Re: [PATCH] Frame pointer for arm with THUMB2 mode

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Denis,

>> Adding support for a frame chain would require an ABI change. It 
> would have to
> > work across GCC, LLVM, Arm, Thumb-1 and Thumb-2 - not a trivial amount of
> > effort.

> Clang already works that way.

No, that's incorrect like Richard pointed out. Only a single register can be used
as the frame chain.

> If we change the size of the traces to 2, it could be something like this:
...
> At the first example we lost the full context, from where the 
> control/data flow comes from.

If 2 is not sufficient, then try 3 or 4. It may also be feasible to only enable
deeper unwinding for particular libraries so you only pay an extra cost for 
leaks you are interested in.

> The stack layout like this enables only with compile time flag 
> (-mthumb-fp and works only together with -mthumb and
> -fno-omit-frame-pointer). It does not affect other codegen.

But any code built like that will *always* run slower even if you don't use
the sanitizer.

Wilco