Add initial pattern matching for SCEV constants. Follow-up patches will
add additional matchers for various SCEV expressions.
This patch only converts a few instances to use the new matchers to make
sure everything builds as expected for now.
PR: https://github.com/llvm/llvm-project/pull/119389
De-duplicate the functions getSignedPredicate and getUnsignedPredicate,
nearly identical versions of which were present in CmpInst and ICmpInst,
creating less confusion.
This fixes all the places that hit the new assertion added in
https://github.com/llvm/llvm-project/pull/106524 in tests. That is,
cases where the value passed to the APInt constructor is not an N-bit
signed/unsigned integer, where N is the bit width and signedness is
determined by the isSigned flag.
The fixes either set the correct value for isSigned, set the
implicitTrunc flag, or perform more calculations inside APInt.
Note that the assertion is currently still disabled by default, so this
patch is mostly NFC.
There are a number of places where we call getSmallConstantMaxTripCount
without passing a vector of predicates:
getSmallBestKnownTC
isIndvarOverflowCheckKnownFalse
computeMaxVF
isMoreProfitable
I've changed all of these to now pass in a predicate vector so that
we get the benefit of making better vectorisation choices when we
know the max trip count for loops that require SCEV predicate checks.
I've tried to add tests that cover all the cases affected by these
changes.
Store predicates in ExitLimit and ExitNotTaken in a SmallVector instead
of a SmallPtrSet. This guarantees the predicates can be iterated on in a
predictable manner. This ensures the predicates can be printed and
generated in a predictable order.
This shifts de-duplication of predicates to construction time for
ExitLimit. ExitNotTaken just takes predicates from ExitLimit, so they
should also be free of duplicates.
This was exposed by 2f7ccaf4a8565628a4c7d2b5a49bb45478940be6
(https://github.com/llvm/llvm-project/pull/108777).
Should fix https://lab.llvm.org/buildbot/#/builders/110/builds/1494.
This time with 100% more building unit tests. Original commit message follows.
[NFC] Switch a number of DenseMaps to SmallDenseMaps for speedup (#109417)
If we use SmallDenseMaps instead of DenseMaps at these locations,
we get a substantial speedup because there's less spurious malloc
traffic. Discovered by instrumenting DenseMap with some accounting
code, then selecting sites where we'll get the most bang for our buck.
If we use SmallDenseMaps instead of DenseMaps at these locations,
we get a substantial speedup because there's less spurious malloc
traffic. Discovered by instrumenting DenseMap with some accounting
code, then selecting sites where we'll get the most bang for our buck.
Currently if a loop contains loads that we can prove at compile time
are dereferenceable when certain conditions are satisfied the function
isDereferenceableAndAlignedInLoop will still return false because
getSmallConstantMaxTripCount will return 0 when SCEV predicates
are required. This patch changes getSmallConstantMaxTripCount to take
an optional Predicates pointer argument so that we can permit
functions such as isDereferenceableAndAlignedInLoop to consider more
cases.
It is almost always simpler to use {} instead of std::nullopt to
initialize an empty ArrayRef. This patch changes all occurrences I could
find in LLVM itself. In future the ArrayRef(std::nullopt_t) constructor
could be deprecated or removed.
There are a few places in ScalarEvolution.cpp where we copy predicates
from one list to another and they have a similar pattern:
for (const auto *P : ENT.Predicates)
Predicates->push_back(P);
We can avoid the loop by writing them like this:
Predicates->append(ENT.Predicates.begin(), ENT.Predicates.end());
which may end up being more efficient since we only have to try
reserving more space once.
Due to a reviewer request on PR #88385 I have created this patch
to add a getPredicatedExitCount function, which is similar to
getExitCount except that it uses the predicated backedge taken
information. With PR #88385 we will start to care about more
loops with multiple exits, and want the ability to query exit
counts for a particular exiting block. Such loops may require
predicates in order to be vectorised.
New tests added here:
Analysis/ScalarEvolution/predicated-exit-count.ll
Whilst dealing with review comments on
https://github.com/llvm/llvm-project/pull/96752
I discovered that SCEV does not know about the dereferenceable attribute
on function arguments so I have updated getRangeRef to make use of it
by calling getPointerDereferenceableBytes.
Use computeConstantDifference() instead of casting getMinusSCEV() to
SCEVConstant. This can be much faster in some cases, because
computeConstantDifference() computes the result without creating new
SCEV expressions.
This improves LTO/ThinLTO compile-time for lencod by more than 10%.
I've verified that computeConstantDifference() does not produce worse
results than the previous code for anything in llvm-test-suite. This
required raising the iteration cutoff to 6. I ended up increasing it to
8 just to be on the safe side (for code outside llvm-test-suite), and
because this doesn't materially affect compile-time anyway (we'll almost
always bail out earlier).
Inside computeConstantDifference(), handle the case where both sides are
of the form `C * %x`, in which case we can strip off the common
multiplication (as long as we remember to multiply by it for the
following difference calculation).
There is an obvious alternative implementation here, which would be to
directly decompose multiplies inside the "Multiplicity" accumulation.
This does work, but I've found this to be both significantly slower
(because everything has to work on APInt) and more complex in
implementation (e.g. because we now need to match back the new More/Less
with an arbitrary factor) without providing more power in practice. As
such, I went for the simpler variant here.
