SCEVs ExprValueMap currently tracks not only which IR Values
correspond to a given SCEV expression, but additionally stores that
it may be expanded in the form X+Offset. In theory, this allows
reusing existing IR Values in more cases.
In practice, this doesn't seem to be particularly useful (the test
changes are rather underwhelming) and adds a good bit of complexity.
Per https://github.com/llvm/llvm-project/issues/53905, we have an
invalidation issue with these offseted expressions.
Differential Revision: https://reviews.llvm.org/D120311
D118090 causes a pretty significant (19%) regression in some Eigen
benchmarks. Investigating is a bit time consuming as the compilation
unit where this occurs is large. Rather than revert, this patch adds a
flag controlling that behavior (enabled by default).
This patch fixes a logical error in how we work with `LoopUsers` map.
It maps a loop onto a set of AddRecs that depend on it. The Addrecs
are added to this map only once when they are created and put to
the UniqueSCEVs` map.
The only purpose of this map is to make sure that, whenever we forget
a loop, all (directly or indirectly) dependent SCEVs get forgotten too.
Current code erases SCEVs from dependent set of a given loop whenever
we forget this loop. This is not a correct behavior due to the following scenario:
1. We have a loop `L` and an AddRec `AR` that depends on it;
2. We modify something in the loop, but don't destroy it. We still call forgetLoop on it;
3. `AR` is no longer dependent on `L` according to `LoopUsers`. It is erased from
ValueExprMap` and `ExprValue map, but still exists in UniqueSCEVs;
4. We can later request the very same AddRec for the very same loop again, and get existing
SCEV `AR`.
5. Now, `AR` exists and is used again, but its notion that it depends on `L` is lost;
6. Then we decide to delete `L`. `AR` will not be forgotten because we have lost it;
7. Just you wait when you run into a dangling pointer problem, or any other kind of problem
because an active SCEV is now referecing a non-existent loop.
The solution to this is to stop erasing values from `LoopUsers`. Yes, we will maybe forget something
that is already not used, but it's cheap.
This fixes a functional bug and potentially may have negative compile time impact on methods with
huge or numerous loops.
Differential Revision: https://reviews.llvm.org/D120303
Reviewed By: nikic
Our current strategy of computing ranges of SCEVUnknown Phis was to simply
compute the union of ranges of all its inputs. In order to avoid infinite recursion,
we mark Phis as pending and conservatively return full set for them. As result,
even simplest patterns of cycled phis always have a range of full set.
This patch makes this logic a bit smarter. We basically do the same, but instead
of taking inputs of single Phi we find its strongly connected component (SCC)
and compute the union of all inputs that come into this SCC from outside.
Processing entire SCC together has one more advantage: we can set range for all
of them at once, because the only thing that happens to them is the same value is
being passed between those Phis. So, despite we spend more time analyzing a
single Phi, overall we may save time by not processing other SCC members, so
amortized compile time spent should be approximately the same.
Differential Revision: https://reviews.llvm.org/D110620
Reviewed By: reames
zext(umin(x,y)) == umin(zext(x),zext(y))
zext(x) == 0 -> x == 0
While it is not a very likely scenario, we probably should not expect
that instcombine already dropped such a redundant zext,
but handle directly. Moreover, perhaps there was no ZExtInst,
and SCEV somehow managed to pull out said zext out of the SCEV expression.
zext(umin(x,y)) == umin(zext(x),zext(y))
zext(x) == 0 -> x == 0
Extra leading zeros do not affect the result of comparison with zero,
nor do they matter for the unsigned min/max,
so we should not be dissuaded when we find a zero-extensions,
but instead we should just skip it.
This mechanism was used for a couple of purposes, but the primary one was keeping track of which predicates in a union might apply to an expression. As these sets are small and agressively deduped, this has little value.
Even if the search is marked as terminated after only looking at
the first operand, we'd still look at the remaining operands
before actually ending the search.
This seems pointless and wasteful, let's not do that.
Since we don't greedily flatten `umin_seq(a, umin(b, c))` into `umin_seq(a, b, c)`,
just looking at the operands of the outer-level `umin` is not sufficient,
and we need to recurse into all same-typed `umin`'s.
This lets us avoid redundant implication work in the constructor of SCEVUnionPredicate which simplifies an upcoming change. If we're actually building a predicate via PSE, that goes through addPredicate which does include the implication check.
Note that this doesn't actually cause the top level predicate to become a non-union just yet.
The * above comes from a case in the LoopVectorizer where a predicate which is later proven no longer blocks vectorization due to a change from checking if predicates exists to whether the predicate is possibly false.
We'd catch the tautological select pattern later anyways
due to constant folding, so that leaves PHI-like select,
but it does not appear to fire there.
