[arm] needs porting to thumb2

[Alexander Sack]

https://wiki.ubuntu.com/ARM/Thumb2PackageReviewList

"uses mov; also "add" assumes arm"

[Alexander Sack (asac)]

identified during sprint:

17:23 < asac> 1. fix mov in the .asm files (special casing using m4)
17:24 < asac> 2. fix the invert_limb thing to use adr rather than add

3. if its easy, switch AS to -mthumb

[Alexander Sack (asac)]

dmart said:details about how to properly do 3. will land on the wiki page:
  https://wiki.ubuntu.com/ARM/Thumb2PortingHowto

[Dave Martin (dave-martin-arm)]

Some info is now posted; see:

<https://wiki.ubuntu.com/ARM/Thumb2PortingHowto#Building%20Stand-Alone%20Ass
embler%20Files%20in%20Thumb-2>

[Dave Martin (dave-martin-arm)]

Trying that again:

https://wiki.ubuntu.com/ARM/Thumb2PortingHowto#Building%20Stand-Alone%20Assembler%20Files%20in%20Thumb-2

[David Sugar (dyfet-deactivatedaccount)]

I found a simpler way to patch this which leaves configure alone. The debdiff is attached.

[David Sugar (dyfet-deactivatedaccount)]

Opps, forgot to add the actual bug to the changelog ;). Revised patch....

[Dave Martin (dave-martin-arm)]

+ifneq (,$(findstring $(DEB_HOST_ARCH), armel))
+ CFLAGS += -Wa,--defsym,USES_THUMB=1
+endif

To avoid breaking things in Debian, I believe USES_THUMB should not be set for their armel target.

The best way might to be to see whether the compiler defaults to Thumb (which could be done via configure or in the Makefile).

This would work in the Makefile for example (though it's pretty ghastly):

ifneq (,$(findstring $(DEB_HOST_ARCH),armel))
CFLAGS += $(shell $(CC) -dM -E -xc /dev/null | grep -q __thumb__ && echo -Wa,--defsym,INTERWORKING_NEEDED=1)
endif

(CFLAGS is deliberately omitted from the CC command line here: if the toolchain _defaults_ to Thumb, we can expect to have to support interworking, even someone decides to add -marm for this package or specific files. I also renamed the symbol which gets defined to make the meaning a bit clearer; but obviously that's just cosmetic.)

[Dave Martin (dave-martin-arm)]

Talking to a Debian guy about the armel port, it sounds like armel is not expected to run on anything prior to ARMv4T.

So maybe the complexity in my above post isn't needed. However, Debian are pretty unlikely to use Thumb at all until/unless they move to ARMv5 or newer (since adding interworking support introduces some inefficiencies on ARMv4T). So it may still be best to avoid affecting any code generation on Debian for now in armel.

[Loïc Minier (lool)]

The CC test is fine and the right approach IMO; I used something similar in ffmpeg to check whether the compiler defaults to VFP, v6 etc.:
check_asm = $(shell echo 'void foo(void) { __asm__ volatile("$(1)"); }' | $(CC) -x c -c - -o /dev/null 2>/dev/null && echo 1 || echo 0)
vfp_asm := fadds s0, s0, s0
has_vfp := $(call check_asm, $(vfp_asm))
ifneq ($(has_vfp),1)
FLAVORS += vfp
endif

[Alexander Sack (asac)]

was fixed a bit a go ... changelog had a typo in bug numbre ...