This patch adds a new cost heuristic that allows peeling a single
iteration off read-only loops, if the loop contains a load that
1. is feeding an exit condition,
2. dominates the latch,
3. is not already known to be dereferenceable,
4. and has a loop invariant address.
If all non-latch exits are terminated with unreachable, such loads
in the loop are guaranteed to be dereferenceable after peeling,
enabling hoisting/CSE'ing them.
This enables vectorization of loops with certain runtime-checks, like
multiple calls to `std::vector::at` if the vector is passed as pointer.
Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D108114
Removed obsolete DT verification that should not be there because the
strategy of DT updates has changed.
Differential Revision: https://reviews.llvm.org/D110922
Added support for peeling loops with "deoptimizing" exits -
such exits that it or any of its children (or any of their
children, etc) either has a @llvm.experimental.deoptimize call
prior to the terminating return instruction of this basic block
or is terminated with unreachable. All blocks in the the
sequence must have a single successor, maybe except for the last
one.
Previously we only checked the exit block for being deoptimizing.
Now we check if the last reachable block from the exit is deoptimizing.
Patch by Dmitry Makogon!
Differential Revision: https://reviews.llvm.org/D110922
Reviewed By: mkazantsev
I can't seem to wrap my head around the proper fix here,
we should be fine without this requirement, iff we can form this form,
but the naive attempt (https://reviews.llvm.org/D106317) has failed.
So just to unblock the release, put up a restriction.
Fixes https://bugs.llvm.org/show_bug.cgi?id=51125
This patch adds support for unrolling inner loops using epilogue unrolling. The basic issue is that the original latch exit block of the inner loop could be outside the outer loop. When we clone the inner loop and split the latch exit, the cloned blocks need to be in the outer loop.
Differential Revision: https://reviews.llvm.org/D108476
The runtime unroller will try to produce a non-loop if the unroll count is 2 and thus the prolog/epilog loop would only run at most one iteration. The old implementation did this by avoiding loop construction entirely. This patches instead constructs the trivial loop and then explicitly breaks the backedge and simplifies. This does result in some additional code churn when triggered, but a) results in better quality code and b) removes a codepath which didn't work properly for multiple exit epilogs.
One oddity that I want to draw to reviewer attention is that this somehow changes revisit order. The new order looks equivalent to me, but I don't understand how creating and erasing an extra loop here creates this effect.
Differential Revision: https://reviews.llvm.org/D108521
Support for peeling with multiple exit blocks was added in D63921/77bb3a486fa6.
So far it has only been enabled for loops where all non-latch exits are
'de-optimizing' exits (D63923). But peeling of multi-exit loops can be
highly beneficial in other cases too, like if all non-latch exiting
blocks are unreachable.
The motivating case are loops with runtime checks, like the C++ example
below. The main issue preventing vectorization is that the invariant
accesses to load the bounds of B is conditionally executed in the loop
and cannot be hoisted out. If we peel off the first iteration, they
become dereferenceable in the loop, because they must execute before the
loop is executed, as all non-latch exits are terminated with
unreachable. This subsequently allows hoisting the loads and runtime
checks out of the loop, allowing vectorization of the loop.
int sum(std::vector<int> *A, std::vector<int> *B, int N) {
int cost = 0;
for (int i = 0; i < N; ++i)
cost += A->at(i) + B->at(i);
return cost;
}
This gives a ~20-30% increase of score for Geekbench5/HDR on AArch64.
Note that this requires a follow-up improvement to the peeling cost
model to actually peel iterations off loops as above. I will share that
shortly.
Also, peeling of multi-exits might be beneficial for exit blocks with
other terminators, but I would like to keep the scope limited to known
high-reward cases for now.
I removed the option to disable peeling for multi-deopt exits because
the code is more general now. Alternatively, the option could also be
generalized, but I am not sure if there's much value in the option?
Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D108108
This patch extends the runtime unrolling infrastructure to support unrolling a loop with multiple exiting blocks branching to the same exit block used by the latch. It intentionally does not include a cost model change to enable this functionality unless appropriate force flags are used.
This is the prolog companion to D107381. Since this was LGTMed, a problem with DT updating was reported against that patch. I roled in the analogous fix here as it seemed obvious, and not worth re-review.
As an aside, our prolog form leaves a lot of potential value on the floor when there is an invariant load or invariant condition in the loop being runtime unrolled. We should probably consider a "required prolog" heuristic. (Alternatively, maybe we should be peeling these cases more aggressively?)
