Re: question about linker

David Brown <david.brown@hesbynett.no> wrote:

On 11/12/2024 23:26, Michael S wrote:

On Wed, 11 Dec 2024 21:35:06 +0100
David Brown <david.brown@hesbynett.no> wrote:

I don't think there would be much reason to generate Thumb2 code unless
it was for running on microcontrollers.

Code size.  I use Armbian (Debian derivative) on Orange Pi PC and
Orange Pi Zero.  That is on Corex-A.  Armbian binaries by default
are compiled as Thumb2.  I did not check if any sizeable piece of
userland uses 32-bit instructions, but binaries using 32-bit
instructions work fine.

 So it would be a lot of work
for a compiler developer for little purpose.  I don't know if tcc
targeted 32-bit ARM or 64-bit ARM.  If it is the former, then Thumb2
would be less effort to implement since it is mostly a different
encoding of the same instruction set - but it would be of little use
since (if I understand you correctly) 32-bit Cortex-A devices don't
support Thumb2.  And if tcc supports 64-bit ARM, then the Thumb2
generation would be much more work since it is a significantly different
ISA.  And again, how many people actually want Thumb2 binaries for their
64-bit Cortex-A devices?

AFAIK normal 64-bit Linux distributions have no support for 32-bit
code or Thumb code.  But one can run 32-bit Linux distribution on
64-bit Cortex-A and Thumb2 support is part of this capability.
AFAICS this is mostly about code size and "disc use": Thumb2 gives
significant savings, but mixing 64-bit code with 32-bit code in
a single process is problematic and when you require separate
libraries for 32-bit and 64-bit, then all savings are gone.

However, you would not expect binary compatibility between code
generated for a 32-bit Cortex-M device and a Cortex-A platform, even
if it supports Thumb2 instructions - you have major differences in
the ABI, memory layouts, and core features beyond the basic registers.
>

 
There are major difference in floating-point parts of the ABI and in
everything related to interrupts. But for integer, it looks like the
T32 ABI is the same.

 
I'm sure much of it is the same (after all, it is solving the same basic
problem), but the details are critical to making things work.

AFAIK Thumb2 on Cortex-M3 is a subset of Thumb2 on Cortex-A.  To
get best performance you want to use all available instructions,
so compilers have separate targets.

--
                              Waldek Hebisch