[llvm] r367440 - [docs] Reword documentation in terms of SCCs not cycles
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 31 09:24:20 PDT 2019
Author: reames
Date: Wed Jul 31 09:24:20 2019
New Revision: 367440
URL: http://llvm.org/viewvc/llvm-project?rev=367440&view=rev
Log:
[docs] Reword documentation in terms of SCCs not cycles
Given the example:
header:
br i1 %c, label %next, label %header
next:
br i1 %c2, label %exit, label %header
We end up with a loop containing both header and next. Given that, the describing the loop in terms of cycles is confusing since we have multiple distinct cycles within a single Loop. Standardize on the SCC to clarify.
Differential Revision: https://reviews.llvm.org/D65299
Modified:
llvm/trunk/docs/LoopTerminology.rst
Modified: llvm/trunk/docs/LoopTerminology.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/LoopTerminology.rst?rev=367440&r1=367439&r2=367440&view=diff
==============================================================================
--- llvm/trunk/docs/LoopTerminology.rst (original)
+++ llvm/trunk/docs/LoopTerminology.rst Wed Jul 31 09:24:20 2019
@@ -12,18 +12,25 @@ Loops are a core concept in any optimize
of the common terminology used within LLVM code to describe loop
structures.
-First, let's start with the basics. In LLVM, a Loop is a cycle within
-the control flow graph (CFG) where there exists one block (the loop
-header block) which dominates all other blocks within the cycle.
+First, let's start with the basics. In LLVM, a Loop is a set of basic
+blocks that form a strongly connected component (SCC) in the Control
+Flow Graph (CFG) where there exists a dedicated entry/header block that
+dominates all other blocks within the loop. Thus, without leaving the
+loop, one can reach every block in the loop from the header block and
+the header block from every block in the loop.
Note that there are some important implications of this definition:
-* Not all cycles are loops. There exist cycles that do not meet the
+* Not all SCCs are loops. There exist SCCs that do not meet the
dominance requirement and such are not considered loops.
-* Loops can contain non-loop cycles and non-loop cycles may contain
+* Loops can contain non-loop SCCs and non-loop SCCs may contain
loops. Loops may also contain sub-loops.
+* A header block is uniquely associated with one loop. There can be
+ multiple SCC within that loop, but the strongly connected component
+ (SCC) formed from their union must always be unique.
+
* Given the use of dominance in the definition, all loops are
statically reachable from the entry of the function.
@@ -51,9 +58,9 @@ of the other.
Exiting Block - A basic block contained within a given loop which has
at least one successor outside of the loop and one successor inside the
-loop. (The latter is required for the block to be contained within the
-cycle which makes up the loop.) That is, it has a successor which is
-an Exit Block.
+loop. (The latter is a consequence of the block being contained within
+an SCC which is part of the loop.) That is, it has a successor which
+is an Exit Block.
Exit Block - A basic block outside of the associated loop which has a
predecessor inside the loop. That is, it has a predecessor which is
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