Currently `createNodeForSelectOrPHI()` takes an Instruction,
and only works on the Cond that is an ICmpInst,
but that can be relaxed somewhat.
For now, simply rename the existing function,
and add a thin wrapper ontop that still does
the same thing as it used to.
https://alive2.llvm.org/ce/z/ULuZxB
We could transparently handle wider bitwidths,
by effectively casting iN to <N x i1> and performing the `add`
bit/element -wise, the expression will be rather large,
so let's not do that for now.
https://alive2.llvm.org/ce/z/aKAr94
We could transparently handle wider bitwidths,
by effectively casting iN to <N x i1> and performing the `umin`
bit/element -wise, the expression will be rather large,
so let's not do that for now.
https://alive2.llvm.org/ce/z/SMEaoc
We could transparently handle wider bitwidths,
by effectively casting iN to <N x i1> and performing the `umax`
bit/element -wise, the expression will be rather large,
so let's not do that for now.
This is the last major stepping stone before being able to allocate the node via the folding set allocator. That will in turn allow more general SCEV predicate expression trees.
For those curious, the whole reason for tracking the predicate set seperately as opposed to just immediately registering the dependencies appears to be allowing the printing code to print a result without changing the PSE state. It's slightly questionable if this justifies the complexity, but since we can preserve it with local ugliness, I did so.
PredicatedScalarEvolution has a predicate type for representing A == B. This change generalizes it into something which can represent a A <pred> B.
This generality is currently unused, but is motivated by a couple of recent cases which have come up. In particular, I'm currently playing around with using this to simplify the runtime checking code in LoopVectorizer. Regardless of the outcome of that prototyping, generalizing the compare node seemed useful.
Extend scalar evolution to handle >= and <= if a loop is known to be finite and the induction variable guards the condition. Specifically, with these assumptions lhs <= rhs is equivalent to lhs < rhs + 1 and lhs >= rhs to lhs > rhs -1.
In the case of lhs <= rhs, this is true since the only case these are not equivalent
is when rhs == unsigned/signed intmax, which would have resulted in an infinite loop.
In the case of lhs >= rhs, this is true since the only case these are not equivalent
is when rhs == unsigned/signed intmin, which would again have resulted in an infinite loop.
Reviewed By: lebedev.ri
Differential Revision: https://reviews.llvm.org/D118090
Instead use either Type::getPointerElementType() or
Type::getNonOpaquePointerElementType().
This is part of D117885, in preparation for deprecating the API.
This patch adds support for implication inference logic for the
following pattern:
```
lhs < (y >> z) <= y, y <= rhs --> lhs < rhs
```
We should be able to use the fact that value shifted to right is
not greater than the original value (provided it is non-negative).
Differential Revision: https://reviews.llvm.org/D116150
Reviewed-By: apilipenko
Since we don't merge/expand non-sequential umin exprs into umin_seq exprs,
we may have umin_seq(umin(umin_seq())) chain, and the innermost umin_seq
can have duplicate operands still.
We could just merge all umin into umin_seq, but that is likely
a pessimization, so don't do that, but pretend that we did
for the purpose of deduplication.
Having the same operand more than once doesn't change the outcome here,
neither reduction-wise nor poison-wise.
We must keep the first instance specifically though.
Two crashes have been reported. This change disables the new logic while leaving the new node in tree. Hopefully, that's enough to allow investigation without breakage while avoiding massive churn.
As discussed in https://github.com/llvm/llvm-project/issues/53020 / https://reviews.llvm.org/D116692,
SCEV is forbidden from reasoning about 'backedge taken count'
if the branch condition is a poison-safe logical operation,
which is conservatively correct, but is severely limiting.
Instead, we should have a way to express those
poison blocking properties in SCEV expressions.
The proposed semantics is:
```
Sequential/in-order min/max SCEV expressions are non-commutative variants
of commutative min/max SCEV expressions. If none of their operands
are poison, then they are functionally equivalent, otherwise,
if the operand that represents the saturation point* of given expression,
comes before the first poison operand, then the whole expression is not poison,
but is said saturation point.
```
* saturation point - the maximal/minimal possible integer value for the given type
The lowering is straight-forward:
```
compare each operand to the saturation point,
perform sequential in-order logical-or (poison-safe!) ordered reduction
over those checks, and if reduction returned true then return
saturation point else return the naive min/max reduction over the operands
```
https://alive2.llvm.org/ce/z/Q7jxvH (2 ops)
https://alive2.llvm.org/ce/z/QCRrhk (3 ops)
Note that we don't need to check the last operand: https://alive2.llvm.org/ce/z/abvHQS
Note that this is not commutative: https://alive2.llvm.org/ce/z/FK9e97
That allows us to handle the patterns in question.
