Re: What is the precise definition of NOTE_INSN_FUNCTION_BEG?

[Matthew Malcomson <Matthew dot Malcomson at arm dot com>]

On 30/04/19 18:01, Jeff Law wrote:
> On 4/30/19 10:48 AM, Matthew Malcomson wrote:
>> Hi Jeff,
>>
>> On 30/04/19 16:29, Jeff Law wrote:
>>> On 1/4/19 9:03 AM, Matthew Malcomson wrote:
>>>> Hi there,
>>>>
>>>> I'm trying to figure out precisely what NOTE_INSN_FUNCTION_BEG means and
>>>> hoping someone here knows.
>>> It doesn't mean very much anymore.   I believe it was used to
>>> distinguish between stuff like copying incoming arguments into pseudos
>>> and real user code.
>>>
>>> However, in a world with instruction scheduling and other code motion it
>>> just doesn't have much use because it's so imprecise, particularly as we
>>> get deeper into the RTL pipeline.
>>>
>>
>> Ah, I guess an imprecise marker probably needs an imprecise definition ;-)
>>
>>>>
>>> Well, the question I'd ask is precisely what are you trying to mark?
>>
>>
>> I'm wanting to mark the first instruction from "user code".
>> I'm trying to fix PR88432, where gdb puts the breakpoint from the
>> "start" command in the wrong place.
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88432
>>
>> Gdb breaks on the second debug line entry in a function, which in
>> practice is the instruction directly after NOTE_INSN_FUNCTION_BEG.
>>
>>
>> The "fuzzy" definition from gccint.info didn't seem to explain what was
>> to happen with compiler inserted code that was not part of the prologue,
>> and this was what I was hoping to nail down.
>>
>> When looking, it seemed the note was being used in three slightly
>> different ways (I went into more information in the cover letter of my
>> [RFC] patch https://gcc.gnu.org/ml/gcc-patches/2019-01/msg00543.html ),
>> and I was hoping to get a precise definition so I could decide what
>> approach to take.
> ISTM that we should place the stack protector bits before the
> NOTE_INSN_FUNCTION_BEG.  At least in the non-optimizing compile case
> that should make the right thing happen.
> 
> Jeff
> 

Yeah, unfortunately I saw that moving that note would break some other 
places -- where the note is used as a marker for where to insert some 
other code.

The one I can remember (sort of) was that asan stack protection code is 
inserted just before "parm_birth_insn" (which is 
NOTE_INSN_FUNCTION_BEG), and that needs to be done before the non-local 
goto save area as per the solution for bugzilla 88186.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81186

That was why I ended up suggesting multiple notes -- it's currently 
trying to satisfy more than one criteria and they're not quite compatible.


MM