[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

Albert ARIBAUD
2014-11-19T17:48:57Z

Hello Bill,

On Wed, 19 Nov 2014 11:31:05 -0500, Bill Pringlemeir
<bpringlemeir@nbsps.com> wrote:
> On 19 Nov 2014, wd at denx.de wrote:
>
> > Dear Albert,
> >
> > In message <20141119074214.3d414ce6@lilith> you wrote:
> >>
> >> For -mauto-it, it is not documented in the gas documentation online
> >> or in my current as' --target-help. I'll dig this deeper today, but
> >> barring any scream from me, the change above is fine globally in
> >> U-Boot.
>
> > Apparently this [1] is where it is coming from; no further
> > documentation there, though.
>
> > [1] https://sourceware.org/ml/binutils/2009-05/msg00132.html
>
> I would think that if this worked they would make it automatic and not
> an option. Probably this is only in certain binutils/as.
>
> With 4.6.3 and 4.9.1 I do not have this option,

Which option do you mean? -mimplicit-it or -mauto-it?

> [foo.S]
> .syntax unified
> .thumb
> foo:
> cmp r0, #5
> movne r1,#6
> moveq r1,#2
> bx lr
> bar:
> cmp r0, #10
> movhi r1,#3
> movls r1,#7
> moveq r1,#11
> bx lr
>
> $ arm-none-linux-gnueabi.gcc4.6.3/bin/arm-none-linux-gnueabi-as
> -mcpu=cortex-a5 -mimplicit-it=always foo.S -o foo.o
> $ arm-none-linux-gnueabi-vybrid-4.9.1/bin/arm-none-linux-gnueabi-as
> -mcpu=cortex-a5 -mimplicit-it=always foo.S -o foo.o
>
> Both give 'objdump -S foo.o',
> foo.o: file format elf32-littlearm
>
> Disassembly of section .text:
>
> 00000000 <foo>:
> 0: 2805 cmp r0, #5
> 2: bf14 ite ne
> 4: 2106 movne r1, #6
> 6: 2102 moveq r1, #2
> 8: 4770 bx lr
>
> 0000000a <bar>:
> a: 280a cmp r0, #10
> c: bf8c ite hi
> e: 2103 movhi r1, #3
> 10: 2107 movls r1, #7
> 12: bf08 it eq
> 14: 210b moveq r1, #11
> 16: 4770 bx lr
>
> I think before the patch there would be 'it' values before each and
> every condition. In fact, if you change 'bar' to,
>
> bar:
> cmp r0, #10
> movhi r1,#3
> movlo r1,#7
> moveq r1,#11
> bx lr
>
> You get three 'IT' conditions as 'HI' and 'LO' are not opposite. The
> patch seem to detect things that are the exact opposite. The 'bar'
> above ends up with '11' in r1 if the value is zero, but it temporarily
> '7'. The 2nd bar will only place 11 in r1.
>
> Fwiw,
> Bill Pringlemeir.
>
> Ref: https://wiki.ubuntu.com/ARM/Thumb2PortingHowto#Conditional_Execution

All this is about -mimplicit-it, which *is* documented, *is*
consistently present in common toolchains AFAICT and is and (as)
logcical (as it can be when dealing with backward compatibility of
command lines etc).

However, my problem is not with -mimplicit-it; it is with -mauto-it.

My gut feeling is that -mauto-it is a predecessor of -mimplicit-it.

Amicalement,
--
Albert.