RE: [ARM] mno-pic-data-is-text-relative & msingle-pic-base

[Irfan Ahmad <h dot irfanahmad at gmail dot com>]

Nathan,

Sorry for jumping in a relatively old thread. I saw this in mailing list 
archives during a web search (I wasn't on this mailing list before). I 
decided to speak up as I would be an affected party if this patch (or 
some similar future patch) gets accepted or worse yet, the feature 
involved gets disabled.

> Apparently there are legitimate reasons one might want the -mno-PDITR 
behaviour without -mSPB. I don't know what those are, perhaps Joey could 
clarify?

Yes, there are some practical use cases that require 
-mno-pic-data-is-text-relative (-mno-PDITR) without -msingle-pic-base 
(-mSPB).

These are based on two primary principles:

1. In the absence of lazy binding (that is almost always the case in 
embedded systems), GOT is practically read-only - it needs to be 
modified only during linking by the dynamic linker, after that it can be 
considered and marked read-only (e.g. read-only attribute set to be 
enforced by some MMU or MPU).

2. If you only need a simple dynamic object model - where you just need 
dynamic loading and dynamic linking - but do not need to maintain 
multiple data states for the object like you need in a traditional 
shared object model, then one instance of GOT per dynamic object is enough.

From #1: GOT is read-only so keeping it with CODE segment is a natural 
choice. Now as there is no need to move it to RAM so there is no need 
for a mechanism (-mSPB) that would enable controlling the GOT location 
independently of CODE segment.

From #2: Only one instance of GOT is required per dynamic object so 
there is no need for a mechanism (-mSPB) that would enable switching GOTs.

So when both #1 and #2 are met, you only need 
-mno-pic-data-is-text-relative.

Irfan Ahmad