Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

On Mon, 17 Aug 2020 at 17:13, Dan Cross <crossd(a)gmail.com> wrote: Yes, it only uses sbrk(). One consequence I think is that sbrk() expands the process memory without invalidating existing use of memory - so the code is able to periodically expand heap while retaining all existing allocations. A simple workaround I used was to preallocate a heap and just stub out sbrk() calls - so that works. So in essence given a single chunk of memory (if large enough - which is still quite small by today's standards) the compiler manages fine. However I find this unsatisfactory and would like to improve it. But it is a bit difficult to understand how the memory is being used. Memory can be used for declarations, trees (for expressions) and strings as far as I can tell. Strings actually use the tree allocation, and just pretend that a node is a string. It seems that tree memory is allocated in a stack discipline. But what puzzled me is that when a tree starts, about 512 bytes of memory are left as gap for declarations to use. I have been trying to think in what circumstances would you encounter a declaration while parsing an expression - perhaps cast expressions? Anyway - if a declaration occurs inside an expression (i.e. tree) then it only has 512 bytes available. Of course this could be made bigger ... but at the least I would like to have separate heaps for declarations, trees and strings. I guess no one really dug into this code - as presumably once PCC came along the original compiler by Dennis stopped being used. Thanks and Regards Dibyendu