Index: docs/LoopTerminology.rst =================================================================== --- docs/LoopTerminology.rst +++ docs/LoopTerminology.rst @@ -12,18 +12,25 @@ 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 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. @@ -52,7 +59,7 @@ 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 +SCC which makes up 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