For the rtl passes, architecture target macros are not much of an issue with regards to executable code modularity: since the rtl passes are deeply interwoven with the insn-*.c files, we might as well compile one specialized copy of the rtl passes for each target architecture.
Another argument against leaving the macros are their often ill-defined interface types and the call-by-name semantics that make all the identifiers in scope at the call site a potential interface. We could avoid the latter problems without sacrificing the speed that we get from target-specific code by replacing the target macro with an inline hook. E.g. consider HARD_REGNO_MODE_OK. We could have $tm_file define TARGET_HARD_REGNO_MODE_OK as a static inline function, or #define it as the name of a static inline function somewhere else in $tm_file. The function's address will be in TARGET_INITIALIZER, and thus type checking on the function definition will be done. But a file that includes tm.h will be able to use the function TARGET_HARD_REGNO_MODE_OK directly, which can then be inlined, thus giving type safety without performance penalty.
