Re: Importance of GNAT.Source_Info

[Optikos <ZUERCHER_Andreas@outlook.com>]

On Monday, January 6, 2020 at 4:03:56 PM UTC-6, Jere wrote:
> I'm working on a baremetal RTS and while looking at
> https://wiki.osdev.org/Ada_Bare_bones, one of the files
> it suggests is part of the minimum set of RTS files is
> g-souinf.ads which contains the package GNAT.Source_Info.
> 
> Does anyone know what part of the compiler requires this?
> So far I haven't had GNAT barf at me for not having it
> while compiling the files I do have, but I don't want to 
> leave it out if it is indeed necessary for something.  I
> didn't see any info on why it is necessary.  It's definitely
> useful in that it gives a lot of compile time values, but
> not sure why it would be a "required" file.  I'm assuming
> something will break without it, but don't know what.
> 
> Sorry that this isn't a pure "Ada" question and is more
> focused on GNAT specifically, but thought one of yall
> might know why.

https://www.radford.edu/~nokie/classes/320/std_lib_html/gnat-source_info.html

Speaking in general from RTOS knowledge not specific to GNAT or Ada, I observe that GNAT.Source_Info.Enclosing_Entity (utilized transitively to the task's entry point) would provide information useful to knowing the complete call tree for maximum execution-time analysis in static analyzers of static machine code or in empirical analysis tools of a running system or hybrids thereof.  The A#1 goal of hard realtime systems is to definitively know (via mathematical proof of worst-case) the maximum (not typical) execution time of each subroutine invocation—especially this analysis for each  required use-case from system engineering (but in lieu of each use-case, then the absolute maximum execution-time of each subroutine).  Often this analysis starts with a table of all system calls in an RTOS plus all subroutines in a realtime system (including app-domain) that is derived from the map file of each executibile as output at link-time.  It appears that GNAT.Source_Info.Enclosing_Entity provides a dynamic walk with software assist that would otherwise be a manual or semi-manual walk of the map file of subroutine names & their entry points.

Perhaps someone else will have more GNAT-specific meat to put on this general skeleton.