after trivial unswitching.
This PR illustrates that a fundamental analysis update was not performed
with the new loop unswitch. This update is also somewhat fundamental to
the core idea of the new loop unswitch -- we actually *update* the CFG
based on the unswitching. In order to do that, we need to update the
loop nest in addition to the domtree.
For some reason, when writing trivial unswitching, I thought that the
loop nest structure cannot be changed by the transformation. But the PR
helps illustrate that it clearly can. I've expanded this to a number of
different test cases that try to cover the different cases of this. When
we unswitch, we move an exit edge of a loop out of the loop. If this
exit edge changes which loop reached by an exit is the innermost loop,
it changes the parent of the loop. Essentially, this transformation may
hoist the inner loop up the nest. I've added the simple logic to handle
this reliably in the trivial unswitching case. This just requires
updating LoopInfo and rebuilding LCSSA on the impacted loops. In the
trivial case, we don't even need to handle dedicated exits because we're
only hoisting the one loop and we just split its preheader.
I've also ported all of these tests to non-trivial unswitching and
verified that the logic already there correctly handles the loop nest
updates necessary.
Differential Revision: https://reviews.llvm.org/D48851
llvm-svn: 336477
unswitching loops.
Original patch trying to address this was sent in D47624, but that
didn't quite handle things correctly. There are two key principles used
to select whether and how to invalidate SCEV-cached information about
loops:
1) We must invalidate any info SCEV has cached before unswitching as we
may change (or destroy) the loop structure by the act of unswitching,
and make it hard to recover everything we want to invalidate within
SCEV.
2) We need to invalidate all of the loops whose CFGs are mutated by the
unswitching. Notably, this isn't the *entire* loop nest, this is
every loop contained by the outermost loop reached by an exit block
relevant to the unswitch.
And we need to do this even when doing trivial unswitching.
I've added more focused tests that directly check that SCEV starts off
with imprecise information and after unswitching (and simplifying
instructions) re-querying SCEV will produce precise information. These
tests also specifically work to check that an *outer* loop's information
becomes precise.
However, the testing here is still a bit imperfect. Crafting test cases
that reliably fail to be analyzed by SCEV before unswitching and succeed
afterward proved ... very, very hard. It took me several hours and
careful work to build these, and I'm not optimistic about necessarily
coming up with more to cover more elaborate possibilities. Fortunately,
the code pattern we are testing here in the pass is really
straightforward and reliable.
Thanks to Max Kazantsev for the initial work on this as well as the
review, and to Hal Finkel for helping me talk through approaches to test
this stuff even if it didn't come to much.
Differential Revision: https://reviews.llvm.org/D47624
llvm-svn: 336183
unswitching of switches.
This works much like trivial unswitching of switches in that it reliably
moves the switch out of the loop. Here we potentially clone the entire
loop into each successor of the switch and re-point the cases at these
clones.
Due to the complexity of actually doing nontrivial unswitching, this
patch doesn't create a dedicated routine for handling switches -- it
would duplicate far too much code. Instead, it generalizes the existing
routine to handle both branches and switches as it largely reduces to
looping in a few places instead of doing something once. This actually
improves the results in some cases with branches due to being much more
careful about how dead regions of code are managed. With branches,
because exactly one clone is created and there are exactly two edges
considered, somewhat sloppy handling of the dead regions of code was
sufficient in most cases. But with switches, there are much more
complicated patterns of dead code and so I've had to move to a more
robust model generally. We still do as much pruning of the dead code
early as possible because that allows us to avoid even cloning the code.
This also surfaced another problem with nontrivial unswitching before
which is that we weren't as precise in reconstructing loops as we could
have been. This seems to have been mostly harmless, but resulted in
pointless LCSSA PHI nodes and other unnecessary cruft. With switches, we
have to get this *right*, and everything benefits from it.
While the testing may seem a bit light here because we only have two
real cases with actual switches, they do a surprisingly good job of
exercising numerous edge cases. Also, because we share the logic with
branches, most of the changes in this patch are reasonably well covered
by existing tests.
The new unswitch now has all of the same fundamental power as the old
one with the exception of the single unsound case of *partial* switch
unswitching -- that really is just loop specialization and not
unswitching at all. It doesn't fit into the canonicalization model in
any way. We can add a loop specialization pass that runs late based on
profile data if important test cases ever come up here.
Differential Revision: https://reviews.llvm.org/D47683
llvm-svn: 335553
conditions feeding a chain of `and`s or `or`s for a branch.
