Summary:
The code for generating a name for loops for various reporting scenarios
created a name by serializing the loop into a string. This may result in
a very large name for a loop containing many blocks. Use the getName()
function on the loop instead.
Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: Whitney (Whitney Tsang), aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D133587
With profile data, non-trivial LoopUnswitch will only apply on non-cold loops, as unswitching cold loops may not gain much benefit but significantly increase the code size.
Reviewed By: aeubanks, asbirlea
Differential Revision: https://reviews.llvm.org/D129599
The basic problem we have is that we're trying to reuse an instruction which is mapped to some SCEV. Since we can have multiple such instructions (potentially with different flags), this is analogous to our need to drop flags when performing CSE. A trivial implementation would simply drop flags on any instruction we decided to reuse, and that would be correct.
This patch is almost that trivial patch except that we preserve flags on the reused instruction when existing users would imply UB on overflow already. Adding new users can, at most, refine this program to one which doesn't execute UB which is valid.
In practice, this fixes two conceptual problems with the previous code: 1) a binop could have been canonicalized into a form with different opcode or operands, or 2) the inbounds GEP case which was simply unhandled.
On the test changes, most are pretty straight forward. We loose some flags (in some cases, they'd have been dropped on the next CSE pass anyways). The one that took me the longest to understand was the ashr-expansion test. What's happening there is that we're considering reuse of the mul, previously we disallowed it entirely, now we allow it with no flags. The surrounding diffs are all effects of generating the same mul with a different operand order, and then doing simple DCE.
The loss of the inbounds is unfortunate, but even there, we can recover most of those once we actually treat branch-on-poison as immediate UB.
Differential Revision: https://reviews.llvm.org/D112734
Upon further investigation and discussion,
this is actually the opposite direction from what we should be taking,
and this direction wouldn't solve the motivational problem anyway.
Additionally, some more (polly) tests have escaped being updated.
So, let's just take a step back here.
This reverts commit f3190dedeef9da2109ea57e4cb372f295ff53b88.
This reverts commit 749581d21f2b3f53e4fca4eb8728c942d646893b.
This reverts commit f3df87d57e096143670e0fd396e81d43393a2dd2.
This reverts commit ab1dbcecd6f0969976fafd62af34730436ad5944.
Using BPI within loop predication is non-trivial because BPI is only
preserved lossily in loop pass manager (one fix exposed by lossy
preservation is up for review at D111448). However, since loop
predication is only used in downstream pipelines, it is hard to keep BPI
from breaking for incomplete state with upstream changes in BPI.
Also, correctly preserving BPI for all loop passes is a non-trivial
undertaking (D110438 does this lossily), while the benefit of using it
in loop predication isn't clear.
In this patch, we rely on profile metadata to get almost similar benefit as
BPI, without actually using the complete heuristics provided by BPI.
This avoids the compile time explosion we tried to fix with D110438 and
also avoids fragile bugs because BPI can be lossy in loop passes
(D111448).
Reviewed-By: asbirlea, apilipenko
Differential Revision: https://reviews.llvm.org/D111668
This is analogous to D86156 (which preserves "lossy" BFI in loop
passes). Lossy means that the analysis preserved may not be up to date
with regards to new blocks that are added in loop passes, but BPI will
not contain stale pointers to basic blocks that are deleted by the loop
passes.
This is achieved through BasicBlockCallbackVH in BPI, which calls
eraseBlock that updates the data structures in BPI whenever a basic
block is deleted.
This patch does not have any changes in the upstream pipeline, since
none of the loop passes in the pipeline use BPI currently.
However, since BPI wasn't previously preserved in loop passes, the loop
predication pass was invoking BPI *on the entire
function* every time it ran in an LPM. This caused massive compile time
in our downstream LPM invocation which contained loop predication.
See updated test with an invocation of a loop-pipeline containing loop
predication and -debug-pass turned ON.
Reviewed-By: asbirlea, modimo
Differential Revision: https://reviews.llvm.org/D110438
Precommit testcase for D110438. Since we do not preserve BPI in loop
pass manager, we are forced to compute BPI everytime Loop predication is
invoked.
The patch referenced changes that behaviour by preserving lossy BPI for
loop passes.
To make the IR easier to analyze, this pass makes some minor transformations.
After that, even if it doesn't decide to optimize anything, it can't report that
it changed nothing and preserved all the analyses.
Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D109855
The attached testcase crashes without the patch (Not the same accesses
in the same order).
When we move instructions before another instruction, we also need to
update the memory accesses corresponding to it.
Reviewed-By: asbirlea
Differential Revision: https://reviews.llvm.org/D109197
Since LICM has now unconditionally moved to MemorySSA based form, all
passes that run in same LPM as LICM need to preserve MemorySSA (i.e. our
downstream pipeline).
Added loop-mssa to all tests and perform -verify-memoryssa within
LoopPredication itself.
Differential Revision: https://reviews.llvm.org/D108724
These intrinsics, not the icmp+select are the canonical form nowadays,
so we might as well directly emit them.
This should not cause any regressions, but if it does,
then then they would needed to be fixed regardless.
Note that this doesn't deal with `SCEVExpander::isHighCostExpansion()`,
but that is a pessimization, not a correctness issue.
Additionally, the non-intrinsic form has issues with undef,
see https://reviews.llvm.org/D88287#2587863
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