Reviewed By: nikic, reames
Differential Revision: https://reviews.llvm.org/D116766
This ports the logic we generate in instcombine for a single use x.with.overflow check for use in SCEV's analysis. The result is that we can prove trip counts for many checks, and (through existing logic) often discharge them.
Motivation comes from compiling a simple example with -ftrapv.
Differential Revision: https://reviews.llvm.org/D116499
This patch updates applyLoopGuards to first collect all conditions and
then applies them in reverse order. This ensures the SCEVs with the
shortest dependency chains are constructed first, limiting the required
stack size.
This fixes a crash reported in D113578.
Note that the order conditions are applied can impact the accuracy of
the result, mostly due to missing min/max simplifications when
constructing SCEVs.
The changed test highlights the impact of the evaluation order. I will
follow up with a SCEV patch to improve min/max simplifications to get
the same results for both orders.
Fixes verification failure reported at:
https://reviews.llvm.org/rGc9f9be0381d1
The issue is that getSCEVAtScope() might compute a result without
inserting it in the ValuesAtScopes map in degenerate cases,
specifically if the ValuesAtScopes entry is invalidated during the
calculation. Arguably we should still insert the result if no
existing placeholder is found, but for now just tweak the logic
to only update ValuesAtScopesUsers if ValuesAtScopes is updated.
Track which SCEVs are used as ExactNotTaken counts in
BackedgeTakenInfo structures, so we can directly determine which
loops need to be invalidated, rather than iterating over all BECounts.
This gives a small compile-time improvement on average, but the
motivation here is more to ensure there are no degenerate cases,
if the number of backedge taken counts is large.
Differential Revision: https://reviews.llvm.org/D114784
ValuesAtScopes maps a SCEV and a Loop to another SCEV. While we
invalidate entries if the left-hand SCEV is invalidated, we
currently don't do this for the right-hand SCEV. Fix this by
tracking users in a reverse map and using it for invalidation.
This is conceptually the same change as D114738, but using the
reverse map to avoid performance issues.
Differential Revision: https://reviews.llvm.org/D114788
Fix assertion failure reported on D113349 by removing the assert.
While the produced expression should be equivalent, it may not
be strictly the same, e.g. due to lazy nowrap flag updates. Similar
to what the main createSCEV() code does, simply retain the old
value map entry if one already exists.
With the recently introduced tracking as well as D113349, we can
greatly simplify forgetSymbolicName(). In fact, we can simply
replace it with forgetMemoizedResults().
What forgetSymbolicName() used to do is to walk the IR use-def
chain to find all SCEVs that mention the SymbolicName. However,
thanks to use tracking, we can now determine the relevant SCEVs
in a more direct way. D113349 is needed to also clear out the
actual IR to SCEV mapping in ValueExprMap.
Differential Revision: https://reviews.llvm.org/D114263
After backedge taken counts have been calculated, we want to
invalidate all addrecs and dependent expressions in the loop,
because we might compute better results with the newly available
backedge taken counts. Previously this was done with a forgetLoop()
style use-def walk. With recent improvements to SCEV invalidation,
we can instead directly invalidate any SCEVs using addrecs in this
loop. This requires a great deal less subtlety to avoid invalidating
more than necessary, and in particular gets rid of the hack from
D113349. The change is similar to D114263 in spirit.
Relative to the previous landing attempt, this introduces an additional
flag on forgetMemoizedResults() to not remove SCEVUnknown phis from
the value map. The invalidation after BECount calculation wants to
leave these alone and skips them in its own use-def walk, but we can
still end up invalidating them via forgetMemoizedResults() if there
is another IR value with the same SCEV. This is intended as a temporary
workaround only, and the need for this should go away once the
getBackedgeTakenInfo() invalidation is refactored in the spirit of
D114263.
-----
This adds validation for consistency of ValueExprMap and
ExprValueMap, and fixes identified issues:
* Addrec construction directly wrote to ValueExprMap in a few places,
without updating ExprValueMap. Add a helper to ensures they stay
consistent. The adjustment in forgetSymbolicName() explicitly
drops the old value from the map, so that we don't rely on it
being overwritten.
* forgetMemoizedResultsImpl() was dropping the SCEV from
ExprValueMap, but not dropping the corresponding entries from
ValueExprMap.
Differential Revision: https://reviews.llvm.org/D113349
Relative to the previous landing attempt, this makes
insertValueToMap() resilient against the value already being
present in the map -- previously I only checked this for the
createSimpleAffineAddRec() case, but the same issue can also
occur for the general createNodeForPHI(). In both cases, the
addrec may be constructed and added to the map in a recursive
query trying to create said addrec. In this case, this happens
due to the invalidation when the BE count is computed, which
ends up clearing out the symbolic name as well.
