Re: [PATCH] Fix length calculation in aarch64_linux_set_debug_regs

[Simon Marchi <simon dot marchi at ericsson dot com>]

I looked a bit more into the issue and did some testing, and it appears the
current code is correct (as Yao mentioned).  It's probably not as clear as it
could be though.  I think it would be nicer if expressed as

  (size of fixed part) + (size of variable part)


On 15-11-02 11:00 AM, Pedro Alves wrote:
> IIUYC, you're pointing out two issues:
> #1 - the offsetof that doesn't work in C++.

This actually appears to be a bug in g++.

See this, especially towards the end: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14932

The expression is accepted by gcc and clang++, but not g++ (test with 4.8 and 5.2).

> #2 - an off-by-one.

Never mind, I thought wrongly at first that sizeof (regs.dbg_regs [count - 1])
would give the size for "count - 1" elements, but it's actually the size of a
single element.  So scratch that, the current code works fine.

So the only reason to change the code would be to circumvent a bug in g++, which
is probably a valid one (otherwise we can't build).

> I don't know enough about Aarch64 to judge #1, but it does sound right to me.
> 
> On #2, I saw the same on x86.  See my fix here:
> 
>   https://sourceware.org/ml/gdb-patches/2015-02/msg00213.html
> 
> I think it's a little nicer to hide away the offsetof+sizeof.

You mean hide in in a function?  This expression is only used at one place and I think it's
reasonably straightforward if expressed correctly, but if you think it will make the code
clearer I don't mind.