buildLoad(DstReg, SrcReg, **MI.memoperands_begin());
@@ -263,17 +264,19 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
     unsigned NarrowSize = NarrowTy.getSizeInBits();
     int NumParts =
         MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() / NarrowSize;
-    LLT NarrowPtrTy = LLT::pointer(
-        MRI.getType(MI.getOperand(1).getReg()).getAddressSpace(), NarrowSize);
+    LLT OffsetTy = LLT::scalar(
+        MRI.getType(MI.getOperand(1).getReg()).getScalarSizeInBits());
 
     SmallVector<unsigned, 2> SrcRegs;
     extractParts(MI.getOperand(0).getReg(), NarrowTy, NumParts, SrcRegs);
 
     for (int i = 0; i < NumParts; ++i) {
-      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowPtrTy);
-      unsigned Offset = MRI.createGenericVirtualRegister(LLT::scalar(64));
-      MIRBuilder.buildConstant(Offset, i * NarrowSize / 8);
-      MIRBuilder.buildGEP(DstReg, MI.getOperand(1).getReg(), Offset);
+      unsigned DstReg = 0;
+      unsigned Adjustment = i * NarrowSize / 8;
+
+      MIRBuilder.materializeGEP(DstReg, MI.getOperand(1).getReg(), OffsetTy,
+                                Adjustment);
+
       // TODO: This is conservatively correct, but we probably want to split the
       // memory operands in the future.
       MIRBuilder.buildStore(SrcRegs[i], DstReg, **MI.memoperands_begin());
diff --git a/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp b/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
index 54ef7e5c5a1..79d312fb52c 100644
--- a/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
@@ -191,6 +191,24 @@ MachineInstrBuilder MachineIRBuilder::buildGEP(unsigned Res, unsigned Op0,
       .addUse(Op1);
 }
 
+Optional<MachineInstrBuilder>
+MachineIRBuilder::materializeGEP(unsigned &Res, unsigned Op0,
+                                 const LLT &ValueTy, uint64_t Value) {
+  assert(Res == 0 && "Res is a result argument");
+  assert(ValueTy.isScalar()  && "invalid offset type");
+
+  if (Value == 0) {
+    Res = Op0;
+    return None;
+  }
+
+  Res = MRI->createGenericVirtualRegister(MRI->getType(Op0));
+  unsigned TmpReg = MRI->createGenericVirtualRegister(ValueTy);
+
+  buildConstant(TmpReg, Value);
+  return buildGEP(Res, Op0, TmpReg);
+}
+
 MachineInstrBuilder MachineIRBuilder::buildPtrMask(unsigned Res, unsigned Op0,
                                                    uint32_t NumBits) {
   assert(MRI->getType(Res).isPointer() &&