Re: [PATCH, RFC, RFT] ARM relocation fixes

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 03.12.2013 09:47, Leif Lindholm wrote:
> On Tue, Dec 03, 2013 at 09:22:56AM +0100, Vladimir 'φ-coder/phcoder' 
> Serbinenko wrote:
>>>> I've looked through encoding of those instructions and see how much it's
>>>> a mess. b* and b*x don't have similar set of options which makes
>>>> validating them a difficult error-prone task. So I think, I'll just add
>>>> veneers to mkimage, just like we do on ia64 (either by making a
>>>> pc-relative variant of veneers or adding fixup for them)
>>>
>>> Not B, BL.
>>> There is a 1-bit range difference between Thumb BL and BLX, which we
>>> need to check for anyway. This check already exists (and must exist) in
>>> the code. Adding veneers would be pure overhead.
>>
>> I meant that you can use conditions with bl but not blx. So if we have a
>> reloc on ARM bl.e targetting Thumb then we have to add veneers. Since we
>> have only small number of interworking calls it's probably easier to
>> always add veneers on interworking relative relocations rather than
>> having micro-optimisation and get some minor case wrong.
> 
> OK, but the only place we could ever have a problem with this would
> be if we had asm in the kernel _explicitly_ done as .thumb.
> Which we don't. We explicitly moved away from that in order to have
> support for pre-v7 processors.
> 
We also call C code from asm. One such instance (for division
instructions) caused the problem
> All modules will have full 32-bit external references, so will not
> use these instructions anyway. Any internal references within modules
> will be linked with LD, which will fix this up automatically.
> 
In my small test I compiled:
extern void g(void);

void f (int x)
{
  if (!x)
    g();
}
And got following assembly with -Os:

   0:   e3500000        cmp     r0, #0
   4:   e92d4008        push    {r3, lr}
   8:   0bfffffe        bleq    0 <g>
                        8: R_ARM_JUMP24 g
   c:   e8bd4008        pop     {r3, lr}
  10:   e12fff1e        bx      lr

If g is a function in thumb kernel or thumb module then you need a veneer.
> /
>     Leif
> 
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