Blindly following unique-successors chain appeared to be a bad idea.
In a degenerate case when block jumps to itself that goes into endless loop.
Discovered this problem when playing with additional changes,
managed to reproduce it on existing LoopPredication code.
Fix by checking a "visited" set while iterating through unique successors.
Reviewed By: skatkov
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D72908
We may end up with a case where we have a widenable branch above the loop, but not all widenable branches within the loop have been removed. Since a widenable branch inhibit SCEVs ability to reason about exit counts (by design), we have a tradeoff between effectiveness of this optimization and allowing future widening of the branches within the loop. LoopPred is thought to be one of the most important optimizations for range check elimination, so let's pay the cost.
As a reminder, a "widenable branch" is the pattern "br i1 (and i1 X, WC()), label %taken, label %untaken" where "WC" is the widenable condition intrinsics. The semantics of such a branch (derived from the semantics of WC) is that a new condition can be added into the condition arbitrarily without violating legality.
Broaden the definition in two ways:
Allow swapped operands to the br (and X, WC()) form
Allow widenable branch w/trivial condition (i.e. true) which takes form of br i1 WC()
The former is just general robustness (e.g. for X = non-instruction this is what instcombine produces). The later is specifically important as partial unswitching of a widenable range check produces exactly this form above the loop.
Differential Revision: https://reviews.llvm.org/D70502
Unswitch (and other loop transforms) like to generate loop exit blocks with unconditional successors, and phi nodes (LCSSA, or simple multiple exiting blocks sharing an exit). Generalize the "likely very rare exit" check slightly to handle this form.
This implements a version of the predicateLoopExits transform from IndVarSimplify extended to exploit widenable conditions - and thus be much wider in scope of legality. The code structure ends up being almost entirely different, so I chose to duplicate this into the LoopPredication pass instead of trying to reuse the code in the IndVars.
The core notions of the transform are as follows:
If we have a widenable condition which controls entry into the loop, we're allowed to widen it arbitrarily. Given that, it's simply a *profitability* question as to what conditions to fold into the widenable branch.
To avoid pass ordering issues, we want to avoid widening cases that would otherwise be dischargeable. Or... widen in a form which can still be discharged. Thus, we phrase the transform as selecting one analyzeable exit from the set of analyzeable exits to keep. This avoids creating pass ordering complexities.
Since none of the above proves that we actually exit through our analyzeable exits - we might exit through something else entirely - we limit ourselves to cases where a) the latch is analyzeable and b) the latch is predicted taken, and c) the exit being removed is statically cold.
Differential Revision: https://reviews.llvm.org/D69830
A while back, I added support for NE latches formed by LFTR. I didn't think that quite through, as LFTR will also produce the inverse EQ form for some loops and I hadn't handled that. This change just adds handling for that case as well.
llvm-svn: 365419
This reverts commit r365260 which broke the following tests:
Clang :: CodeGenCXX/cfi-mfcall.cpp
Clang :: CodeGenObjC/ubsan-nullability.m
LLVM :: Transforms/LoopVectorize/AArch64/pr36032.ll
llvm-svn: 365284
Without this, we have the unfortunate property that tests are dependent on the order of operads passed the CreateOr and CreateAnd functions. In actual usage, we'd promptly optimize them away, but it made tests slightly more verbose than they should have been.
llvm-svn: 365260
This is a really silly bug that even a simple test w/an unconditional latch would have caught. I tried to guard against the case, but put it in the wrong if check. Oops.
llvm-svn: 362727
At the moment, LoopPredication completely bails out if it sees a latch of the form:
%cmp = icmp ne %iv, %N
br i1 %cmp, label %loop, label %exit
OR
%cmp = icmp ne %iv.next, %NPlus1
br i1 %cmp, label %loop, label %exit
This is unfortunate since this is exactly the form that LFTR likes to produce. So, go ahead and recognize simple cases where we can.
For pre-increment loops, we leverage the fact that LFTR likes canonical counters (i.e. those starting at zero) and a (presumed) range fact on RHS to discharge the check trivially.
For post-increment forms, the key insight is in remembering that LFTR had to insert a (N+1) for the RHS. CVP can hopefully prove that add nsw/nuw (if there's appropriate range on N to start with). This leaves us both with the post-inc IV and the RHS involving an nsw/nuw add, and SCEV can discharge that with no problem.