-----
This adds validation for consistency of ValueExprMap and
ExprValueMap, and fixes identified issues:
* Addrec construction directly wrote to ValueExprMap in a few places,
without updating ExprValueMap. Add a helper to ensures they stay
consistent. The adjustment in forgetSymbolicName() explicitly
drops the old value from the map, so that we don't rely on it
being overwritten.
* forgetMemoizedResultsImpl() was dropping the SCEV from
ExprValueMap, but not dropping the corresponding entries from
ValueExprMap.
Differential Revision: https://reviews.llvm.org/D113349
Accum is guaranteed to be defined outside L (via Loop::isLoopInvariant
checks above). I think that should guarantee that the more powerful
ScalarEvolution::isLoopInvariant also determines that the value is loop
invariant.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D114634
This adds validation for consistency of ValueExprMap and
ExprValueMap, and fixes identified issues:
* Addrec construction directly wrote to ValueExprMap in a few places,
without updating ExprValueMap. Add a helper to ensures they stay
consistent. The adjustment in forgetSymbolicName() explicitly
drops the old value from the map, so that we don't rely on it
being overwritten.
* forgetMemoizedResultsImpl() was dropping the SCEV from
ExprValueMap, but not dropping the corresponding entries from
ValueExprMap.
Differential Revision: https://reviews.llvm.org/D113349
Revert "[SCEV] Defer all work from ea12c2cb as late as possible"
Revert "[SCEV] Defer loop property checks from ea12c2cb as late as possible"
This reverts commit 734abbad79 and 1a5666acb2.
Both of these changes were speculative attempts to address a compile time regression. Neither worked, and both complicated the code in undesirable ways.
This is a second speculative compile time optimization to address a reported regression. My actual suspicion is that availability of no-self-wrap is making some *other* bit of code trigger, but let's rule this out.
This change moves logic which we'd added specifically for less than tests so that it applies to equalities and greater than tests as well. The basic idea is that if we can show an IV cycles infinitely through the same series on self-wrap, and that the exit condition must be taken to prevent UB, we can conclude that it must be taken before self-wrap and thus infer said flag.
The motivation here is simple loops with unsigned induction variables w/non-one steps and inequality tests. A toy example would be:
for (unsigned i = 0; i != N; i += 2) { body; }
If body contains no side effects, and this is a mustprogress function, we can assume that this must be a finite loop and thus that the exit count is N/2.
Differential Revision: https://reviews.llvm.org/D103991
The initial two cases require a SCEVConstant as RHS. Pull up the condition
to check and swap SCEVConstants from below. Also remove a redundant
check & swap if RHS is SCEVUnknown.
This solves the same crash as in D104503, but with a different approach.
The test case test_non_dom demonstrates a case where scev-aa crashes today. (If exercised either by -eval-aa or -licm.) The basic problem is that SCEV-AA expects to be able to compute a pointer difference between two SCEVs for any two pair of pointers we do an alias query on. For (valid, but out of scope) reasons, we can end up asking whether expressions in different sub-loops can alias each other. This results in a subtraction expression being formed where neither operand dominates the other.
The approach this patch takes is to leverage the "defining scope" notion we introduced for flag semantics to detect and disallow the formation of the problematic SCEV. This ends up being relatively straight forward on that new infrastructure. This change does hint that we should probably be verifying a similar property for all SCEVs somewhere, but I'll leave that to a follow on change.
Differential Revision: D114112
Similar other cases in the current function (e.g. when the step is 1 or
-1), applying loop guards can lead to tighter upper bounds for the
backedge-taken counts.
Fixes PR52464.
Reviewed By: reames, nikic
Differential Revision: https://reviews.llvm.org/D113578
There are multiple possible ways to represent the X - urem X, Y pattern. SCEV was not canonicalizing, and thus, depending on which you analyzed, you could get different results. The sub representation appears to produce strictly inferior results in practice, so I decided to canonicalize to the Y * X/Y version.
The motivation here is that runtime unroll produces the sub X - (and X, Y-1) pattern when Y is a power of two. SCEV is thus unable to recognize that an unrolled loop exits because we don't figure out that the new unrolled step evenly divides the trip count of the unrolled loop. After instcombine runs, we convert the the andn form which SCEV recognizes, so essentially, this is just fixing a nasty pass ordering dependency.
The ARM loop hardware interaction in the test diff is opague to me, but the comments in the review from others knowledge of the infrastructure appear to indicate these are improvements in loop recognition, not regressions.
Differential Revision: https://reviews.llvm.org/D114018
So far, applying loop guard information has been restricted to
SCEVUnknown. In a few cases, like PR40961 and PR52464, this leads to
SCEV failing to determine tight upper bounds for the backedge taken
count.
This patch adjusts SCEVLoopGuardRewriter and applyLoopGuards to support
re-writing ZExt expressions.
This is a first step towards fixing PR40961 and PR52464.
Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D113577
attempts
Prevent the selection of IVs that have a SCEV containing an undef. Also
prevent salvaging attempts for values for which a SCEV could not be
created by ScalarEvolution and have only SCEVUknown.
Reviewed by: Orlando
Differential Revision: https://reviews.llvm.org/D111810
The basic idea here is that given a zero extended narrow IV, we can prove the inner IV to be NUW if we can prove there's a value the inner IV must take before overflow which must exit the loop.
Differential Revision: https://reviews.llvm.org/D109457
Data references in a loop should not access elements over the
statically allocated size. So we can infer a loop max trip count
from this undefined behavior.
Reviewed By: reames, mkazantsev, nikic
Differential Revision: https://reviews.llvm.org/D109821
It it now sufficient to track only direct addrec users of a loop,
and let the SCEVUsers mechanism track and invalidate transitive users.
Differential Revision: https://reviews.llvm.org/D112875
This function is used at least in 2 places, to it makes sense to make it separate.
Differential Revision: https://reviews.llvm.org/D112516
Reviewed By: reames
Following discussion in D110390, it seems that we are suffering from unability
to traverse users of a SCEV being invalidated. The result of that is that ScalarEvolution's
inner caches may store obsolete data about SCEVs even if their operands are
forgotten. It creates problems when we try to verify the contents of those caches.
It's also a frequent situation when messing with cache causes very sneaky and
hard-to-analyze bugs related to corruption of memory when dealing with cached
data. They are lurking there because ScalarEvolution's veirfication is not powerful
enough and misses many problematic cases. I plan to make SCEV's verification
much stricter in follow-ups, and this requires dangling-pointers-free caches.
This patch makes sure that, whenever we forget cached information for a SCEV,
we also forget it for all SCEVs that (transitively) use it.
This may have negative compile time impact. It's a sacrifice we are more
than willing to make to enforce correctness. We can also save some time by
reworking invokers of forgetMemoizedResults (maybe we can forget multiple
SCEVs with single query).
Differential Revision: https://reviews.llvm.org/D111533
Reviewed By: reames
Make sure that, for every living SCEV, we have all its direct
operand tracking it as their user.
Differential Revision: https://reviews.llvm.org/D112402
Reviewed By: reames
Always insert values into ExprValueMap, and instead skip using them
in SCEVExpander if poison-generating flags have been lost. This
ensures that all values that are in ValueExprMap are also in
ExprValueMap, so we can use the latter to invalidate the former.
This change is probably not entirely NFC for the case where
originally the SCEV had no nowrap flags but they were inferred
later, in which case that would now allow reusing the existing
value for expansion.
Differential Revision: https://reviews.llvm.org/D112389
Mass forgetMemoizedResults can be done more efficiently than bunch
of individual invocations of helper because we can traverse maps being
updated just once, rather than doing this for each invidivual SCEV.
Should be NFC and supposedly improves compile time.
Differential Revision: https://reviews.llvm.org/D112294
Reviewed By: reames
When forgetting multiple SCEVs, rather than doing this one by one, we can
instead use mass updates. We plan to make them more efficient than they
are now, potentially improving compile time.
Differential Revision: https://reviews.llvm.org/D111602
Reviewed By: reames
This patch changes signature of forgetMemoizedResults to be able to work with
multiple SCEVs. Usage will come in follow-ups. We also plan to optimize it in the
future to work faster than individual invalidation updates. Should not change
behavior in any sense.
Split-off from D111602.
Differential Revision: https://reviews.llvm.org/D112293
Reviewed By: reames
Follow-up from D112295, suggested by Nikita: we can avoid tracking
users of SCEVConstants because dropping their cached info is unlikely
to give any new prospects for fact inference, and it should not introduce
any correctness problems.
This patch introduces API that keeps track of SCEVs users of
another SCEVs, required to handle invalidations of users along
with operands that comes in follow-up patches.
Differential Revision: https://reviews.llvm.org/D112295
Reviewed By: reames
The functionality of this method is already covered by
computeExitCountExhaustively() in a more general fashion. It was
added at a time when exhaustive exit count calculation did not
support constant folding loads yet. I double checked that dropping
this code causes no binary changes in test-suite.
Differential Revision: https://reviews.llvm.org/D112343
As seen in PR51869 the ScalarEvolution::isImpliedCond function might
end up spending lots of time when doing the isKnownPredicate checks.
Calling isKnownPredicate for example result in isKnownViaInduction
being called, which might result in isLoopBackedgeGuardedByCond being
called, and then we might get one or more new calls to isImpliedCond.
Even if the scenario described here isn't an infinite loop, using
some random generated C programs as input indicates that those
isKnownPredicate checks quite often returns true. On the other hand,
the third condition that needs to be fulfilled in order to "prove
implications via truncation", i.e. the isImpliedCondBalancedTypes
check, is rarely fulfilled.
