This patch extends applyLoopGuards to detect a single-cond range check
idiom that InstCombine generates.
It extends applyLoopGuards to detect conditions of the form
(-C1 + X < C2). InstCombine will create this form when combining two
checks of the form (X u< C2 + C1) and (X >=u C1).
In practice, this enables us to correctly compute a tight trip count
bounds for code as in the function below. InstCombine will fold the
minimum iteration check created by LoopRotate with the user check (< 8).
void unsigned_check(short *pred, unsigned width) {
if (width < 8) {
for (int x = 0; x < width; x++)
pred[x] = pred[x] * pred[x];
}
}
As a consequence, LLVM creates dead vector loops for the code above,
e.g. see https://godbolt.org/z/cb8eTcqEThttps://alive2.llvm.org/ce/z/SHHW4d
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D104741
This patch generalizes MatchBinaryAddToConst to support matching
(A + C1), (A + C2), instead of just matching (A + C1), A.
The existing cases can be handled by treating non-add expressions A as
A + 0.
Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D104634
getPointerBase should only be looking through Add and AddRec
expressions; other expressions either aren't pointers, or can't be
looked through.
Technically, this is a functional change. For a multiply or min/max
expression, if they have exactly one pointer operand, and that operand
is the first operand, the behavior here changes. Similarly, if an AddRec
has a pointer-type step, the behavior changes. But that shouldn't be
happening in practice, and we plan to make such expressions illegal.
SCEVNAryExpr::getType() could return the wrong type for a SCEVAddExpr.
Remove it, and add getType() methods to the relevant subclasses.
NFC because nothing uses it directly, as far as I know; this is just
future-proofing.
This adds handling for signed predicates, similar to how unsigned
predicates are already handled.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D104732
Currently we drop wrapping flags for expressions like (A + C1)<flags> - C2.
But we can retain flags under certain conditions:
* Adding a smaller constant is NUW if the original AddExpr was NUW.
* Adding a constant with the same sign and small magnitude is NSW, if the
original AddExpr was NSW.
This can improve results after using `SimplifyICmpOperands`, which may
subtract one in order to use stricter predicates, as is the case for
`isKnownPredicate`.
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D104319
A backedge-taken count doesn't refer to memory; returning a pointer type
is nonsense. So make sure we always return an integer.
The obvious way to do this would be to just convert the operands of the
icmp to integers, but that doesn't quite work out at the moment:
isLoopEntryGuardedByCond currently gets confused by ptrtoint operations.
So we perform the ptrtoint conversion late for lt/gt operations.
The test changes are mostly innocuous. The most interesting changes are
more complex SCEV expressions of the form "(-1 * (ptrtoint i8* %ptr to
i64)) + %ptr)". This is expected: we can't fold this to zero because we
need to preserve the pointer base.
The call to isLoopEntryGuardedByCond in howFarToZero is less precise
because of ptrtoint operations; this shows up in the function
pr46786_c26_char in ptrtoint.ll. Fixing it here would require more
complex refactoring. It should eventually be fixed by future
improvements to isImpliedCond.
See https://bugs.llvm.org/show_bug.cgi?id=46786 for context.
Differential Revision: https://reviews.llvm.org/D103656
The old version of this code would blindly perform arithmetic without
paying attention to whether the types involved were pointers or
integers. This could lead to weird expressions like negating a pointer.
Explicitly handle simple cases involving pointers, like "x < y ? x : y".
In all other cases, coerce the operands of the comparison to integer
types. This avoids the weird cases, while handling most of the
interesting cases.
Differential Revision: https://reviews.llvm.org/D103660
As per (committed without review) @reames's rGac81cb7e6dde9b0890ee1780eae94ab96743569b change,
we are now allowed to produce `ptrtoint` for non-integral pointers.
This will unblock further unbreaking of SCEV regarding int-vs-pointer type confusion.
Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D104322
Essentially, the cover function simply combines the loop level check and the function level scope into one call. This simplifies several callers and is (subjectively) less error prone.
This addresses a performance regression reported against 3c6e4191. That change (correctly) limited a transform based on assumed finiteness to mustprogress loops, but the previous change (38540d7) which introduced the mustprogress check utility only handled function attributes, not the loop metadata form.
It turns out that clang uses the function attribute form for C++, and the loop metadata form for C. As a result, 3c6e4191 ended up being a large regression in practice for C code as loops weren't being considered mustprogress despite the language semantics.
Currently, NoWrapFlags are dropped if we inline operands of SCEVAddExpr
operands. As a consequence, we always drop flags when building
expressions like `getAddExpr(A, getAddExpr(B, C, NUW), NUW)`.
We should be able to retain NUW flags common among all inlined
SCEVAddExpr and the original flags.
Reviewed By: nikic, mkazantsev
Differential Revision: https://reviews.llvm.org/D103877
Noticed via code inspection. We changed the semantics of the IR when we added mustprogress, and we appear to have not updated this location.
Differential Revision: https://reviews.llvm.org/D103834
The motivation here is simple loops with unsigned induction variables w/non-one steps. A toy example would be:
for (unsigned i = 0; i < N; i += 2) { body; }
Given C/C++ semantics, we do not get the nuw flag on the induction variable. Given that lack, we currently can't compute a bound for this loop. We can do better for many cases, depending on the contents of "body".