This is the last step to make computeConstantDifference() sufficiently
powerful to replace existing uses of
`cast<SCEVConstant>(getMinusSCEV())` with it.
computeConstantDifference() can currently look through addrecs with
identical steps, and then through adds with identical operands (apart
from constants).
However, it fails to handle minor variations, such as two nested add
recs, or an outer add with an inner addrec (rather than the other way
around).
This patch supports these cases by adding a loop over the
simplifications, limited to a small number of iterations. The motivation
is the same as in #101339, to make
computeConstantDifference() powerful enough to replace existing uses of
`dyn_cast<SCEVConstant>(getMinusSCEV())` with it. Though as the IR test
diff shows, other callers may also benefit.
The canAssumeNoSelfWrap routine in howManyLessThans was doing two subtly
inter-related things. First, it was proving no-self-wrap. This exactly
duplicates the existing logic in the caller. Second, it was establishing
the precondition for the nw->nsw/nuw inference. Specifically, we need to
know that *this* exit must be taken for the inference to be sound.
Otherwise, another (possible abnormal) exit could be taken in the
iteration where this IV would become poison.
This change moves all of that logic into the caller, and caches the
resulting nuw/nsw flags in the AddRec. This centralizes the logic in one
place, and makes it clear that it all depends on controlling the sole
exit.
We do loose a couple cases with SCEV predication. Specifically, if SCEV
predication was able to convert e.g. zext(addrec) into an addrec(zext)
using predication, but didn't record the nuw fact on the new addrec,
then the consuming code can no longer fix this up. I don't think this
case particularly matters.
---------
Co-authored-by: Nikita Popov <github@npopov.com>
The Mul factor was zero-extended here, resulting in incorrect
results for integers larger than 64-bit.
As we currently only multiply by 1 or -1, just split this into
two cases -- there's no need for a full multiplication here.
Fixes https://github.com/llvm/llvm-project/issues/102597.
Without this patch, the constructor arguments come from
SmallVectorImpl, not ArrayRef. This patch switches them to ArrayRef
so that we can construct SmallVector with a single argument.
Note that LLVM Programmer’s Manual prefers ArrayRef to SmallVectorImpl
for flexibility.
Currently it only deals with the case where we're subtracting adds with
at most one non-constant operand. This patch extends it to cancel out
common operands for the subtraction of arbitrary add expressions.
The background here is that I want to replace a getMinusSCEV() call in
LAA with computeConstantDifference():
93fecc2577/llvm/lib/Analysis/LoopAccessAnalysis.cpp (L1602-L1603)
This particular call is very expensive in some cases (e.g. lencod with
LTO) and computeConstantDifference() could achieve this much more
cheaply, because it does not need to construct new SCEV expressions.
However, the current computeConstantDifference() implementation is too
weak for this and misses many basic cases. This is a step towards making
it more powerful while still keeping it pretty fast.
Add a common constantFoldAndGroupOps() helper that takes care of
constant folding and grouping transforms that are common to all nary
ops. This moves the constant folding prior to grouping, which is more
efficient, and excludes any constant from the sort.
The constant folding has hooks for folding, identity constants and
absorber constants.
This gives a compile-time improvement for SCEV-heavy workloads like
lencod.
We have existing code which reasons about a step evenly dividing the
iteration space is a finite loop with a single exit implying
no-self-wrap. The sign of the step doesn't effect this.
---------
Co-authored-by: Nikita Popov <github@npopov.com>
SCEV has logic for inferring wrap flags on AddRecs which are known to
control an exit based on whether the step is a power of two. This logic
only considered constants, and thus did not trigger for steps such as (4
x vscale) which are common in scalably vectorized loops.
The net effect is that we were very sensative to the preservation of
nsw/nuw flags on such IVs, and could not infer trip counts if they got
lost for any reason.
---------
Co-authored-by: Nikita Popov <github@npopov.com>
The cache triggers almost never, and seems unlikely to help with
performance. However, when it does, it is likely to cause the comparator
to become inconsistent due to a bad interaction of the depth limit and
cache hits. This leads to crashes in debug builds. See the new unit test
for a reproducer.
The use-def walk in forgetValue() was skipping instructions with
non-SCEVable types. However, SCEV may look past with.overflow
intrinsics returning aggregates.
Fixes#97586.
PR #96919 caused a minor compile-time regression, mostly because
SCEV now goes through an extra out-of-line function to fetch the
data layout, and does this a lot. Cache the DataLayout in SCEV
to avoid these repeated calls.
Fixes#92554 (std::reverse will auto-vectorize now)
When calculating number of times a exit condition containing a
comparison is executed, we mostly assume that RHS of comparison should
be loop invariant, but it may be another add-recurrence.
~In that case, we can try the computation with `LHS = LHS - RHS` and
`RHS = 0`.~ (It is not valid unless proven that it doesn't wrap)
**Edit:**
We can calculate back edge count for loop structure like:
```cpp
left = left_start
right = right_start
while(left < right){
// ...do something...
left += s1; // the stride of left is s1 (> 0)
right -= s2; // the stride of right is -s2 (s2 > 0)
}
// left and right converge somewhere in the middle of their start values
```
We can calculate the backedge-count as ceil((End - left_start) /u (s1-
(-s2)) where, End = max(left_start, right_start).
**Alive2**: https://alive2.llvm.org/ce/z/ggxx58
Namespaces are terminated with a closing comment in the majority of the
codebase so do the same here for consistency. Also format code within
some namespaces to make clang-format happy.
