[Buildroot] [PATCH 04/16] uclibc: Enable compile in thumb mode when selected

[Peter Korsgaard <peter@korsgaard.com>]

>>>>> "Thomas" == Thomas Petazzoni <thomas.petazzoni@free-electrons.com> writes:

 > From: Paul Enman <paul.enman@gmail.com>
 > Until now, when thumb was selected with threads, we were forcing ARM
 > mode to be used, to workaround some problems building uClibc with
 > threads in Thumb mode. Also, we were never enabling Thumb mode in
 > uClibc when Thumb2 was selected in the configuration.

 > Therefore, this commit updates the uclibc.mk logic to:

 >  - Force ARM mode only when gcc 4.7 or 4.8 is selected with Thumb1 and
 >    threads. We also explicitly disable COMPILE_IN_THUMB_MODE when we
 >    fall in this situation. With newer gcc versions, starting 4.9, this
 >    problem is fixed and we can build uClibc in Thumb1 mode.

 >  - Enable COMPILE_IN_THUMB_MODE when Thumb1 or Thumb2 is selected. We
 >    need to disable context functions since they don't build in Thumb
 >    mode.

Why do we need to tell uClibc that we are building in thumb1/2 mode?
From a quick grep this just adds -mthumb to CFLAGS, which shouldn't be
needed as we've already configured the compiler to do so by default and
enables USE_BX + USE_LDREXSTREX.

Now, I don't know enough of the finer details about this. I thought BX
was for thumb1 interworking with classical ARM code and not needed for
thumb2, so isn't it just adding overhead for thumb2? I also see that
enabling this option will break thumb compilation on arm920t.

Luckily Yann is the author of this logic in uClibc, so lets ask him ;)

 >  - Disable COMPILE_IN_THUMB_MODE explicitly otherwise.

 > This allows to build a Thumb2 toolchain on ARMv7-M for example.

What wasn't working before?

-- 
Bye, Peter Korsgaard