Re: __builtin_return_address for ARM

[Andrew Haley <aph at redhat dot com>]

Paul Brook wrote:
>>>> As I understand it, the ARM kernel can now do something similar.  So,
>>>> the only use for a __builtin_return_address(N) that used the frame
>>>> pointer chain would be if the code were compiled with nonstandard
>>>> options.
>>> Correct.
>>   Well, but wouldn't it still be nice if __builtin_return_address(N) was
>> implemented for N>0 by libcalling into the unwinder for you?  Obviously
>> this would still have to return NULL at runtime when you're running on a
>> DW2 target without any EH frame data present in memory (and I guess it
>> wouldn't work on SjLj targets either), but wouldn't it still be a nice
>> convenience feature for users?
> 
> There are sufficiently many caveats and system specific bits of weirdness that 
> you probably just have to know what you're doing (or rely on backtrace(3) to 
> do it for you).
> 
> IMHO builtins are for things that you can't do in normal C. So 
> __builtin_return_address(0) makes a lot of sense. Having it start guessing 
> how to do N>0 much less so.

I suggest we could contribute a version of backtrace.c for ARM to glibc.
An example to follow is libc/sysdeps/ia64/backtrace.c.

Andrew.