This does still need to be extended to handle non-one steps, or other harder patterns of variable (but range restricted) starting values. That'll come later.
Differential Revision: https://reviews.llvm.org/D62748
llvm-svn: 362282
The bug is that I didn't check whether the operand of the invariant_loads were themselves invariant. I don't know how this got missed in the patch and review. I even had an unreduced test case locally, and I remember handling this case, but I must have lost it in one of the rebases. Oops.
llvm-svn: 358688
The purpose of this patch is to eliminate a pass ordering dependence between LoopPredication and LICM. To understand the purpose, consider the following snippet of code inside some loop 'L' with IV 'i'
A = _a.length;
guard (i < A)
a = _a[i]
B = _b.length;
guard (i < B);
b = _b[i];
...
Z = _z.length;
guard (i < Z)
z = _z[i]
accum += a + b + ... + z;
Today, we need LICM to hoist the length loads, LoopPredication to make the guards loop invariant, and TrivialUnswitch to eliminate the loop invariant guard to establish must execute for the next length load. Today, if we can't prove speculation safety, we'd have to iterate these three passes 26 times to reduce this example down to the minimal form.
Using the fact that the array lengths are known to be invariant, we can short circuit this iteration. By forming the loop invariant form of all the guards at once, we remove the need for LoopPredication from the iterative cycle. At the moment, we'd still have to iterate LICM and TrivialUnswitch; we'll leave that part for later.
As a secondary benefit, this allows LoopPred to expose peeling oppurtunities in a much more obvious manner. See the udiv test changes as an example. If the udiv was not hoistable (i.e. we couldn't prove speculation safety) this would be an example where peeling becomes obviously profitable whereas it wasn't before.
A couple of subtleties in the implementation:
- SCEV's isSafeToExpand guarantees speculation safety (i.e. let's us expand at a new point). It is not a precondition for expansion if we know the SCEV corresponds to a Value which dominates the requested expansion point.
- SCEV's isLoopInvariant returns true for expressions which compute the same value across all iterations executed, regardless of where the original Value is located. (i.e. it can be in the loop) This implies we have a speculation burden to prove before expanding them outside loops.
- invariant_loads and AA->pointsToConstantMemory are two cases that SCEV currently does not handle, but meets the SCEV definition of invariance. I plan to sink this part into SCEV once this has baked for a bit.
Differential Revision: https://reviews.llvm.org/D60093
llvm-svn: 358684
As it's causing some bot failures (and per request from kbarton).
This reverts commit r358543/ab70da07286e618016e78247e4a24fcb84077fda.
llvm-svn: 358546
If we have multiple range checks which can be predicated, hoist the and of the results outside the loop. This minorly cleans up the resulting IR, but the main motivation is as a building block for D60093.
llvm-svn: 358419
The code was failing to actually check for the presence of the call to widenable_condition. The whole point of specifying the widenable_condition intrinsic was allowing widening transforms. A normal branch is not widenable. A normal branch leading to a deopt is not widenable (in general).
I added a test case via LoopPredication, but GuardWidening has an analogous bug. Those are the only two passes actually using this utility just yet. Noticed while working on LoopPredication for non-widenable branches; POC in D60111.
llvm-svn: 357493
We'd been optimizing the case where the predicate was obviously true, do the same for the false case. Mostly just for completeness sake, but also may improve compile time in loops which will exit through the guard. Such loops are presumed rare in fastpath code, but may be present down untaken paths, so optimizing for them is still useful.
llvm-svn: 357408
LoopPredication was replacing the original condition, but leaving the instructions to compute the old conditions around. This would get cleaned up by other passes of course, but we might as well do it eagerly. That also makes the test output less confusing.
llvm-svn: 357406
I'm about to make some changes to the pass which cause widespread - but uninteresting - test diffs. Prepare the tests for easy updating.
llvm-svn: 357404
As highlighted by tests, if one of the operands is loop variant, but guaranteed to have the same value on all iterations, we have a missed oppurtunity.
llvm-svn: 357403
This patch adds support of guards expressed as branches by widenable
conditions in Loop Predication.
Differential Revision: https://reviews.llvm.org/D56081
Reviewed By: reames
llvm-svn: 351805
Summary:
LoopPredication is not profitable when the loop is known to always exit
through some block other than the latch block.
A coarse grained latch check can cause loop predication to predicate the
loop, and unconditionally deoptimize.
