Re: [dpdk-dev] [RFC] toolchain specific macro expansion

[Thomas Monjalon <thomas@monjalon.net>]

23/06/2021 20:26, Tyler Retzlaff:
> today rte_common.h defines common macros for use by dpdk and consuming
> applications. most expansions are specific to the gcc toolchain.
> 
> example
> // lib/eal/include/rte_common.h
> 
> /**
>  * Hint never returning function
>  */
> #define __rte_noreturn __attribute__((noreturn))
> 
> there is an anticipated need rte_common.h to expose alternate
> expansions for non-gcc toolchains in the future and would like
> comments on how to achieve this in an agreeable manner.
> 
> 
> option 1 add conditional compilation directly to rte_common.h
> 
> example
> // lib/eal/include/rte_common.h
> 
> /**
>  * Hint never returning function
>  */
> #ifdef RTE_TOOLCHAIN_GCC
> #define __rte_noreturn __attribute__((noreturn))
> #endif
> 
> #ifdef RTE_TOOLCHAIN_FOO
> #define __rte_noreturn __foo_expansion_for_noreturn
> #endif
> 
> represents the simplest approach technically but may be tedious to
> maintain due to the number of macros and number of conditional
> expansions per macro.

Macros are simple so the option 1 is not that bad.


> option 2 add toolchain specific files (follow existing platform/os
>          includes pattern)
> 
> example:
> // lib/eal/gcc/rte_toolchain_common.h
> #define __rte_noreturn __attribute__((noreturn))

We should keep a macro in rte_common.h which triggers an explicit error
if not overriden for the toolchain.
That way we can have a single doxygen comment in rte_common.h
like it is done in lib/eal/include/generic/

> // lib/eal/include/rte_common.h
> #include <rte_toolchain_common.h>

The include may be done in the reverse order:
the toolchain-specific .h in lib/eal/gcc/include/
includes generic/rte_common.h in lib/eal/include/generic/.
If both have the same name, we keep #include <rte_common.h>

> // meson.build (illustrative change)
> host_toolchain = cc.get_id() # e.g. gcc
> 
> global_inc = include_directories('.', 'config',
>     'lib/eal/include',
>     'lib/eal/@0@/include'.format(host_machine.system()),
>     'lib/eal/@0@/include'.format(arch_subdir),
>     'lib/eal/@0@/include'.format(host_toolchain),
> )
> 
> results in the introduction of more files that need to be
> published/installed for applications but separate files per
> implementation allow for easier maintainability.