[DF] RFC: obstacks in DF

[Dimitrios Apostolou <jimis at gmx dot net>]

Hi,

while I was happy using obstacks in other parts of the compiler I thought 
they would provide a handy solution for the XNEWVECs/XRESIZEVECs in 
df-scan.c, especially df_install_refs() which is the heaviest malloc() 
user after the rest of my patches.

In the process I realised that obstacks weren't exactly suitable (thanks 
matz for helping on IRC), nevertheless I have a patch which is stable, a 
bit faster and more memory friendly (don't know why, but RSS memory for 
common non-pathological compilations, was smaller). However after trying 
various incorrect changes I'm aware that there are leaks in there that 
can't be closed without dirty hacks, so I gave up.

I'm posting the simplest but stable version of my changes and would really 
like to hear ideas. There are "holes" in the obstack that should have been 
free'd, but in the end I didn't see memory increase. I don't know what 
would happen for huge functions that keep the obstack alive for long, I 
didn't have such testcase. Finally, I think my next try will involve 
converting the chains to pool_alloc'ed linked list, do you think that 
makes sense?


Thanks,
Dimitris

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