1.. _loop-terminology:
2
3===========================================
4LLVM Loop Terminology (and Canonical Forms)
5===========================================
6
7.. contents::
8   :local:
9
10Introduction
11============
12
13Loops are a core concept in any optimizer.  This page spells out some
14of the common terminology used within LLVM code to describe loop
15structures.
16
17First, let's start with the basics. In LLVM, a Loop is a maximal set of basic
18blocks that form a strongly connected component (SCC) in the Control
19Flow Graph (CFG) where there exists a dedicated entry/header block that
20dominates all other blocks within the loop. Thus, without leaving the
21loop, one can reach every block in the loop from the header block and
22the header block from every block in the loop.
23
24Note that there are some important implications of this definition:
25
26* Not all SCCs are loops.  There exist SCCs that do not meet the
27  dominance requirement and such are not considered loops.
28
29* Loops can contain non-loop SCCs and non-loop SCCs may contain
30  loops.  Loops may also contain sub-loops.
31
32* A header block is uniquely associated with one loop.  There can be
33  multiple SCC within that loop, but the strongly connected component
34  (SCC) formed from their union must always be unique.
35
36* Given the use of dominance in the definition, all loops are
37  statically reachable from the entry of the function.
38
39* Every loop must have a header block, and some set of predecessors
40  outside the loop.  A loop is allowed to be statically infinite, so
41  there need not be any exiting edges.
42
43* Any two loops are either fully disjoint (no intersecting blocks), or
44  one must be a sub-loop of the other.
45
46A loop may have an arbitrary number of exits, both explicit (via
47control flow) and implicit (via throwing calls which transfer control
48out of the containing function).  There is no special requirement on
49the form or structure of exit blocks (the block outside the loop which
50is branched to).  They may have multiple predecessors, phis, etc...
51
52Key Terminology
53===============
54
55Header Block - The basic block which dominates all other blocks
56contained within the loop.  As such, it is the first one executed if
57the loop executes at all.  Note that a block can be the header of
58two separate loops at the same time, but only if one is a sub-loop
59of the other.
60
61Exiting Block - A basic block contained within a given loop which has
62at least one successor outside of the loop and one successor inside the
63loop.  (The latter is a consequence of the block being contained within
64an SCC which is part of the loop.)  That is, it has a successor which
65is an Exit Block.
66
67Exit Block - A basic block outside of the associated loop which has a
68predecessor inside the loop.  That is, it has a predecessor which is
69an Exiting Block.
70
71Latch Block - A basic block within the loop whose successors include
72the header block of the loop.  Thus, a latch is a source of backedge.
73A loop may have multiple latch blocks.  A latch block may be either
74conditional or unconditional.
75
76Backedge(s) - The edge(s) in the CFG from latch blocks to the header
77block.  Note that there can be multiple such edges, and even multiple
78such edges leaving a single latch block.
79
80Loop Predecessor -  The predecessor blocks of the loop header which
81are not contained by the loop itself.  These are the only blocks
82through which execution can enter the loop.  When used in the
83singular form implies that there is only one such unique block.
84
85Preheader Block - A preheader is a (singular) loop predecessor which
86ends in an unconditional transfer of control to the loop header.  Note
87that not all loops have such blocks.
88
89Backedge Taken Count - The number of times the backedge will execute
90before some interesting event happens.  Commonly used without
91qualification of the event as a shorthand for when some exiting block
92branches to some exit block. May be zero, or not statically computable.
93
94Iteration Count - The number of times the header will execute before
95some interesting event happens.  Commonly used without qualification to
96refer to the iteration count at which the loop exits.  Will always be
97one greater than the backedge taken count.  *Warning*: Preceding
98statement is true in the *integer domain*; if you're dealing with fixed
99width integers (such as LLVM Values or SCEVs), you need to be cautious
100of overflow when converting one to the other.
101
102It's important to note that the same basic block can play multiple
103roles in the same loop, or in different loops at once.  For example, a
104single block can be the header for two nested loops at once, while
105also being an exiting block for the inner one only, and an exit block
106for a sibling loop.  Example:
107
108.. code-block:: C
109
110  while (..) {
111    for (..) {}
112    do {
113      do {
114        // <-- block of interest
115        if (exit) break;
116      } while (..);
117    } while (..)
118  }
119
120LoopInfo
121========
122
123LoopInfo is the core analysis for obtaining information about loops.
124There are few key implications of the definitions given above which
125are important for working successfully with this interface.
126
127* LoopInfo does not contain information about non-loop cycles.  As a
128  result, it is not suitable for any algorithm which requires complete
129  cycle detection for correctness.
130
131* LoopInfo provides an interface for enumerating all top level loops
132  (e.g. those not contained in any other loop).  From there, you may
133  walk the tree of sub-loops rooted in that top level loop.
134
135* Loops which become statically unreachable during optimization *must*
136  be removed from LoopInfo. If this can not be done for some reason,
137  then the optimization is *required* to preserve the static
138  reachability of the loop.
139
140
141Loop Simplify Form
142==================
143
144TBD
145
146
147Loop Closed SSA (LCSSA)
148=======================
149
150TBD
151
152"More Canonical" Loops
153======================
154
155TBD
156