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Lots of blocks had "llvm" or "nasm" syntax types but either weren't following the syntax, or the syntax has changed (and sphinx hasn't keep up) or the type doesn't even exist (nasm?). Other documents had :options: what were invalid. I only removed those that had warnings, and left the ones that didn't, in order to follow the principle of least surprise. This is like this for ages, but the buildbot is now failing on errors. It may take a while to upgrade the buildbot's sphinx, if that's even possible, but that shouldn't stop us from getting docs updates (which seem down for quite a while). Also, we're not losing any syntax highlight, since when it doesn't parse, it doesn't colour. Ie. those blocks are not being highlighted anyway. I'm trying to get all docs in one go, so that it's easy to revert later if we do fix, or at least easy to know what's to fix. llvm-svn: 276109
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3.1 KiB
ReStructuredText
78 lines
3.1 KiB
ReStructuredText
========================
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Segmented Stacks in LLVM
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========================
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.. contents::
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:local:
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Introduction
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============
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Segmented stack allows stack space to be allocated incrementally than as a
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monolithic chunk (of some worst case size) at thread initialization. This is
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done by allocating stack blocks (henceforth called *stacklets*) and linking them
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into a doubly linked list. The function prologue is responsible for checking if
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the current stacklet has enough space for the function to execute; and if not,
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call into the libgcc runtime to allocate more stack space. Segmented stacks are
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enabled with the ``"split-stack"`` attribute on LLVM functions.
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The runtime functionality is `already there in libgcc
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<http://gcc.gnu.org/wiki/SplitStacks>`_.
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Implementation Details
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======================
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.. _allocating stacklets:
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Allocating Stacklets
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--------------------
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As mentioned above, the function prologue checks if the current stacklet has
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enough space. The current approach is to use a slot in the TCB to store the
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current stack limit (minus the amount of space needed to allocate a new block) -
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this slot's offset is again dictated by ``libgcc``. The generated
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assembly looks like this on x86-64:
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.. code-block:: text
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leaq -8(%rsp), %r10
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cmpq %fs:112, %r10
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jg .LBB0_2
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# More stack space needs to be allocated
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movabsq $8, %r10 # The amount of space needed
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movabsq $0, %r11 # The total size of arguments passed on stack
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callq __morestack
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ret # The reason for this extra return is explained below
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.LBB0_2:
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# Usual prologue continues here
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The size of function arguments on the stack needs to be passed to
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``__morestack`` (this function is implemented in ``libgcc``) since that number
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of bytes has to be copied from the previous stacklet to the current one. This is
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so that SP (and FP) relative addressing of function arguments work as expected.
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The unusual ``ret`` is needed to have the function which made a call to
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``__morestack`` return correctly. ``__morestack``, instead of returning, calls
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into ``.LBB0_2``. This is possible since both, the size of the ``ret``
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instruction and the PC of call to ``__morestack`` are known. When the function
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body returns, control is transferred back to ``__morestack``. ``__morestack``
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then de-allocates the new stacklet, restores the correct SP value, and does a
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second return, which returns control to the correct caller.
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Variable Sized Allocas
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----------------------
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The section on `allocating stacklets`_ automatically assumes that every stack
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frame will be of fixed size. However, LLVM allows the use of the ``llvm.alloca``
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intrinsic to allocate dynamically sized blocks of memory on the stack. When
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faced with such a variable-sized alloca, code is generated to:
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* Check if the current stacklet has enough space. If yes, just bump the SP, like
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in the normal case.
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* If not, generate a call to ``libgcc``, which allocates the memory from the
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heap.
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The memory allocated from the heap is linked into a list in the current
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stacklet, and freed along with the same. This prevents a memory leak.
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