I also made some similar experiments to look at how often we would
get the same result when using isKnownViaNonRecursiveReasoning instead
of isKnownPredicate. So far I haven't seen a single case when codegen
is negatively impacted by using isKnownViaNonRecursiveReasoning. On
the other hand, it seems like we get rid of the compile time explosion
seen in PR51869 that way. Hence this patch.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D112080
This patch teaches SCEV two implication rules:
x <u y && y >=s 0 --> x <s y,
x <s y && y <s 0 --> x <u y.
And all equivalents with signs/parts swapped.
Differential Revision: https://reviews.llvm.org/D110517
Reviewed By: nikic
Current implementations of DFS in SCEV check unique-visited of traversed
values on pop, and not on push. As result, the same value may be pushed
multiple times just to be thrown away when popped. These operations are
meaningless and only waste time and increase memory footprint of the
worklist.
This patch reworks the DFS strategy to check uniqueness before push.
Should be NFC.
Differential Revision: https://reviews.llvm.org/D111774
Reviewed By: nikic, reames
Replace check with
if ((ExitIfTrue && CI->isZero()) || (!ExitIfTrue && CI->isOne()))
with equivalent and simpler version
if (ExitIfTrue == CI->isZero())
As a brief reminder, an "exit count" is the number of times the backedge executes before some event. It can be zero if we exit before the backedge is reached. A "trip count" is the number of times the loop header is entered if we branch into the loop. In general, TC = BTC + 1 and thus a zero trip count is ill defined
There is a cornercases which we don't handle well. Let's assume i8 for our examples to keep things simple. If BTC = 255, then the correct trip count is 256. However, 256 is not representable in i8.
In theory, code which needs to reason about trip counts is responsible for checking for this cornercase, and either bailing out, or handling it correctly. Historically, we don't have a great track record about actually doing so.
When reviewing D109676, I found myself asking a basic question. Was there any good reason to preserve the current wrap-to-zero behavior when converting from backedge taken counts to trip counts? After reviewing existing code, I could not find a single case which appears to correctly and precisely handle the overflow case.
This patch changes the default behavior to extend instead of wrap. That is, if the result might be 256, we return a value of i9 type to ensure we interpret the count correctly. I did leave the legacy behavior as an option since a) loop-flatten stops triggering if I extend due to weirdly specific pattern matching I didn't understand and b) we could reasonably use the mode if we'd externally established a lack of overflow.
I want to emphasize that this change is *not* NFC. There are two call sites (one in ScalarEvolution.cpp, one in LoopCacheAnalysis.cpp) which are switched to the extend semantics. The former appears imprecise (but correct) for a constant 255 BTC. The later appears incorrect, though I don't have a test case.
Differential Revision: https://reviews.llvm.org/D110587
This factors out utilities for scanning a bounded block of instructions since we have this code repeated in a bunch of places. The change to InlineFunction isn't strictly NFC as the limit mechanism there didn't handle debug instructions correctly.
When checking to see if we can apply IR flags to a SCEV, we need to identify a bound on the defining scope of the SCEV to be produced. We'd previously added support for a couple SCEVExpr types which trivially imply bounds, but hadn't handled types such as umax where the bounds come from the bounds of the operands. This does the obvious thing, and recurses through operands searching for a tighter bound on the defining scope.
I'm honestly surprised by how little this seems to mater on existing tests, but it's worth doing for completeness sake alone.
Differential Revision: https://reviews.llvm.org/D111191
When determining the defining scope, avoid repeatedly querying
dominationg against the function entry instruction. This ends up
begin a very common case that we can handle more efficiently.
This patch removes a compile time restriction from isSCEVExprNeverPoison. We've strengthened our ability to reason about flags on scopes other than addrecs, and this bailout prevents us from using it. The comment is also suspect as well in that we're in the middle of constructing a SCEV for I. As such, we're going to visit all operands *anyways*.
Differential Revision: https://reviews.llvm.org/D111186
Behavior wise, this patch should be mostly NFC. The only behavior difference known is that on the isSCEVExprNeverPoison path we'll consider a bound imposed by the SCEVable operands (if any).
Algorithmically, it's an invert of the existing code. Previously, we checked for each operand if we could find a bound, then checked for must-execute given that bound. With the patch, we use dominance to refine the innermost bound, then check must execute once. The interesting case is when we have multiple unknowns within a single basic block. While both dominance and must-execute are worst-case linear walks within the block, only dominance is cached. As such, refining based on dominance should be more efficient.
Stop using APInt constructors and methods that were soft-deprecated in
D109483. This fixes all the uses I found in llvm, except for the APInt
unit tests which should still test the deprecated methods.
