[fpc-pascal] Re: Fix & Continue in FPC [was: off-topic discussion)

[Jonas Maebe]

On 28 Feb 2012, at 14:36, Lukasz Sokol wrote:

> On 28/02/2012 10:45, Jonas Maebe wrote:
>>
>> Apple supported that in older versions of their development tools,
>> and it worked as well (or as badly) with FPC as it did with GCC. It
>> required changes to their dynamic linker (for the code replacement)
>> and to GDB. It was extremely flakey and limited though, and they
>> abandoned it fairly quickly.
>
> (Un)fortunately I don't know much about the internals, as I  
> mentioned :)
>
> The closest idea that looks similar is a loadable library: you can  
> unload a dll
> (or a .so), edit/recompile/rebuild it while the caller program is  
> paused/waiting,
> then reload it (with an obvious constraint that you can't modify the  
> interfaces
> that the already-running program knows)
> - now imagine each code/data block is separate 'object' that can be  
> loaded/unloaded...?

That is basically what Apple did. But if you want to do that at a  
reasonable speed and with support for e.g. existing addresses in  
memory (someone may have taken the address of a routine you are about  
to replace, and if the new code is larger it may get stored somewhere  
else), then you need lots of extra support (e.g. via trampolines for  
code; for data it's even harder). Not to mention that your debugger  
has to be able to deal with code changing addresses all the time and  
debug info changing.

In the end, people spent more time debugging the inconsistencies and  
limitations of "fix and continue" than it helped them. Maybe  
Microsoft's implementation is better, but
a) it is extremely hard to get right
b) as mentioned, it cannot be implemented in FPC itself; it mainly  
requires support from a lot of other things (and if that support is in  
place and requires some help from the compiler, then we can talk about  
making FPC support such an environment)


Jonas
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