Much like with full non-trivial unswitching, we rely on the pass manager
to handle iterating until all of the profitable unswitches have been
done. This is to allow other more profitable unswitches to fire on any
of the cloned, simpler versions of the loop if viable.
Threading the partial unswiching through the non-trivial unswitching
logic motivated some minor refactorings. If those are too disruptive to
make it reasonable to review this patch, I can separate them out, but
it'll be somewhat timeconsuming so I wanted to send it for initial
review as-is. Feel free to tell me whether it warrants pulling apart.
I've tried to re-use (and factor out) logic form the partial trivial
unswitching, but not as much could be shared as I had haped. Still, this
wasn't as bad as I naively expected.
Some basic testing is added, but I probably need more. Suggestions for
things you'd like to see tested more than welcome. One thing I'd like to
do is add some testing that when we schedule this with loop-instsimplify
it effectively cleans up the cruft created.
Last but not least, this uncovered a bug that has been in loop cloning
the entire time for non-trivial unswitching. Specifically, we didn't
correctly add the outer-most cloned loop to the list of cloned loops.
This meant that LCSSA wouldn't be updated for it hypothetically, and
more significantly that we would never visit it in the loop pass
manager. I noticed this while checking loop-instsimplify by hand. I'll
try to separate this bugfix out into its own patch with a more focused
test. But it is just one line, so shouldn't significantly confuse the
review here.
After this patch, the only missing "feature" in this unswitch I'm aware
of us non-trivial unswitching of switches. I'll try implementing *full*
non-trivial unswitching of switches (which is at least a sound thing to
implement), but *partial* non-trivial unswitching of switches is
something I don't see any sound and principled way to implement. I also
have no interesting test cases for the latter, so I'm not really
worried. The rest of the things that need to be ported are bug-fixes and
more narrow / targeted support for specific issues.
Differential Revision: https://reviews.llvm.org/D47522
llvm-svn: 335203
The idea of partial unswitching is to take a *part* of a branch's
condition that is loop invariant and just unswitching that part. This
primarily makes sense with i1 conditions of branches as opposed to
switches. When dealing with i1 conditions, we can easily extract loop
invariant inputs to a a branch and unswitch them to test them entirely
outside the loop.
As part of this, we now create much more significant cruft in the loop
body, so this relies on adding cleanup passes to the loop pipeline and
revisiting unswitched loops to do that cleanup before continuing to
process them.
This already appears to be more powerful at unswitching than the old
loop unswitch pass, and so I'd appreciate pretty careful review in case
I'm just missing some correctness checks. The `LIV-loop-condition` test
case is not unswitched by the old unswitch pass, but is with this pass.
Thanks to Sanjoy and Fedor for the review!
Differential Revision: https://reviews.llvm.org/D46706
llvm-svn: 335156
Summary:
I noticed this issue because we didn't put the primary cloned loop into
the `NonChildClonedLoops` vector and so never iterated on it. Once
I fixed that, it made it clear why I had to do a really complicated and
unnecesasry dance when updating the loops to remain in canonical form --
I was unwittingly working around the fact that the primary cloned loop
wasn't in the expected list of cloned loops. Doh!
Now that we include it in this vector, we don't need to return it and we
can consolidate the update logic as we correctly have a single place
where it can be handled.
I've just added a test for the iteration order aspect as every time
I changed the update logic partially or incorrectly here, an existing
test failed and caught it so that seems well covered (which is also
evidenced by the extensive working around of this missing update).
Reviewers: asbirlea, sanjoy
Subscribers: mcrosier, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D47647
llvm-svn: 333811
loop-cleanup passes at the beginning of the loop pass pipeline, and
re-enqueue loops after even trivial unswitching.
This will allow us to much more consistently avoid simplifying code
while doing trivial unswitching. I've also added a test case that
specifically shows effective iteration using this technique.
I've unconditionally updated the new PM as that is always using the
SimpleLoopUnswitch pass, and I've made the pipeline changes for the old
PM conditional on using this new unswitch pass. I added a bunch of
comments to the loop pass pipeline in the old PM to make it more clear
what is going on when reviewing.
Hopefully this will unblock doing *partial* unswitching instead of just
full unswitching.
Differential Revision: https://reviews.llvm.org/D47408
llvm-svn: 333493
The DEBUG() macro is very generic so it might clash with other projects.