Differential Revision: https://reviews.llvm.org/D110807
This addresses a comment from review on D109845. The concern was raised that an unbounded scan would be expensive. Long term plan is to cache this search - likely reusing the existing mechanism for loop side effects - but let's be simple and conservative for now.
This addresses a comment from review on D109845. Even for SCEVs which we can't find true bounds without recursing through operands, entry to the function forms a trivial upper bound. In some cases, this trivial bound is enough to prove safety of flag inference.
This is a followon to D109845. With that landed, we will have fixed all known instances of pr51817, and can thus start inferring flags more aggressively with greatly reduced risk of miscompiles. This patch simply applies the same inference logic used in that patch to our other major flag inference path.
We can still do much better here (on both paths), but this is our first step.
Differential Revision: https://reviews.llvm.org/D111003
This fixes a violation of the wrap flag rules introduced in c4048d8f. This is an alternate fix to D106852.
The basic problem being fixed is that we infer a set of flags which is valid at some inner scope S1 (usually by correctly propagating them from IR), and then (incorrectly) extend them to a SCEV in scope S2 where S1 != S2. This is not in general safe per the wrap flags semantics recently defined.
In this patch, I include a simple inference step to handle the case where we can prove that S2 is the preheader of the loop S1, and that entry into S2 implies execution of S1. See the code for a more detailed explanation.
One worry I have with this patch is that I might be over-fitting what shows up in tests - and thus hiding negative impact we'd see in the real world. My best defense is that the rule used here very closely follows the one used to propagate the flags from IR to the inner add to start with, and thus if one is reasonable, so probably is the other. Curious what others think about that piece.
The test diffs are roughly as expected. Mostly analysis only, with two transform changes. Oddly, the result looks better in the loop-idiom test, and I don't understand the PPC output enough to have tell. Nothing terrible looking though. (For context, without the scope inference peephole, the test delta includes a couple of vectorization tests. Again, not super concerning, but slightly more so.)
Differential Revision: https://reviews.llvm.org/D109845
This fixes a violation of the wrap flag rules introduced in c4048d8f. This was also noted in the (very old) PR23527.
The issue being fixed is that we assume the inbound flag on any GEP assumes that all users of *any* gep (or add) which happens to map to that SCEV would also be UB if the (other) gep overflowed. That's simply not true.
In terms of the test diffs, I don't see anything seriously problematic. The lost flags are expected (given the semantic restriction on when its legal to tag the SCEV), and there are several cases where the previously inferred flags are unsound per the new semantics.
The only common trend I noticed when looking at the deltas is that by not considering branch on poison as immediate UB in ValueTracking, we do miss a few cases we could reclaim. We may be able to claw some of these back with the follow ideas mentioned in PR51817.
It's worth noting that most of the changes are analysis result only changes. The two transform changes are pretty minimal. In one case, we miss the opportunity to infer a nuw (correctly). In the other, we fail to fold an exit and produce a loop invariant form instead. This one is probably over-reduced as the program appears to be undefined in practice, and neither before or after exploits that.
Differential Revision: https://reviews.llvm.org/D109789
This code is attempting to prove that I must execute if we enter the defining scope of the SCEV which will be created from I. In the case where it found a defining addrec scope, it had a rather odd restriction that all of the other operands must be loop invariant in that addrec's loop.
As near as I can tell here, we really only need a upper bound on the defining scope. If we can prove the stronger property, then we must also have proven the property on the exact defining scope as well.
In practice, the actual effect of this change is narrow. The compile time restriction at the top of the routine basically limits us to I being an arithmetic in some loop L with both an addrec operand in L, and a unknown operands in L. Possible to demonstrate, but the main value of the change is removing unneeded code.
Differential Revision: https://reviews.llvm.org/D110892
This reverts commit 8fdac7cb7a.
The issue causing the revert has been fixed a while ago in 60b852092c.
Original message:
Now that SCEVExpander can preserve LCSSA form,
we do not have to worry about LCSSA form when
trying to look through PHIs. SCEVExpander will take
care of inserting LCSSA PHI nodes as required.
This increases precision of the analysis in some cases.
Reviewed By: mkazantsev, bmahjour
Differential Revision: https://reviews.llvm.org/D71539
The logic in howManyLessThans is fishy. It first checks invariance of
RHS, and then uses OrigRHS as argument for isLoopEntryGuardedByCond, which
is, strictly saying, a different thing. We are seeing a very rare intermittent
failure of availability checks, and it looks like this precondition is
sometimes broken. Before we can figure out what's going on, adding asserts
that all involved values that may possibly to to isLoopEntryGuardedByCond
are available at loop entry.
If either of these asserts fails (OrigRHS is the most likely suspect), it
means that the logic here is flawed.
