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annotate docs/SegmentedStacks.rst @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents 54457678186b
children 1172e4bd9c6f
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1 ========================
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2 Segmented Stacks in LLVM
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3 ========================
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4
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5 .. contents::
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6 :local:
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7
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8 Introduction
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9 ============
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10
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11 Segmented stack allows stack space to be allocated incrementally than as a
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12 monolithic chunk (of some worst case size) at thread initialization. This is
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13 done by allocating stack blocks (henceforth called *stacklets*) and linking them
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14 into a doubly linked list. The function prologue is responsible for checking if
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15 the current stacklet has enough space for the function to execute; and if not,
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16 call into the libgcc runtime to allocate more stack space. Segmented stacks are
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17 enabled with the ``"split-stack"`` attribute on LLVM functions.
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18
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19 The runtime functionality is `already there in libgcc
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20 <http://gcc.gnu.org/wiki/SplitStacks>`_.
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21
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22 Implementation Details
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23 ======================
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25 .. _allocating stacklets:
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26
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27 Allocating Stacklets
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28 --------------------
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29
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30 As mentioned above, the function prologue checks if the current stacklet has
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31 enough space. The current approach is to use a slot in the TCB to store the
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32 current stack limit (minus the amount of space needed to allocate a new block) -
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33 this slot's offset is again dictated by ``libgcc``. The generated
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34 assembly looks like this on x86-64:
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36 .. code-block:: nasm
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37
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38 leaq -8(%rsp), %r10
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39 cmpq %fs:112, %r10
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40 jg .LBB0_2
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41
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42 # More stack space needs to be allocated
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43 movabsq $8, %r10 # The amount of space needed
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44 movabsq $0, %r11 # The total size of arguments passed on stack
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45 callq __morestack
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46 ret # The reason for this extra return is explained below
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47 .LBB0_2:
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48 # Usual prologue continues here
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49
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50 The size of function arguments on the stack needs to be passed to
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51 ``__morestack`` (this function is implemented in ``libgcc``) since that number
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52 of bytes has to be copied from the previous stacklet to the current one. This is
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53 so that SP (and FP) relative addressing of function arguments work as expected.
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54
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55 The unusual ``ret`` is needed to have the function which made a call to
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56 ``__morestack`` return correctly. ``__morestack``, instead of returning, calls
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57 into ``.LBB0_2``. This is possible since both, the size of the ``ret``
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58 instruction and the PC of call to ``__morestack`` are known. When the function
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59 body returns, control is transferred back to ``__morestack``. ``__morestack``
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60 then de-allocates the new stacklet, restores the correct SP value, and does a
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61 second return, which returns control to the correct caller.
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62
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63 Variable Sized Allocas
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64 ----------------------
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65
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66 The section on `allocating stacklets`_ automatically assumes that every stack
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67 frame will be of fixed size. However, LLVM allows the use of the ``llvm.alloca``
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68 intrinsic to allocate dynamically sized blocks of memory on the stack. When
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69 faced with such a variable-sized alloca, code is generated to:
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70
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71 * Check if the current stacklet has enough space. If yes, just bump the SP, like
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72 in the normal case.
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73 * If not, generate a call to ``libgcc``, which allocates the memory from the
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74 heap.
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75
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76 The memory allocated from the heap is linked into a list in the current
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77 stacklet, and freed along with the same. This prevents a memory leak.