The renaming was done as follows:
- git grep -l 'DEBUG' | xargs sed -i 's/\bDEBUG\s\?(/LLVM_DEBUG(/g'
- git diff -U0 master | ../clang/tools/clang-format/clang-format-diff.py -i -p1 -style LLVM
- Manual change to APInt
- Manually chage DOCS as regex doesn't match it.
In the transition period the DEBUG() macro is still present and aliased
to the LLVM_DEBUG() one.
Differential Revision: https://reviews.llvm.org/D43624
llvm-svn: 332240
unswitch and replace it with the amazingly simple update API code.
This addresses piles of FIXMEs around the update logic here and makes
everything substantially simpler.
llvm-svn: 331247
code review.
It turns out this *is* necessary, and I read the comment on the API
correctly the first time. ;]
The `applyUpdates` routine requires that updates are "balanced". This is
in order to cleanly handle cycles like inserting, removing, nad then
re-inserting the same edge. This precludes inserting the same edge
multiple times in a row as handling that would cause the insertion logic
to become *ordered* instead of *unordered* (which is what the API
provides).
It happens that in this specific case nothing (other than an assert and
contract violation) goes wrong because we're never inserting and
removing the same edge. The implementation *happens* to do the right
thing to eliminate redundant insertions in that case.
But the requirement is there and there is an assert to catch it.
Somehow, after the code review I never did another asserts-clang build
testing loop-unswich for a long time. As a consequence, I didn't notice
this despite a bunch of testing going on, but it shows up immediately
with an asserts build of clang itself.
llvm-svn: 331246
update API for dominators rather than doing manual, hacky updates.
This is just the first step, but in some ways the most important as it
moves the non-trivial unswitching to update the domtree rather than
fully recalculating it each time.
Subsequent patches should remove the custom update logic used by the
trivial unswitch and replace it with uses of the update API.
This also fixes a number of bugs I was seeing when testing non-trivial
unswitch due to it querying the quasi-correct dominator tree. Now the
tree is 100% correct and safe to query. That said, there are still more
bugs I can see with non-trivial unswitch just running over the test
suite, so more bugfix patches are needed as well.
Thanks to both Sanjoy and Fedor for reviews and testing!
Differential Revision: https://reviews.llvm.org/D45943
llvm-svn: 330787
loop unswitch.
This code incorrectly added the header to the loop block set early. As
a consequence we would incorrectly conclude that a nested loop body had
already been visited when the header of the outer loop was the preheader
of the nested loop. In retrospect, adding the header eagerly doesn't
really make sense. It seems nicer to let the cycle be formed naturally.
This will catch crazy bugs in the CFG reconstruction where we can't
correctly form the cycle earlier rather than later, and makes the rest
of the logic just fall out.
I've also added various asserts that make these issues *much* easier to
debug.
llvm-svn: 330707
This code path can very clearly be called in a context where we have
baselined all the cloned blocks to a particular loop and are trying to
handle nested subloops. There is no harm in this, so just relax the
assert. I've added a test case that will make sure we actually exercise
this code path.
llvm-svn: 330680
The condition this was asserting doesn't actually hold. I've added
comments to explain why, removed the assert, and added a fun test case
reduced out of 403.gcc.
llvm-svn: 330564
Summary:
This fixes the bug pointed out in review with non-trivial unswitching.
This also provides a basis that should make it pretty easy to finish
fleshing out a routine to scan an entire function body for irreducible
control flow, but this patch remains minimal for disabling loop
unswitch.
Reviewers: sanjoy, fedor.sergeev
Subscribers: mcrosier, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D45754
llvm-svn: 330357
Summary:
r327219 added wrappers to std::sort which randomly shuffle the container before sorting.
This will help in uncovering non-determinism caused due to undefined sorting
order of objects having the same key.
To make use of that infrastructure we need to invoke llvm::sort instead of std::sort.
Note: This patch is one of a series of patches to replace *all* std::sort to llvm::sort.
Refer the comments section in D44363 for a list of all the required patches.
Reviewers: kcc, pcc, danielcdh, jmolloy, sanjoy, dberlin, ruiu
Reviewed By: ruiu
Subscribers: ruiu, llvm-commits
Differential Revision: https://reviews.llvm.org/D45142
llvm-svn: 330059
Removes verifyDomTree, using assert(verify()) everywhere instead, and
changes verify a little to always run IsSameAsFreshTree first in order
to print good output when we find errors. Also adds verifyAnalysis for
PostDomTrees, which will allow checking of PostDomTrees it the same way
we check DomTrees and MachineDomTrees.