The implication logic for two values that are both negative or non-negative
says that it doesn't matter whether their predicate is signed and unsigned,
but only flips unsigned into signed for further inference. This patch adds
support for flipping a signed predicate into unsigned as well.
Differential Revision: https://reviews.llvm.org/D109959
Reviewed By: nikic
There is a piece of logic that uses the fact that signed and unsigned
versions of the same predicate are equivalent when both values are
non-negative. It's also true when both of them are negative.
Differential Revision: https://reviews.llvm.org/D109957
Reviewed By: nikic
This fixes a violation of the wrap flag rules introduced in c4048d8f. As noted in the original review, the NUW is legal to infer from the structure of the replacee, but a) there's no test coverage, and b) this should be done generically for all multiplies.
Differential Revision: https://reviews.llvm.org/D109782
It's possible in some cases for the LHS to be a pointer where the RHS is not. This isn't directly possible for an icmp, but the analysis mixes up operands of different icmp expressions in some cases.
This does not include a test case as the smallest reduced case we've managed is extremely fragile and unlikely to test anything meaningful in the long term.
Also add an assertion to getNotSCEV() to make tracking down this sort of issue a bit easier in the future.
Fixes https://bugs.llvm.org/show_bug.cgi?id=51787 .
Differential Revision: https://reviews.llvm.org/D109546
This bit of code is incredibly suspicious. It allows fully unknown (but potentially negative) steps, but not steps known to be negative. The comment about scev flag inference is worrying, but also not correct to my knowledge.
At best, this might be covering up some related miscompile. However, there's no test in tree for it, the review history doesn't include obvious motivation, and the C++ example doesn't appear to give wrong results when hand translated to IR. I think it's time to remove this and see what falls out.
During review, there were concerns raised about the correctness of the corresponding signed case. This change was deliberately narrowed to the unsigned case which has been auditted and appears correct for negative values. We need to get back to the known-negative signed case, but that'll be a future patch if nothing falls out from this one.
Differential Revision: https://reviews.llvm.org/D104140
In general, howManyLessThans doesn't really want to work with pointers
at all; the result is an integer, and the operands of the icmp are
effectively integers. However, isLoopEntryGuardedByCond doesn't like
extra ptrtoint casts, so the arguments to isLoopEntryGuardedByCond need
to be computed without those casts.
Somehow, the values got mixed up with the recent howManyLessThans
improvements; fix the confused values, and add a better comment to
explain what's happening.
Differential Revision: https://reviews.llvm.org/D109465
We were returning a tuple when all but one caller only cared about one piece of the return value. That one caller can inline the complexity, and we can simplify all other uses.
None of this logic has anything to do with SCEV's internals, it just uses the existing public APIs. As a result, we can move the code from ScalarEvolution.cpp/hpp to Delinearization.cpp/hpp with only minor changes.
This was discussed in advance on today's loop opt call. It turned out to be easy as hoped.
The basic problem being solved is that we largely give up when encountering a trip count involving an IV which is not an addrec. We will fall back to the brute force constant eval, but that doesn't have the information about the fact that we can't cycle back through the same set of values.
There's a high level design question of whether this is the right place to handle this, and if not, where that place is. The major alternative here would be to return a conservative upper bound, and then rely on two invocations of indvars to add the facts to the narrow IV, and then reconstruct SCEV. (I have not implemented the alternative and am not 100% sure this would work out.) That's arguably more in line with existing code, but I find this substantially easier to reason about. During review, no one expressed a strong opinion, so we went with this one.
Differential Revision: D108651
Follow on to D109029. I realized we had no mention of mustprogrress in the comment (as it prexisted mustprogress in the codebase). In the process of adding it, I tweaked the preconditions into something I think is more clear. Note that mustprogress is checked in the code.
Differential Revision: https://reviews.llvm.org/D109091
Due to a typo, this replaced %x with umax(C1, umin(C2, %x + C3))
rather than umax(C1, umin(C2, %x)). This didn't make a difference
for the existing tests, because the result is only used for range
calculation, and %x will usually have an unknown starting range,
and the additional offset keeps it unknown. However, if %x already
has a known range, we may compute a result range that is too
small.
There's a silent bug in our reasoning about zero strides. We assume that having a single static exit implies that if that exit is not taken, then the loop must be infinite. This ignores the potential for abnormal exits via exceptions. Consider the following example:
for (uint_8 i = 0; i < 1; i += 0) {
throw_on_thousandth_call();
}
Our reasoning is such that we'd conclude this loop can't take the backedge as that would lead to a (presumed) infinite loop.
In practice, this is a silent bug because the loopIsFiniteByAssumption returns false strictly more often than the loopHaNoAbnormalExits property. We could reasonable want to change that in the future, so fixing the codeflow now is worthwhile.
Differential Revision: https://reviews.llvm.org/D109029
Nikita Popov