[U-Boot] [RFC PATCH] Provide a mechanism to avoid using #ifdef everywhere

Simon Glass
2013-02-19T05:18:25Z

Hi Tom,

On Mon, Feb 18, 2013 at 1:36 PM, Tom Rini <trini@ti.com> wrote:
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> On 02/18/2013 02:23 PM, Wolfgang Denk wrote:
>> Dear Simon,
>>
>> In message <1361207920-24983-1-git-send-email-sjg@chromium.org> you
>> wrote:
>>> Many parts of the U-Boot code base are sprinkled with #ifdefs.
>>> This makes different boards compile different versions of the
>>> source code, meaning that we must build all boards to check for
>>> failures. It is easy to misspell
>> ...
>>> +# Create a C header file where every '#define CONFIG_XXX value'
>>> becomes +# '#define config_xxx() value', or '#define config_xxx()
>>> 0' where the CONFIG +# is not used by this board configuration.
>>> This allows C code to do things +# like 'if (config_xxx())' and
>>> have the compiler remove the dead code, +# instead of using
>>> '#ifdef CONFIG_XXX...#endif'. Note that in most cases +# if the
>>> config_...() returns 0 then the option is not enabled. In some
>>> rare +# cases such as CONFIG_BOOTDELAY, the config can be enabled
>>> but still have a +# a value of 0. So in addition we a #define
>>> config_xxx_enabled(), setting the +# value to 0 if the option is
>>> disabled, 1 if enabled. This last feature will +# hopefully be
>>> deprecated soon.
>>
>> I think config_* is not a good name to use here - it has never been
>> a reserved prefix so far, so it is IMO a bad idea to turn it into
>> one now . We already have quite a number such variable names in
>> the code all over the place (not sure I caught them all):
>
> Indeed. It's not a good choice to suddenly reserve now either.
> build_has_xxx() ? I'm not sure.

So config_bootdelay(), or cfg_bootdelay() for the value, and
build_has_bootdelay() for the enable?

>
> [snip]
>>> +The compiler will see that config_xxx() evalutes to a constant
>>> and will +eliminate the dead code. The resulting code (and code
>>> size) is the same.
>>
>> Did you measure the impact on compile time?
>
> Probably won't have a good handle on this without converting a whole
> build target's needs (just about).

Possibly, but it seems like the biggest impact is the slower
reconfigure - sed runs for several seconds.

>
> [snip]
>>> -#if defined CONFIG_ZERO_BOOTDELAY_CHECK /* * Check if key
>>> already pressed * Don't check if bootdelay < 0 */ - if (bootdelay
>>> >= 0) { + if (config_zero_bootdelay_check() && bootdelay >= 0) {
>>> if (tstc()) { /* we got a key press */ (void) getc(); /* consume
>>> input */ puts ("\b\b\b 0"); abort = 1; /* don't auto boot */ } }
>>> -#endif
>>
>> I think code like this needs more careful editing.
>>
>> With the #ifdef, it was clear that the comment only applies if
>> CONFIG_ZERO_BOOTDELAY_CHECK is defined, but now it suddenly
>> becomes valid _always_, which is pretty much misleading to someone
>> trying to understand this code. I think it would be necessary to
>> rephrase the commend, and move it partially into the if() block.
>
> Exactly. This type of change will require a lot of re-commenting to
> make it clear what's going on now, and after the change even more-so.

I suspect cleaning up main.c would involve a number of patches in the end.

BTW this patch also is interesting from the point of view of the
generic board series. At the moment I am using a list of function
pointers, which is quite nice, but does defeat the compiler
optimisations, so adds about 1KB of code size. Also it's hard to
imagine removing the #ifdefs from a list of function pointers using
this mechanism.

Regards,
Simon

>
> - --
> Tom
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