Re: Some questions

[Szabolcs Nagy <nsz@port70.net>]

* Andre McCurdy <armccurdy@gmail.com> [2018-05-01 14:49:00 -0700]:
> On Tue, May 1, 2018 at 10:35 AM, Rich Felker <dalias@libc.org> wrote:
> > I'm considering applying the attached patch, which would make it so
> > VLAs don't break thumb syscalls and eliminate the need to force frame
> > pointer off when building as thumb. This is all a workaround for gcc
> > being wrong about not letting you use r7, but it seems reasonable and
> > non-invasive. It just omits r7 from the constraints and uses a temp
> > register to save/restore it.
> 
> This seems to fail when compiling src/thread/arm/__set_thread_area.c:
> 
>   {standard input}: Assembler messages:
>   {standard input}:45: Error: invalid constant (f0005) after fixup
>   make: *** [obj/src/thread/arm/__set_thread_area.o] Error 1
> 
> Without the patch, __set_thread_area() effectively compiles to:
> 
> __set_thread_area:
>     push    {r7, lr}
>     ldr    r7, .L2
>     pop    {r7, pc}
> .L2:
>     .word    983045
> 
> With the patch:
> 
> __set_thread_area:
>     push    {r7, lr}
>     add    r7, sp, #0
>     mov r3,r7 ; mov r7,#983045 ; svc 0 ; mov r7,r3
>     pop    {r7, pc}
> 
> ie the immediate value 0xf0005 can't be loaded directly into r7 with a
> single Thumb2 mov instruction.
> 
> I tried a quick test to replace the single mov instruction in
> __asm_syscall() with a movw + movt pair:
> 

i think the syscall can be just inline asm here,
since __set_thread_area is arm specific code.

in generic code the mov r7,.. hack should work
and fixes the vla issue.

(alternatively using just "r" operand instead
of "rI" does not generate immediate, but will
use more registers/instructions)