However, without predicating the loop, the guard may never fail within the
loop during the dynamic execution because the non-latch loop termination
condition exits the loop before the latch condition causes the loop to
exit.
We teach LP about this using BranchProfileInfo pass.
Reviewers: apilipenko, skatkov, mkazantsev, reames
Reviewed by: skatkov
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D44667
llvm-svn: 328210
Add support of uge and sge latch condition to Loop Prediction for
reverse loops.
Reviewers: apilipenko, mkazantsev, sanjoy, anna
Reviewed By: anna
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D42837
llvm-svn: 324589
Summary:
Currently, we only support predication for forward loops with step
of 1. This patch enables loop predication for reverse or
countdownLoops, which satisfy the following conditions:
1. The step of the IV is -1.
2. The loop has a singe latch as B(X) = X <pred>
latchLimit with pred as s> or u>
3. The IV of the guard is the decrement
IV of the latch condition (Guard is: G(X) = X-1 u< guardLimit).
This patch was downstream for a while and is the last series of patches
that's from our LP implementation downstream.
Reviewers: apilipenko, mkazantsev, sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D40353
llvm-svn: 319659
Summary:
This patch allows us to predicate range checks that have a type narrower than
the latch check type. We leverage SCEV analysis to identify a truncate for the
latchLimit and latchStart.
There is also safety checks in place which requires the start and limit to be
known at compile time. We require this to make sure that the SCEV truncate expr
for the IV corresponding to the latch does not cause us to lose information
about the IV range.
Added tests show the loop predication over range checks that are of various
types and are narrower than the latch type.
This enhancement has been in our downstream tree for a while.
Reviewers: apilipenko, sanjoy, mkazantsev
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D39500
llvm-svn: 317269
This is a follow up change for D37569.
Currently the transformation is limited to the case when:
* The loop has a single latch with the condition of the form: ++i <pred> latchLimit, where <pred> is u<, u<=, s<, or s<=.
* The step of the IV used in the latch condition is 1.
* The IV of the latch condition is the same as the post increment IV of the guard condition.
* The guard condition is of the form i u< guardLimit.
This patch enables the transform in the case when the latch is
latchStart + i <pred> latchLimit, where <pred> is u<, u<=, s<, or s<=.
And the guard is
guardStart + i u< guardLimit
Reviewed By: anna
Differential Revision: https://reviews.llvm.org/D39097
llvm-svn: 316768
This is a follow up for the loop predication change 313981 to support ule, sle latch predicates.
Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D38177
llvm-svn: 315616
We've found a serious issue with the current implementation of loop predication.
The current implementation relies on SCEV and this turned out to be problematic.
To fix the problem we had to rework the pass substantially. We have had the
reworked implementation in our downstream tree for a while. This is the initial
patch of the series of changes to upstream the new implementation.
For now the transformation is limited to the following case:
* The loop has a single latch with either ult or slt icmp condition.
* The step of the IV used in the latch condition is 1.
* The IV of the latch condition is the same as the post increment IV of the guard condition.
* The guard condition is ult.
See the review or the LoopPredication.cpp header for the details about the
problem and the new implementation.
Reviewed By: sanjoy, mkazantsev
Differential Revision: https://reviews.llvm.org/D37569
llvm-svn: 313981
This is a fix for a loop predication bug which resulted in malformed IR generation.
Loop invariant side of the widened condition is not guaranteed to be available in the preheader as is, so we need to expand it as well. See added unsigned_loop_0_to_n_hoist_length test for example.
Reviewed By: sanjoy, mkazantsev
Differential Revision: https://reviews.llvm.org/D30099
llvm-svn: 296345
This patch introduces guard based loop predication optimization. The new LoopPredication pass tries to convert loop variant range checks to loop invariant by widening checks across loop iterations. For example, it will convert
for (i = 0; i < n; i++) {
guard(i < len);
...
}
to
for (i = 0; i < n; i++) {
guard(n - 1 < len);
...
}
After this transformation the condition of the guard is loop invariant, so loop-unswitch can later unswitch the loop by this condition which basically predicates the loop by the widened condition:
if (n - 1 < len)
for (i = 0; i < n; i++) {
...
}
else
deoptimize
This patch relies on an NFC change to make ScalarEvolution::isMonotonicPredicate public (revision 293062).
Reviewed By: sanjoy
Differential Revision: https://reviews.llvm.org/D29034
llvm-svn: 293064
Fedor Sergeev