[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;