[RISCV] Assume no-op addrspacecasts by default

[frasercrmck]

Summary

To support OpenCL, which typically uses SPIR as an IR, non-zero address
spaces must be accounted for. This patch makes the RISC-V target assume
no-op address space casts across the board, which effectively removes
the need to support addrspacecast instructions in the backend.

For a RISC-V implementation with different configurations or specialized
address spaces where casts aren't no-ops, the function can be adjusted
as required.

llvm/lib/Target/RISCV/RISCVTargetMachine.h

public:
 
   TargetPassConfig *createPassConfig(PassManagerBase &PM) override;
 
   TargetLoweringObjectFile *getObjFileLowering() const override {
     return TLOF.get();
   }
 
   TargetTransformInfo getTargetTransformInfo(const Function &F) override;
+
+  virtual bool isNoopAddrSpaceCast(unsigned SrcAS,
+                                   unsigned DstAS) const override;
 };
 }
 
 #endif

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

RISCVTargetMachine::getSubtargetImpl(const Function &F) const {
   return I.get();
 }
 
 TargetTransformInfo
 RISCVTargetMachine::getTargetTransformInfo(const Function &F) {
   return TargetTransformInfo(RISCVTTIImpl(this, F));
 }
 
+// A RISC-V hart has a single byte-addressable address space of 2^XLEN bytes
+// for all memory accesses, so it is reasonable to assume that an
+// implementation has no-op address space casts. If an implementation makes a
+// change to this, they can override it here.
+bool RISCVTargetMachine::isNoopAddrSpaceCast(unsigned SrcAS,
+                                             unsigned DstAS) const {
+  return true;
+}
+
 namespace {
 class RISCVPassConfig : public TargetPassConfig {
 public:
   RISCVPassConfig(RISCVTargetMachine &TM, PassManagerBase &PM)
       : TargetPassConfig(TM, PM) {}
 
   RISCVTargetMachine &getRISCVTargetMachine() const {
     return getTM<RISCVTargetMachine>();

llvm/test/CodeGen/RISCV/addrspacecast.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
+; RUN:   | FileCheck %s --check-prefix=RV32I
+; RUN: llc -mtriple=riscv64 -verify-machineinstrs < %s \
+; RUN:   | FileCheck %s --check-prefix=RV64I
+
+define void @cast0(i32 addrspace(1)* %ptr) {
+; RV32I-LABEL: cast0:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    sw zero, 0(a0)
+; RV32I-NEXT:    ret
+;
+; RV64I-LABEL: cast0:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    sw zero, 0(a0)
+; RV64I-NEXT:    ret
+  %ptr0 = addrspacecast i32 addrspace(1)* %ptr to i32 addrspace(0)*
+  store i32 0, i32* %ptr0
+  ret void
+}
+
+define void @cast1(i32* %ptr) {
+; RV32I-LABEL: cast1:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    addi sp, sp, -16
+; RV32I-NEXT:    .cfi_def_cfa_offset 16
+; RV32I-NEXT:    sw ra, 12(sp) # 4-byte Folded Spill
+; RV32I-NEXT:    .cfi_offset ra, -4
+; RV32I-NEXT:    call foo@plt
+; RV32I-NEXT:    lw ra, 12(sp) # 4-byte Folded Reload
+; RV32I-NEXT:    addi sp, sp, 16
+; RV32I-NEXT:    ret
+;
+; RV64I-LABEL: cast1:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    addi sp, sp, -16
+; RV64I-NEXT:    .cfi_def_cfa_offset 16
+; RV64I-NEXT:    sd ra, 8(sp) # 8-byte Folded Spill
+; RV64I-NEXT:    .cfi_offset ra, -8
+; RV64I-NEXT:    call foo@plt
+; RV64I-NEXT:    ld ra, 8(sp) # 8-byte Folded Reload
+; RV64I-NEXT:    addi sp, sp, 16
+; RV64I-NEXT:    ret
+  %castptr = addrspacecast i32* %ptr to i32 addrspace(10)*
+  call void @foo(i32 addrspace(10)* %castptr)
+  ret void
+}
+
+declare void @foo(i32 addrspace(10)*)