Differential Revision: https://reviews.llvm.org/D41298
llvm-svn: 326315
making it no longer even remotely simple.
The pass will now be more of a "full loop unswitching" pass rather than
anything substantively simpler than any other approach. I plan to rename
it accordingly once the dust settles.
The key ideas of the new loop unswitcher are carried over for
non-trivial unswitching:
1) Fully unswitch a branch or switch instruction from inside of a loop to
outside of it.
2) Update the CFG and IR. This avoids needing to "remember" the
unswitched branches as well as avoiding excessively cloning and
reliance on complex parts of simplify-cfg to cleanup the cfg.
3) Update the analyses (where we can) rather than just blowing them away
or relying on something else updating them.
Sadly, #3 is somewhat compromised here as the dominator tree updates
were too complex for me to want to reason about. I will need to make
another attempt to do this now that we have a nice dynamic update API
for dominators. However, we do adhere to #3 w.r.t. LoopInfo.
This approach also adds an important principls specific to non-trivial
unswitching: not *all* of the loop will be duplicated when unswitching.
This fact allows us to compute the cost in terms of how much *duplicate*
code is inserted rather than just on raw size. Unswitching conditions
which essentialy partition loops will work regardless of the total loop
size.
Some remaining issues that I will be addressing in subsequent commits:
- Handling unstructured control flow.
- Unswitching 'switch' cases instead of just branches.
- Moving to the dynamic update API for dominators.
Some high-level, interesting limitationsV that folks might want to push
on as follow-ups but that I don't have any immediate plans around:
- We could be much more clever about not cloning things that will be
deleted. In fact, we should be able to delete *nothing* and do
a minimal number of clones.
- There are many more interesting selection criteria for which branch to
unswitch that we might want to look at. One that I'm interested in
particularly are a set of conditions which all exit the loop and which
can be merged into a single unswitched test of them.
Differential revision: https://reviews.llvm.org/D34200
llvm-svn: 318549
I did this a long time ago with a janky python script, but now
clang-format has built-in support for this. I fed clang-format every
line with a #include and let it re-sort things according to the precise
LLVM rules for include ordering baked into clang-format these days.
I've reverted a number of files where the results of sorting includes
isn't healthy. Either places where we have legacy code relying on
particular include ordering (where possible, I'll fix these separately)
or where we have particular formatting around #include lines that
I didn't want to disturb in this patch.
This patch is *entirely* mechanical. If you get merge conflicts or
anything, just ignore the changes in this patch and run clang-format
over your #include lines in the files.
Sorry for any noise here, but it is important to keep these things
stable. I was seeing an increasing number of patches with irrelevant
re-ordering of #include lines because clang-format was used. This patch
at least isolates that churn, makes it easy to skip when resolving
conflicts, and gets us to a clean baseline (again).
llvm-svn: 304787
pass.
The original logic only considered direct successors of the hoisted
domtree nodes, but that isn't really enough. If there are other basic
blocks that are completely within the subtree, their successors could
just as easily be impacted by the hoisting.
The more I think about it, the more I think the correct update here is
to hoist every block on the dominance frontier which has an idom in the
chain we hoist across. However, this is subtle enough that I'd
definitely appreciate some more eyes on it.
Sadly, if this is the correct algorithm, it requires computing a (highly
localized) dominance frontier. I've done this in the simplest (IE, least
code) way I could come up with, but that may be too naive. Suggestions
welcome here, dominance update algorithms are not an area I've studied
much, so I don't have strong opinions.
In good news, with this patch, turning on simple unswitch passes the
LLVM test suite for me with asserts enabled.
Differential Revision: https://reviews.llvm.org/D32740
llvm-svn: 303843
invariant PHI inputs and to rewrite PHI nodes during the actual
unswitching.
The checking is quite easy, but rewriting the PHI nodes is somewhat
surprisingly challenging. This should handle both branches and switches.
I think this is now a full featured trivial unswitcher, and more full
featured than the trivial cases in the old pass while still being (IMO)
somewhat simpler in how it works.
Next up is to verify its correctness in more widespread testing, and
then to add non-trivial unswitching.
Thanks to Davide and Sanjoy for the excellent review. There is one
remaining question that I may address in a follow-up patch (see the
review thread for details) but it isn't related to the functionality
specifically.
