mirror of
https://github.com/jeremysrand/llvm-65816.git
synced 2024-11-16 09:07:36 +00:00
79 lines
3.1 KiB
ReStructuredText
79 lines
3.1 KiB
ReStructuredText
========================
|
|
Segmented Stacks in LLVM
|
|
========================
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
Introduction
|
|
============
|
|
|
|
Segmented stack allows stack space to be allocated incrementally than as a
|
|
monolithic chunk (of some worst case size) at thread initialization. This is
|
|
done by allocating stack blocks (henceforth called *stacklets*) and linking them
|
|
into a doubly linked list. The function prologue is responsible for checking if
|
|
the current stacklet has enough space for the function to execute; and if not,
|
|
call into the libgcc runtime to allocate more stack space. When using ``llc``,
|
|
segmented stacks can be enabled by adding ``-segmented-stacks`` to the command
|
|
line.
|
|
|
|
The runtime functionality is `already there in libgcc
|
|
<http://gcc.gnu.org/wiki/SplitStacks>`_.
|
|
|
|
Implementation Details
|
|
======================
|
|
|
|
.. _allocating stacklets:
|
|
|
|
Allocating Stacklets
|
|
--------------------
|
|
|
|
As mentioned above, the function prologue checks if the current stacklet has
|
|
enough space. The current approach is to use a slot in the TCB to store the
|
|
current stack limit (minus the amount of space needed to allocate a new block) -
|
|
this slot's offset is again dictated by ``libgcc``. The generated
|
|
assembly looks like this on x86-64:
|
|
|
|
.. code-block:: nasm
|
|
|
|
leaq -8(%rsp), %r10
|
|
cmpq %fs:112, %r10
|
|
jg .LBB0_2
|
|
|
|
# More stack space needs to be allocated
|
|
movabsq $8, %r10 # The amount of space needed
|
|
movabsq $0, %r11 # The total size of arguments passed on stack
|
|
callq __morestack
|
|
ret # The reason for this extra return is explained below
|
|
.LBB0_2:
|
|
# Usual prologue continues here
|
|
|
|
The size of function arguments on the stack needs to be passed to
|
|
``__morestack`` (this function is implemented in ``libgcc``) since that number
|
|
of bytes has to be copied from the previous stacklet to the current one. This is
|
|
so that SP (and FP) relative addressing of function arguments work as expected.
|
|
|
|
The unusual ``ret`` is needed to have the function which made a call to
|
|
``__morestack`` return correctly. ``__morestack``, instead of returning, calls
|
|
into ``.LBB0_2``. This is possible since both, the size of the ``ret``
|
|
instruction and the PC of call to ``__morestack`` are known. When the function
|
|
body returns, control is transferred back to ``__morestack``. ``__morestack``
|
|
then de-allocates the new stacklet, restores the correct SP value, and does a
|
|
second return, which returns control to the correct caller.
|
|
|
|
Variable Sized Allocas
|
|
----------------------
|
|
|
|
The section on `allocating stacklets`_ automatically assumes that every stack
|
|
frame will be of fixed size. However, LLVM allows the use of the ``llvm.alloca``
|
|
intrinsic to allocate dynamically sized blocks of memory on the stack. When
|
|
faced with such a variable-sized alloca, code is generated to:
|
|
|
|
* Check if the current stacklet has enough space. If yes, just bump the SP, like
|
|
in the normal case.
|
|
* If not, generate a call to ``libgcc``, which allocates the memory from the
|
|
heap.
|
|
|
|
The memory allocated from the heap is linked into a list in the current
|
|
stacklet, and freed along with the same. This prevents a memory leak.
|