[RISCV] Implement isLegalAddressingMode for RISC-V

This has no impact on codegen for the current RISC-V unit tests or my small 
benchmark set and very minor changes in a few programs in the GCC torture 
suite. Based on this, I haven't been able to produce a representative test 
program that demonstrates a benefit from isLegalAddressingMode. I'm committing 
the patch anyway, on the basis that presenting accurate information to the 
target-independent code is preferable to relying on incorrect generic 
assumptions.

llvm-svn: 330932

lib/Target/RISCV/RISCVISelLowering.cpp

@@ -157,6 +157,32 @@ EVT RISCVTargetLowering::getSetCCResultType(const DataLayout &DL, LLVMContext &,
   return VT.changeVectorElementTypeToInteger();
 }
 
+bool RISCVTargetLowering::isLegalAddressingMode(const DataLayout &DL,
+                                                const AddrMode &AM, Type *Ty,
+                                                unsigned AS,
+                                                Instruction *I) const {
+  // No global is ever allowed as a base.
+  if (AM.BaseGV)
+    return false;
+
+  // Require a 12-bit signed offset.
+  if (!isInt<12>(AM.BaseOffs))
+    return false;
+
+  switch (AM.Scale) {
+  case 0: // "r+i" or just "i", depending on HasBaseReg.
+    break;
+  case 1:
+    if (!AM.HasBaseReg) // allow "r+i".
+      break;
+    return false; // disallow "r+r" or "r+r+i".
+  default:
+    return false;
+  }
+
+  return true;
+}
+
 // Changes the condition code and swaps operands if necessary, so the SetCC
 // operation matches one of the comparisons supported directly in the RISC-V
 // ISA.

lib/Target/RISCV/RISCVISelLowering.h

@@ -39,6 +39,10 @@ public:
   explicit RISCVTargetLowering(const TargetMachine &TM,
                                const RISCVSubtarget &STI);
 
+  bool isLegalAddressingMode(const DataLayout &DL, const AddrMode &AM, Type *Ty,
+                             unsigned AS,
+                             Instruction *I = nullptr) const override;
+
   // Provide custom lowering hooks for some operations.
   SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;