[LLVMdev] RFC: GSoC Project

Talin viridia at gmail.com
Sun Apr 10 21:13:57 PDT 2011

On Sun, Apr 10, 2011 at 4:16 PM, Chris Lattner <clattner at apple.com> wrote:

> On Apr 10, 2011, at 2:45 PM, Talin wrote:
> I wonder - would something like this allow for multiple stacks for a single
> thread? I'm thinking of something like continuations / fibers / green
> threads, which would be very handy.
> I haven't looked at the proposal, but yes, this would be very useful
> functionality for LLVM to provide.

Another thing I'd like is for any proposed segmented stack mechanism to be
garbage-collector friendly - which essentially means an API that can
reliably iterate through all of the stack frames, starting from the current
function, and calculate the correct return address for each stack frame. I'm
envisioning something like this:

    frame = llvm.currentframe();
    while frame != NULL {
      retaddr = llvm.returnaddr(frame);
      // do stack tracing using frame and retaddr to identify stack roots.
      frame = llvm.nextframe(frame);

This is essentially what I do now with the EBP register - but it would be
better if it were encapsulated behind an API, so that frontends wouldn't
have to know about specific registers.

> -Chris
> On Wed, Mar 23, 2011 at 3:07 AM, Sanjoy Das <
> sanjoy at playingwithpointers.com> wrote:
>> Hi All!
>> I will be applying to the LLVM project for this GSoC, and I wanted some
>> preliminary sanity check on my project idea.
>> I intend to implement split (segmented) stacks for LLVM (like we have in
>> Go, and as being implemented for GCC [1]). A lot of what follows is
>> lifted from [1]; I will progressively add more details as I get more
>> familiar with the LLVM codebase.
>> I intend to start with the simplest possible approach - representing the
>> stack as a doubly linked list of _block_s, the size of each _block_
>> being a power of two. This can later be modified to improve performance
>> and accommodate other factors. Blocks will be chained together into a
>> doubly linked list structure (using the first two words in the block as
>> the next and previous pointers).
>> In the prologue, a function will check whether the current block has
>> enough stack space. This is easily done for function which don't have
>> variable sized allocas, and for ones which do, we can assume some
>> worst-case upper bound. The prologue can then call an intrinsic (let's
>> call it llvm.adjust_stack) which allocates a new block (possibly by
>> delegating this to a user-provided callback), copies the arguments,
>> saves the previous stack pointer (in the new block), and adjusts the
>> next and previous pointers. It will also have to adjust the stack
>> pointer, and the frame pointer, if it is being maintained. Cleanup can
>> be done by hijacking the return value, as also mentioned in [1]. It
>> might make sense to leave the allocated blocks around, to prevent
>> re-allocating the next time the program needs more stack space.
>> DWARF info can be generated as follows: since we know the offset of base
>> of the stack frame from the stack pointer (or we are maintaining a frame
>> pointer), we can always say whether the concerned call frame is the
>> first call frame or not. In the second case, all the previous register
>> values can be computed as usual, and in the first case, we will add an
>> extra indirection, involving looking up the stack pointer saved in this
>> block's header.
>> One thing I'd really like some input on is whether implementing split
>> stacks would be useful enough to warrant the effort (especially keeping
>> in mind that this is pretty useless on 64 bit architectures).
>> [1] http://gcc.gnu.org/wiki/SplitStacks
>> --
>> Sanjoy Das
>> http://playingwithpointers.com
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> --
> -- Talin
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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-- Talin
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