Re: [RFC] [gdb/tdep] Assume epilogue unwind info is valid unless gcc < 4.5.0

[Tom Tromey <tom@tromey.com>]

>>>>> "Tom" == Tom de Vries via Gdb-patches <gdb-patches@sourceware.org> writes:

Tom> Fix these two issues by reversing the burden of proof:
Tom> - currently we assume epilogue unwind info is invalid unless we can proof that
Tom>   gcc >= 4.5.0.
Tom> - instead, assume epilogue unwind info is valid unless we can proof that
Tom>   gcc < 4.5.0.

FWIW this approach makes sense to me.

It's pretty lame that there's no way to detect this failure from the
frame section -- it can't be producer-sniffed and the augmentation
strings can't really be changed.

gcc 4.5 was released in 2010, and so it's not like we're inconveniencing
a lot of users.  If needed I guess we could add a user setting to switch
this behavior back on.

Note there is a similar issue for the prologue, see:

https://sourceware.org/bugzilla/show_bug.cgi?id=25696
https://sourceware.org/bugzilla/show_bug.cgi?id=17265
https://sourceware.org/bugzilla/show_bug.cgi?id=21470

Also worth seeing the hilarious:

https://github.com/rust-lang/rust/issues/41252#issuecomment-293676579

I think that in this area we should assume the debug info is correct,
and keep a list of known-bad producers rather than assuming the debug
info is wrong and having a list of known-good ones.

Tom> +  if (/* In absence of producer information, optimistically assume that we're
Tom> +	 not dealing with gcc < 4.5.0.  */

This placement of the comment is pretty weird, it seems fine to just
stick it before the 'if'.

Tom> +      if (cu->producer == nullptr)
Tom> +	/* In absence of producer information, optimistically assume that we're
Tom> +	   not dealing with gcc < 4.5.0.  */
Tom> +	cust->set_epilogue_unwind_valid (true);
Tom> +      if (!producer_is_gcc (cu->producer, nullptr, nullptr))

Normally if there is a comment and a line of code as the consequence of
an 'if', we put them both in a block.

Anyway I was also thinking that the second one should say 'else if'.

Tom