The basic intuition behind this patch is as follows:
* A step which evenly divides the iteration space must wrap through the same numbers repeatedly. And thus, we can ignore potential cornercases where we exit after the n-th wrap through uint32_max.
* Per C++ rules, infinite loops without side effects are UB. We already have code in SCEV which relies on this. In LLVM, this is tied to the mustprogress attribute.
Together, these let us conclude that the trip count of this loop must come before unsigned overflow unless the body would form a well defined infinite loop.
A couple notes for those reading along:
* I reused the loop properties code which is overly conservative for this case. I may follow up in another patch to generalize it for the actual UB rules.
* We could cache the n(s/u)w facts. I left that out because doing a pre-patch which cached existing inference showed a lot of diffs I had trouble fully explaining. I plan to get back to this, but I don't want it on the critical path.
Differential Revision: https://reviews.llvm.org/D103118
We might want to use it when creating SCEV proper in createSCEV(),
now that we don't `forgetValue()` in `SimplifyIndvar::strengthenOverflowingOperation()`,
which might have caused us to loose some optimization potential.
When we're remapping an AddRec, the AddRec constructed by a partial
rewrite might not make sense. This triggers an assertion complaining
it's not loop-invariant.
Instead of constructing the partially rewritten AddRec, just skip
straight to calling evaluateAtIteration.
Testcase was automatically reduced using llvm-reduce, so it's a little
messy, but hopefully makes sense.
Differential Revision: https://reviews.llvm.org/D102959
ExprValueMap is a map from SCEV * to a set-vector of (Value *, ConstantInt *) pair,
and while the map itself will likely be big-ish (have many keys),
it is a reasonable assumption that each key will refer to a small-ish
number of pairs.
In particular looking at n=512 case from
https://bugs.llvm.org/show_bug.cgi?id=50384,
the small-size of 4 appears to be the sweet spot,
it results in the least allocations while minimizing memory footprint.
```
$ for i in $(ls heaptrack.opt.*.gz); do echo $i; heaptrack_print $i | tail -n 6; echo ""; done
heaptrack.opt.0-orig.gz
total runtime: 14.32s.
calls to allocation functions: 8222442 (574192/s)
temporary memory allocations: 2419000 (168924/s)
peak heap memory consumption: 190.98MB
peak RSS (including heaptrack overhead): 239.65MB
total memory leaked: 67.58KB
heaptrack.opt.1-n1.gz
total runtime: 13.72s.
calls to allocation functions: 7184188 (523705/s)
temporary memory allocations: 2419017 (176338/s)
peak heap memory consumption: 191.38MB
peak RSS (including heaptrack overhead): 239.64MB
total memory leaked: 67.58KB
heaptrack.opt.2-n2.gz
total runtime: 12.24s.
calls to allocation functions: 6146827 (502355/s)
temporary memory allocations: 2418997 (197695/s)
peak heap memory consumption: 163.31MB
peak RSS (including heaptrack overhead): 211.01MB
total memory leaked: 67.58KB
heaptrack.opt.3-n4.gz
total runtime: 12.28s.
calls to allocation functions: 6068532 (494260/s)
temporary memory allocations: 2418985 (197017/s)
peak heap memory consumption: 155.43MB
peak RSS (including heaptrack overhead): 201.77MB
total memory leaked: 67.58KB
heaptrack.opt.4-n8.gz
total runtime: 12.06s.
calls to allocation functions: 6068042 (503321/s)
temporary memory allocations: 2418992 (200646/s)
peak heap memory consumption: 166.03MB
peak RSS (including heaptrack overhead): 213.55MB
total memory leaked: 67.58KB
heaptrack.opt.5-n16.gz
total runtime: 12.14s.
calls to allocation functions: 6067993 (499958/s)
temporary memory allocations: 2418999 (199307/s)
peak heap memory consumption: 187.24MB
peak RSS (including heaptrack overhead): 233.69MB
total memory leaked: 67.58KB
```
While that test may be an edge worst-case scenario,
https://llvm-compile-time-tracker.com/compare.php?from=dee85d47d9f15fc268f7b18f279dac2774836615&to=98a57e31b1947d5bcdf4a5605ac2ab32b4bd5f63&stat=instructions
agrees that this also results in improvements in the usual situations.
This patch implements getSmallConstantTripMultiple(L) correctly for multiple exit loops. The previous implementation was both imprecise, and violated the specified behavior of the method. This was fine in practice, because it turns out the function was both dead in real code, and not tested for the multiple exit case.
Differential Revision: https://reviews.llvm.org/D103189
This came up in review for another patch, see https://reviews.llvm.org/D102982#2782407 for full context.
I've reviewed the callers to make sure they can handle multiple exit loops w/non-zero returns. There's two cases in target cost models where results might change (Hexagon and PowerPC), but the results looked legal and reasonable. If a target maintainer wishes to back out the effect of the costing change, they should explicitly check for multiple exit loops and handle them as desired.
