[PATCH] gdb: Remove LANG_MAGIC
Pedro Alves
palves@redhat.com
Fri Apr 12 11:47:00 GMT 2019
On 4/12/19 12:08 PM, Andrew Burgess wrote:
> * Pedro Alves <palves@redhat.com> [2019-04-12 10:50:00 +0100]:
>
>> On 4/12/19 12:23 AM, Andrew Burgess wrote:
>>> The language_defn structure has an la_magic field, this used to be
>>> used as a basic check that the language_defn structure had the
>>> expected layout - at least the end of the structure was where we
>>> expected it to be.
>>>
>>> This feature only really makes sense if we imagine GDB dynamically
>>> loading language support from dynamic libraries, where a version
>>> mismatch might cause problems.
>>
>> We used to have more magic fields like these in other structures,
>> like target_ops, for example. IIRC, their intended purpose was to
>> catch miscompilation, before we had Makefile auto-dependencies. Before
>> that, it was common to change the structure's definition, rebuild, and
>> then find out that gdb hit a "failed internal consistency check" assertion,
>> because some of the .o files hadn't been rebuilt to reflect the new layout.
>>
>> For example, the target_ops magic said:
>>
>> /* Magic number for checking ops size. If a struct doesn't end with this
>> number, somebody changed the declaration but didn't change all the
>> places that initialize one. */
>>
>> #define OPS_MAGIC 3840
>>
>> With auto-dependencies, the problem this "solved" is largely a
>> thing of the past.
>
> Thanks for the explanation, that makes sense.
>
> I assume you're happy that these days we should "assume a working
> build system", and that if any errors like the above cropped up we
> should fix those in the build dependencies, not by adding magic fields
> to gdb.
Yes, of course. I was the one who removed OPS_MAGIC (in f6ac5f3d63e0) :-)
Thanks,
Pedro Alves
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