Differential Revision: https://reviews.llvm.org/D32699
llvm-svn: 302867
Currently, this pass only focuses on *trivial* loop unswitching. At that
reduced problem it remains significantly better than the current loop
unswitch:
- Old pass is worse than cubic complexity. New pass is (I think) linear.
- New pass is much simpler in its design by focusing on full unswitching. (See
below for details on this).
- New pass doesn't carry state for thresholds between pass iterations.
- New pass doesn't carry state for correctness (both miscompile and
infloop) between pass iterations.
- New pass produces substantially better code after unswitching.
- New pass can handle more trivial unswitch cases.
- New pass doesn't recompute the dominator tree for the entire function
and instead incrementally updates it.
I've ported all of the trivial unswitching test cases from the old pass
to the new one to make sure that major functionality isn't lost in the
process. For several of the test cases I've worked to improve the
precision and rigor of the CHECKs, but for many I've just updated them
to handle the new IR produced.
My initial motivation was the fact that the old pass carried state in
very unreliable ways between pass iterations, and these mechansims were
incompatible with the new pass manager. However, I discovered many more
improvements to make along the way.
This pass makes two very significant assumptions that enable most of these
improvements:
1) Focus on *full* unswitching -- that is, completely removing whatever
control flow construct is being unswitched from the loop. In the case
of trivial unswitching, this means removing the trivial (exiting)
edge. In non-trivial unswitching, this means removing the branch or
switch itself. This is in opposition to *partial* unswitching where
some part of the unswitched control flow remains in the loop. Partial
unswitching only really applies to switches and to folded branches.
These are very similar to full unrolling and partial unrolling. The
full form is an effective canonicalization, the partial form needs
a complex cost model, cannot be iterated, isn't canonicalizing, and
should be a separate pass that runs very late (much like unrolling).
2) Leverage LLVM's Loop machinery to the fullest. The original unswitch
dates from a time when a great deal of LLVM's loop infrastructure was
missing, ineffective, and/or unreliable. As a consequence, a lot of
complexity was added which we no longer need.
With these two overarching principles, I think we can build a fast and
effective unswitcher that fits in well in the new PM and in the
canonicalization pipeline. Some of the remaining functionality around
partial unswitching may not be relevant today (not many test cases or
benchmarks I can find) but if they are I'd like to add support for them
as a separate layer that runs very late in the pipeline.
Purely to make reviewing and introducing this code more manageable, I've
split this into first a trivial-unswitch-only pass and in the next patch
I'll add support for full non-trivial unswitching against a *fixed*
threshold, exactly like full unrolling. I even plan to re-use the
unrolling thresholds, as these are incredibly similar cost tradeoffs:
we're cloning a loop body in order to end up with simplified control
flow. We should only do that when the total growth is reasonably small.
One of the biggest changes with this pass compared to the previous one
is that previously, each individual trivial exiting edge from a switch
was unswitched separately as a branch. Now, we unswitch the entire
switch at once, with cases going to the various destinations. This lets
us unswitch multiple exiting edges in a single operation and also avoids
numerous extremely bad behaviors, where we would introduce 1000s of
branches to test for thousands of possible values, all of which would
take the exact same exit path bypassing the loop. Now we will use
a switch with 1000s of cases that can be efficiently lowered into
a jumptable. This avoids relying on somehow forming a switch out of the
branches or getting horrible code if that fails for any reason.
Another significant change is that this pass actively updates the CFG
based on unswitching. For trivial unswitching, this is actually very
easy because of the definition of loop simplified form. Doing this makes
the code coming out of loop unswitch dramatically more friendly. We
still should run loop-simplifycfg (at the least) after this to clean up,
but it will have to do a lot less work.
Finally, this pass makes much fewer attempts to simplify instructions
based on the unswitch. Something like loop-instsimplify, instcombine, or
GVN can be used to do increasingly powerful simplifications based on the
now dominating predicate. The old simplifications are things that
something like loop-instsimplify should get today or a very, very basic
loop-instcombine could get. Keeping that logic separate is a big
simplifying technique.
Most of the code in this pass that isn't in the old one has to do with
achieving specific goals:
- Updating the dominator tree as we go
- Unswitching all cases in a switch in a single step.
I think it is still shorter than just the trivial unswitching code in
the old pass despite having this functionality.
Differential Revision: https://reviews.llvm.org/D32409
llvm-svn: 301576
Chandler Carruth