[LLVMdev] Stack roots and function parameters

[Jon Harrop]

Foo

 

Here's another way to think about the issue: Compare this whole approach to
stack roots to that of a conservative collector. Since we know conservative
collectors work, the whole reason for making an accurate collector in the
first place is efficiency, right? If we couldn't make an accurate collector
that was more efficient than a conservative collector, then no one would
bother because it's such a pain.

 

However, looking strictly at stack frames, a conservative collector has a
much more compact stack layout. There's no need to copy anything to local
variables, because by the time you call into the collector proper,
everything - every register variable, every function parameter, and so on -
is already on the stack *somewhere*.

 

The stack frame layout dictated by llvm.gcroot, by contrast, is much, much
fatter. In a typical deeply-nested call stack, there will be many copies of
the same value on the stack. Let's take the 'this' pointer as an example.
Say we have some method that calls itself recursively, and we are currently
10 levels deep. That means that we have 20 copies of the 'this' pointer on
the stack: Because each invocation of the method has to push the 'this'
pointer on the stack as part of the calling protocol (even if it's passed in
registers, it still has to save the prior value of the register somewhere),
and in addition we also have to save the 'this' pointer as a stack root. A
conservative collector, on the other hand, would be able to get by with only
10 copies of the 'this' pointer.

 

As far as the issue of eliminating redundant stores, I'm pretty sure that
the optimizers cannot eliminate redundant roots. Figuring out the minimal
set of temporary roots needed for a function call is a non-trivial problem
(you don't want to just blindly make every pointer argument to a function a
temporary root, since that just bloats the size of the call frame
needlessly.)

 

My entire goal here is to get a GC that has high performance and efficiency.
I want my GC to be able to run on everything from a data center to a beagle
board, and use the minimum resources possible.

-- 
-- Talin

 

Foo

 

Foo

 

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