Differential Revision: https://reviews.llvm.org/D108262
This patch extends the runtime unrolling infrastructure to support unrolling a loop with multiple exiting blocks branching to the same exit block used by the latch. It intentionally does not include a cost model change to enable this functionality unless appropriate force flags are used.
I decided to restrict this to the epilogue case. Given the changes ended up being pretty generic, we may be able to unblock the prolog case too, but I want to do that in a separate change to reduce the amount of code we all have to understand at one time.
Differential Revision: https://reviews.llvm.org/D107381
I'm not sure this is the best way to approach this,
but the situation is rather not very detectable unless we explicitly call it out when refusing to advise to unroll.
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D107271
There was an alias between 'simplifycfg' and 'simplify-cfg' in the
PassRegistry. That was the original reason for this patch, which
effectively removes the alias.
This patch also replaces all occurrances of 'simplify-cfg'
by 'simplifycfg'. Reason for choosing that form for the name is
that it matches the DEBUG_TYPE for the pass, and the legacy PM name
and also how it is spelled out in other passes such as
'loop-simplifycfg', and in other options such as
'simplifycfg-merge-cond-stores'.
I for some reason the name should be changed to 'simplify-cfg' in
the future, then I think such a renaming should be more widely done
and not only impacting the PassRegistry.
Reviewed By: aeubanks
Differential Revision: https://reviews.llvm.org/D105627
Based ontop of D104598, which is a NFCI-ish refactoring.
Here, a restriction, that only empty blocks can be merged, is lifted.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D104597
v6m cores only have a limited number of registers available. Unrolling
can mean we spend more on stack spills and reloads than we save from the
unrolling. This patch adds an extra heuristic to put a limit on the
unroll count for loops with multiple live out values, as measured from
the LCSSA phi nodes.
Differential Revision: https://reviews.llvm.org/D104659
This changes the approach taken to tail-merge the blocks
to always create a new block instead of trying to reuse some block,
and generalizes it to support dealing not with just the `ret` in the future.
This effectively lifts the CallBr restriction, although this isn't really intentional.
That is the only non-NFC change here, i'm not sure if it's reasonable/feasible to temporarily retain it.
Other restrictions of the transform remain.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D104598
This is a more general alternative/extension to D102635. Rather than
handling the special case of "header exit with non-exiting latch",
this unrolls against the smallest exact trip count from any exit.
The latch exit is no longer treated as priviledged when it comes to
full unrolling.
The motivating case is in full-unroll-one-unpredictable-exit.ll.
Here the header exit is an IV-based exit, while the latch exit is
a data comparison. This kind of loop does not get rotated, because
the latch is already exiting, and loop rotation doesn't try to
distinguish IV-based/analyzable latches.
Differential Revision: https://reviews.llvm.org/D102982
Remove dependence on ULO.TripCount/ULO.TripMultiple from ORE and
debug code. For debug code, print information about all exits.
For optimization remarks, only include the unroll count and the
type of unroll (complete, partial or runtime), but omit detailed
information about exit folding, now that more than one exit may
be folded.
Differential Revision: https://reviews.llvm.org/D104482
Fold all exits based on known trip count/multiple information from
SCEV. Previously only the latch exit or the single exit were folded.
This doesn't yet eliminate ULO.TripCount and ULO.TripMultiple
entirely: They're still used to a) decide whether runtime unrolling
should be performed and b) for ORE remarks. However, the core
unrolling logic is independent of them now.
Differential Revision: https://reviews.llvm.org/D104203
Addition of this pass has been botched.
There is no particular reason why it had to be sold as an inseparable part
of new-pm transition. It was added when old-pm was still the default,
and very *very* few users were actually tracking new-pm,
so it's effects weren't measured.
Which means, some of the turnoil of the new-pm transition
are actually likely regressions due to this pass.
Likewise, there has been a number of post-commit feedback
(post new-pm switch), namely
* https://reviews.llvm.org/D37467#2787157 (regresses HW-loops)
* https://reviews.llvm.org/D37467#2787259 (should not be in middle-end, should run after LSR, not before)
* https://reviews.llvm.org/D95789 (an attempt to fix bad loop backedge metadata)
and in the half year past, the pass authors (google) still haven't found time to respond to any of that.
Hereby it is proposed to backout the pass from the pipeline,
until someone who cares about it can address the issues reported,
and properly start the process of adding a new pass into the pipeline,
with proper performance evaluation.