Differential Revision: https://reviews.llvm.org/D103182
When memoized values for a SCEV expressions are dropped, we also
drop all BECounts that make use of the SCEV expression. This is done
by iterating over all the ExitNotTaken counts and (recursively)
checking whether they use the SCEV expression. If there are many
exits, this will take a lot of time.
This patch improves the situation by pre-computing a set of all
used operands, so that we can determine whether a certain BEInfo
needs to be invalidated using a simple set lookup. Will still need
to loop over all BEInfos though.
This makes for a mild improvement on non-degenerate cases:
https://llvm-compile-time-tracker.com/compare.php?from=b661a55a253f4a1cf5a0fbcb86e5ba7b9fb1387b&to=be1393f450e594c53f0ad7e62339a6bc831b16f6&stat=instructions
For the degenerate case from https://bugs.llvm.org/show_bug.cgi?id=50384,
for n=128 I'm seeing run time drop from 1.6s to 1.1s.
Differential Revision: https://reviews.llvm.org/D102796
We already apply loop-guards when computing the maximum with unitary
steps. This extends the code to also do so when dealing with non-unitary
steps.
This allows us to infer a tighter maximum in some cases.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D102267
applyLoopGuards() already combines conditions from multiple nested
guards. However, it cannot use multiple conditions on the same guard,
combined using and/or. Add support for this by recursing into either
`and` or `or`, depending on the direction of the branch.
Differential Revision: https://reviews.llvm.org/D101692
I think currently isImpliedViaMerge can incorrectly return true for phis
in a loop/cycle, if the found condition involves the previous value of
Consider the case in exit_cond_depends_on_inner_loop.
At some point, we call (modulo simplifications)
isImpliedViaMerge(<=, %x.lcssa, -1, %call, -1).
The existing code tries to prove IncV <= -1 for all incoming values
InvV using the found condition (%call <= -1). At the moment this succeeds,
but only because it does not compare the same runtime value. The found
condition checks the value of the last iteration, but the incoming value
is from the *previous* iteration.
Hence we incorrectly determine that the *previous* value was <= -1,
which may not be true.
I think we need to be more careful when looking at the incoming values
here. In particular, we need to rule out that a found condition refers to
any value that may refer to one of the previous iterations. I'm not sure
there's a reliable way to do so (that also works of irreducible control
flow).
So for now this patch adds an additional requirement that the incoming
value must properly dominate the phi block. This should ensure the
values do not change in a cycle. I am not entirely sure if will catch
all cases and I appreciate a through second look in that regard.
Alternatively we could also unconditionally bail out in this case,
instead of checking the incoming values
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D101829
This seems to be a leftover from when the BackedgeTakenInfo
stored multiple exit counts with manual memory management. At
some point this was switchted to a simple vector, and there should
be no need to micro-manage the clearing anymore. We can simply
drop the loop from the map and the the destructor do its job.
Straight forward extension to the recently added infrastructure which was pioneered with shl. This was originally posted as part of D99687, but split off for ease of review.
(I also decided to exclude the unknown start sign case explicitly for simplicity of understanding.)
Differential Revision: https://reviews.llvm.org/D101181
These can be handled the same way as ule/ult, just using umax
instead of umin. This is useful in cases where the umax prevents
the upper bound from overflowing.
Differential Revision: https://reviews.llvm.org/D101196
ICMP_NE predicates directly overwrote the rewritten result,
instead of chaining it with previous rewrites, as was done for
ICMP_ULT and ICMP_ULE. This means that some guards were effectively
discarded, depending on their order.
Adding the switches to reduce diffs. I'm about to split that into an lshr part and an ashr part, doing the NFC part first makes it easier to maintain both diffs.
As being discussed in https://reviews.llvm.org/D100721,
this modelling is lossy, we can't reconstruct `ash`/`ashr exact`
from it, which means that whenever we actually expand the IR,
we've just pessimized the code..
It would be good to model this pattern, after all it comes up every time
you want to compute a distance between two pointers, but not at this cost.
This reverts commit ec54867df5.
I've run into some cases where a large fraction of compile-time is
spent invalidating SCEV. One of the causes is forgetLoop(), which
walks all values that are def-use reachable from the loop header
phis. When invalidating a topmost loop, that might be close to all
values in a function. Additionally, it's fairly common for there to
not actually be anything to invalidate, but we'll still be performing
this walk again and again.
My first thought was that we don't need to continue walking the uses
if the current value doesn't have a SCEV expression. However, this
isn't quite right, because SCEV construction can skip over values
(e.g. for a chain of adds, we might only create a SCEV expression
for the final value).
What this patch does instead is to only walk the (full) def-use chain
of loop phis that have a SCEV expression. If there's no expression
for a phi, then we also don't have any dependent expressions to
invalidate.
Differential Revision: https://reviews.llvm.org/D100264
"Does the predicate hold between two ranges?"
Not very surprisingly, some places were already doing this check,
without explicitly naming the algorithm, cleanup them all.
"Does the predicate hold between two ranges?"
Not very surprisingly, some places were already doing this check,
without explicitly naming the algorithm, cleanup them all.
Eli Friedman