How-to add custom errata workaround to the assembler?

[Sebastian Huber]

Hello Nick,

----- Am 29. Jul 2024 um 13:15 schrieb Nick Clifton nickc@redhat.com:

> Hi Sebastian,
> 
>> I have a system on a chip with an Armv8-R core (Cortex-R52) which needs an
>> additional load after each load exclusive instruction (ldrex, ...) as a
>> hardware errata workaround. I would like to add a custom assembler option
> 
> I assume that this cannot just be a patch to the compiler as you need the
> workaround to
> support hand written assembler files too.  Is that correct ?

yes, the workaround needs to cover also hand written assembler files.

> 
>> to enable this workaround and then emit for each load exclusive instruction
>> which gets assembled an additional load instruction. It would be great if
>> someone could give me a hint if this sounds feasible
> 
> Yes, it is feasible.  Not sure how much work it will be, but it certainly can be
> done.
> 
>> and ideally which function or file in binutils could be changed to to do this
>> work.
> 
> Pretty much all of the work will be in the gas/config/tc-arm.c file in the
> binutils
> sources.
> 
> New command line options should be added to the arm_opts[] array.
> 
> Create a new local variable to be enabled/disabled by your new command line
> option
> and then test it in functions like do_ldrex() and do_ldrexd().  (You may need to
> scan the tc-arm.c source to find other places that need updating).
> 
> You should be able to use emit_insn() to add the extra instruction into the
> output
> stream, if you have a binary form of the instruction available.  (Creating an
> expression with the O_constant type and an X_add_number field containing the
> binary
> value).  Otherwise you might have to use md_assemble(), temporarily overriding
> the
> input stream, in order to assemble the new instruction.
> 
> I would strongly recommend that you add one or more test files to the
> gas/testsuite/gas/arm directory.  The convention is the *.d files are the test
> drivers and *.s are the test sources.
> 
> It would also be good to update the gas/doc/c-arm.texi file to describe the new
> option.
> (Unless you plan on keeping your update private) and the gas/NEWS file to
> mention the
> new feature.

Thanks for the valuable hints. It turned out to be pretty simple. I just added something similar to output_inst() after this call in md_assemble(). The additional work is guarded by a new option. I added a GCC multilib for this as well. Overall, I guess it is quite a hack, but works well.

With respect to making it public, I have the issue that the hardware manufacturer wants to keep it private for now.

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