Re: [gc-improv] Permanent vs function RTL obstack fix

[Jeff Law <law@redhat.com>]

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On 04/11/11 18:21, Mike Stump wrote:
> On Apr 11, 2011, at 1:03 PM, Jeff Law wrote:
>> The obvious solution is you copy the object, but then you have to be
>> able to distinguish within the object, what fields point to other
>> temporary objects vs permanent objects so that you can copy the
>> referenced temporary objects, but not the permanent objects (other parts
>> of the compiler may expect those permanent objects to be unique/shared).
>>   Not fun, not at all fun.  Been there, done that.
> 
> And then someone tells you that you can't copy...  Usually a nasty bug report much later.  So the choice is then, not solve a problem, or make everything permanent or add GC (back).  :-)  Been there, done that too.
Right.  Hence the old hack to mark the temporary obstacks and make them
permanent (wasting an enormous amount of space in the process).  I don't
recall the name of that function, but it certainly made me barf.

And yes, I remember all too well the problems with deep copying having
written one of the earliest tree deep copy routines to support
- -fcombine-statics .  It was the source of more problems than I could
ever count -- all related to objects allocated to different obstacks
hanging off a single tree node or objects of two different types being
stored into the same field within tree nodes.  That was circa 1992/1993.


These were the kind of problems that ultimately led to the GC system we
have today.  There are clearly things that can be better handled with
different allocation models and I'll support moving to better models
where it makes sense.  Where it doesn't, obviously I won't support it.

jeff
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