Furthermore, neither google nor facebook reports any perf changes
from this change, so i'm dropping the pass completely.
It can always be re-reverted should/if anyone want to pick it up again.
Reviewed By: aeubanks
Differential Revision: https://reviews.llvm.org/D104099
Unrolling with more iterations than MaxTripCount is pointless, as
those iterations can never be executed. As such, we clamp ULO.Count
to MaxTripCount if it is known. This means we no longer need to
consider iterations after MaxTripCount for exit folding, and the
CompletelyUnroll flag becomes independent of ULO.TripCount.
Differential Revision: https://reviews.llvm.org/D103748
Loop peeling is currently performed as part of UnrollLoop().
Outside test scenarios, it is always performed with an unroll
count of 1. This means that unrolling doesn't actually do anything
apart from performing post-unroll simplification.
When testing, it's currently possible to specify both an explicit
peel count and an explicit unroll count. This doesn't perform any
sensible operation and may result in miscompiles, see
https://bugs.llvm.org/show_bug.cgi?id=45939.
This patch moves peeling from UnrollLoop() into tryToUnrollLoop(),
so that peeling does not also perform a susequent unroll. We only
run the post-unroll simplifications. Specifying both an explicit
peel count and unroll count is forbidden.
In the future, we may want to support both (non-PGO) peeling a
loop and unrolling it, but this needs to be done by first performing
the peel and then recalculating unrolling heuristics on a now
possibly analyzable loop.
Differential Revision: https://reviews.llvm.org/D103362
This builds on D103584. The change eliminates the coupling between unroll heuristic and implementation w.r.t. knowing when the passed in trip count is an exact trip count or a max trip count. In theory the new code is slightly less powerful (since it relies on exact computable trip counts), but in practice, it appears to cover all the same cases. It can also be extended if needed.
The test change shows what appears to be a bug in the existing code around the interaction of peeling and unrolling. The original loop only ran 8 iterations. The previous output had the loop peeled by 2, and then an exact unroll of 8. This meant the loop ran a total of 10 iterations which appears to have been a miscompile.
Differential Revision: https://reviews.llvm.org/D103620
One exit is unpredictable, the other has a known trip count. For
one function the predictable exit is the latch exit, for the other
the non-latch exit. Currently they are treated differently.
This is to show that we currently only convert the terminator to
unreachable, but don't clean up instructions before it (unless
trivial DCE removes them).
Also clean up excessive whitespace in this test.
The current full unroll cost model does a symbolic evaluation of the loop up to a fixed limit. That symbolic evaluation currently simplifies to constants, but we can generalize to arbitrary Values using the InstructionSimplify infrastructure at very low cost.
By itself, this enables some simplifications, but it's mainly useful when combined with the branch simplification over in D102928.
Differential Revision: https://reviews.llvm.org/D102934
Initially it failed an assertion with "Do actual DCE in LoopUnroll (try 2)"
which was later reverted. Make sure that when this patch is returned, the
test works fine.
This test case would get miscompiled by the current version of
D102982, because unrolling does not respect the PreserveCondBr
flag for partial unrolling.
This test case requires unrolling against a non-latch exit in
a multiple-exit loop with exiting latch. It's not covered by
exiting heuristics or the extension in D102635.
Turns out simplifyLoopIVs sometimes returns a non-dead instruction in it's DeadInsts out param. I had done a bit of NFC cleanup which was only NFC if simplifyLoopIVs obeyed it's documentation. I'm simplfy dropping that part of the change.
Commit message from try 3:
Recommitting after fixing a bug found post commit. Amusingly, try 1 had been correct, and by reverting to incorporate last minute review feedback, I introduce the bug. Oops. :)
Original commit message:
The problem was that recursively deleting an instruction can delete instructions beyond the current iterator (via a dead phi), thus invalidating iteration. Test case added in LoopUnroll/dce.ll to cover this case.
LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.
Differential Revision: https://reviews.llvm.org/D102511
Recommitting after fixing a bug found post commit. Amusingly, try 1 had been correct, and by reverting to incorporate last minute review feedback, I introduce the bug. Oops. :)
The problem was that recursively deleting an instruction can delete instructions beyond the current iterator (via a dead phi), thus invalidating iteration. Test case added in LoopUnroll/dce.ll to cover this case.
LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.
Differential Revision: https://reviews.llvm.org/D102511
Florian Hahn