Conditionalize make behavior e.g. disable --gc-sections

[Chris Johns]

On 10/5/19 10:27 pm, dufault at hda.com wrote:
> 
> 
>> On May 10, 2019, at 08:07 , Sebastian Huber
>> <sebastian.huber at embedded-brains.de
>> <mailto:sebastian.huber at embedded-brains.de>> wrote:
>>
>> On 10/05/2019 13:58, Gedare Bloom wrote:
>>> On Tue, May 7, 2019 at 5:03 AM Peter Dufault<dufault at hda.com
>>> <mailto:dufault at hda.com>>  wrote:
>>>> What is best practice to change build behavior?  e.g. I need to use
>>>> --whole-archive/--no-whole-archive in one place but --gc-sections breaks it
>>>> and that's turned on for the BSP (xilinx_zynq_a9_qemu).
>>>>
>>>> I’ve done this in the .cfg files for the BSP:
>>>>
>>>> ifneq ($(RTEMS_GC_SECTIONS_DISABLE),1)
>>>> LDFLAGS_GC_SECTIONS_DISABLE = -Wl,--gc-sections
>>>> endif
>>>>
>>>> (...)
>>>>
>>>> LDFLAGS = $(LDFLAGS_GC_SECTIONS_DISABLE)
>>>>
>>>> and then I set RTEMS_GC_SECTIONS_DISABLE=1 in the one Makefile where I need
>>>> it.  I don’t see similar patterns, is this the way to do it?
>>>>
>>> Historically a new BSP variant has been introduced when there needs to
>>> be a change at this .cfg level. But, I could see a rationale to
>>> include this kind of switching behavior at a higher level. Someone
>>> might consider how to integrate it into our configure.ac/Makefile.am
>>> black magic, but that is beyond me:).
>>>
>>> An alternative approach that may work from what you propose is to use
>>> following in the xilinx zynq .cfg file:
>>> LDFLAGS ?= -Wl, --gc-sections
>>> then, you can provide own LDFLAGS = -Wl on your make command line or
>>> in your Makefile, I think that would work bit more elegant
>>
>> Nobody forces you to use -Wl, --gc-sections to link your application. For the
>> RTEMS tests and samples this shouldn't be an issue?
>>
> My make pulls in those settings.  I think it is best to use the same settings
> that the RTEMS tests and samples unless I have a good reason to change them,
> e.g., I need —whole-archive in one application in one place.
> 
> What do you recommend?  Where is an example to build from?

We do not have simple examples with documentation to support them. I receive a
stream of requests about building libdl applications and all I can do is point
user to the testsuite Makefile.am's.

The Makefile.inc approach has a range of problems so adding documentation and
examples based on it is something I do not support. The approach I would like to
see happen is documented in https://devel.rtems.org/ticket/3302.

You do not need to build applications with the exact same flags that the kernel
uses however you need to be careful in managing what you do use. As I state in
the ticket RTEMS Tools [1] and RTEMS Waf [2] both have support to filter the
CFLAGS into parts so you can manage what is used when building applications. I
would prefer this is formalised when exported by RTEMS rather than having to
have this fragile code which does break from time to time.

Chris

[1] https://git.rtems.org/rtems-tools/tree/rtemstoolkit/rld-cc.cpp#n79
[2] https://git.rtems.org/rtems_waf/tree/rtems.py#n793