[clang][CodeGen] Add query for a target's flat address space

clang/include/clang/Basic/TargetInfo.h

@@ -1759,6 +1759,17 @@ class TargetInfo : public TransferrableTargetInfo,
  return 0;
  }
 
+ /// \returns If a target exposes a flat ptr address space (a flat ptr is a ptr
+ /// that can be casted to / from all other target address spaces), then return
+ /// it. Otherwise, return std::nullopt and leave it up to the FE to correctly
+ /// insert specific address spaces. Note that this is target specific and
+ /// might not have a corresponding LangAS equivalent, thus it is not safe to
+ /// unconditionally invoke getLangASFromTargetAS on the value returned by this
+ /// interface.
+ virtual std::optional<unsigned> getFlatPtrAddressSpace() const {
+ return std::nullopt;
+ }
+
  /// \returns If a target requires an address within a target specific address
  /// space \p AddressSpace to be converted in order to be used, then return the
  /// corresponding target specific DWARF address space.

clang/lib/Basic/Targets/AMDGPU.h

@@ -379,6 +379,12 @@ class LLVM_LIBRARY_VISIBILITY AMDGPUTargetInfo final : public TargetInfo {
  return static_cast<unsigned>(llvm::AMDGPUAS::CONSTANT_ADDRESS);
  }
 
+ /// \returns Target specific flat ptr address space; a flat ptr is a ptr that
+ /// can be casted to / from all other target address spaces.
+ std::optional<unsigned> getFlatPtrAddressSpace() const override {
+ return static_cast<unsigned>(llvm::AMDGPUAS::FLAT_ADDRESS);
+ }
+
  /// \returns If a target requires an address within a target specific address
  /// space \p AddressSpace to be converted in order to be used, then return the
  /// corresponding target specific DWARF address space.

clang/lib/Basic/Targets/SPIR.h

@@ -182,6 +182,10 @@ class LLVM_LIBRARY_VISIBILITY BaseSPIRTargetInfo : public TargetInfo {
  return TargetInfo::VoidPtrBuiltinVaList;
  }
 
+ std::optional<unsigned> getFlatPtrAddressSpace() const override {
+ return 4u; // 4 is generic i.e. flat for SPIR & SPIR-V.
+ }
+
  std::optional<unsigned>
  getDWARFAddressSpace(unsigned AddressSpace) const override {
  return AddressSpace;