[PATCH] D75233: [LoopTerminology] LCSSA Form

[Michael Kruse via Phabricator via llvm-commits]

Meinersbur added inline comments.


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Comment at: llvm/docs/LoopTerminology.rst:211-212
+This form is ensured by the LCSSA (:ref:`-loop-simplify <passes-lcssa>`)
+pass and is added automatically by the LoopPassManager when
+scheduling a LoopPass.
+After the loop optimizations are done, these extra phi nodes
----------------
Note that this applies to the new pass manager only (by `FunctionToLoopPassAdaptor`): This does not happen with the legacy (current default) pass manager.


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Comment at: llvm/docs/LoopTerminology.rst:248-251
+If we did not have Loop Closed SSA form, we would have had to
+do deep analysis of the control flow graph to figure out where
+to place the X4 phi node.  With it, we just loop for these
+PHIs and update them.
----------------
For the placement of the PHINode, LCSSA needs to do the deep analysis (finding the dominating exit block) as well. LCSSA makes it a two-step process. I don't think it would be complicated to do it in a single step on-demand when unswitching. There are other advantages.

LLVM uses a linked linked list to list all the uses of an instruction. There is a convenience function `replaceAllUsesWith` (or short: RAUW) does this replacement. For loops however, it had to distinguish between inside/outside users (1), but this is no different that when versioning a non-loop BasicBlock. For the placement of the PHINode, LCSSA needs to do the deep analysis (finding the exit block) as well.

I think the primary reason is that loop analysis become more independent (3). Say we perform a loop analysis on all loops in a function that stores reference to `llvm::Value`s used in the loops, such as ScalarEvolution. In a second step, we transform all loops using the analysis' result which does RAUW. Without LCSSA it might replace values in other loops and invalidate the analysis already applied to the. With LCSSA, only value of the PHINode in the exit block is changed, but the PHINode is the same instance used in other loops. However, if we have loop transformations that transform inner as well as outer loops, we still need to handle the case that transforming the inner (respectively outer) loop may invalidate an analysis on the other.

Another reason is that with LCSSA, `ScalarEvolution::getSCEV` is sufficient (2). Otherwise, one needs to use `getSCEVAtScope` to specify whether we are using the SCEV inside or outside. Polly does not require LCSSA, hence uses mostly `getSCEVAtScope`.

GCC has documentation about the advantages of LCSSA: https://gcc.gnu.org/onlinedocs/gccint/LCSSA.html . The numbers (1-3) correspond to the bullet list.


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