Re: Question: default linker scripts and emulations

[Georg-Johann Lay <avr@gjlay.de>]

Ni Nick

Am 06.12.23 um 12:21 schrieb Nick Clifton:
> Hi Johann,
> 
>> GCC could generate better code for some devices if it could use an
>> adjusted linker description file for these devices.
>>
>> Is it possible to add a new option to the linker so it picks a different
>> ld script, such that:
>>
>> o The same emulation is used.
>>
>> o All works a usual, e.g. when the user request -T myscript.ld, use
>> myscript.ld provided it is a complete ld script and not just INSERT 
>> AFTER etc.
>  >
>> In the case a new emulation is required:
>>
>> The compiler has currently around 60 multilib variants, and a new 
>> emulation
>> would add yet another 6 multilibs. Therefore, it would be desirable to
>> re-use the old multilib (which still has some devices in it).
>>
>> Say the old machine is avr:102 and the new one is avr:108, but the 
>> generated
>> code is all the same, only difference is ld script. The compiler would
>> map avr:108 to avr:102 multilibs.
>>
>> What's the best way to implement this in Binutils?
> 
> Actually it might be easier to implement this using gcc's spec file syntax
> rather than adding a new linker option.  For example
> 
>    gcc [...other gcc options...] --specs=avr108.spec
> 
> and avr108.spec could contain:
> 
>    *self_spec:
>    + %{!T{-T avr108.ld}}
> 
> The spec file syntax allows for options to be conditionally added, so in 
> this
> case a "-T avr108.ld" would be added to the linker command line only if the
> user has not already provided a -T option of their own.
> 
> [This is totally not tested by me, but I think that in theory it should 
> work].
> 
> Cheers
>    Nick

I don't see how this can work, because there is no way to inspect the
contents of the provided file:

* When the user provides a complete ld-script, then use his script instead
of avr108.ld.

* When the user provides an ld-script snipped with INSERT AFTER etc, then use
avr108.ld but with the augmentation provided by the user.

But the compiler (or specs handling) cannot tell which is which.

Then there's the technical problem that the compiler needs to know the install
path (in configure and when running) and provide the complete path to avr108.ld.

Just dropping avr108.ld in $install/avr/lib/ldscripts is not enough that the
linker can find it with -Tavr108.ld

Then the specs would need to be able to select different ld-script flavours
like *.x, *.xn, *.xr, etc depending on options like -r and whatnot.

And the default ld-scripts are contained in the ld executable, so that
changing the scripts on file won't have any effect (except you select them
explicitly with -T).

IMO specs cannot resolve that in a reasonable way, in  particular the
distinction between complete ld-scripts and snippets.

Cheers,

Johann