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
This extends D108921 into a generic rule applied to constructing ExitLimits along all paths. The remaining paths (primarily howFarToZero) don't have the same reasoning about UB sensitivity as the howManyLessThan ones did. Instead, the remain cause for max counts being more precise than exact counts is that we apply context sensitive loop guards on the max path, and not on the exact path. That choice is mildly suspect, but out of scope of this patch.
The MVETailPredication.cpp change deserves a bit of explanation. We were previously figuring out that two SCEVs happened to be equal because the happened to be identical. When we optimized one with context sensitive information, but not the other, we lost the ability to prove them equal. So, cover this case by subtracting and then applying loop guards again. Without this, we see changes in test/CodeGen/Thumb2/mve-blockplacement.ll
Differential Revision: https://reviews.llvm.org/D109015
This patch is specifically the howManyLessThan case. There will be a couple of followon patches for other codepaths.
The subtle bit is explaining why the two codepaths have a difference while both are correct. The test case with modifications is a good example, so let's discuss in terms of it.
* The previous exact bounds for this example of (-126 + (126 smax %n))<nsw> can evaluate to either 0 or 1. Both are "correct" results, but only one of them results in a well defined loop. If %n were 127 (the only possible value producing a trip count of 1), then the loop must execute undefined behavior. As a result, we can ignore the TC computed when %n is 127. All other values produce 0.
* The max taken count computation uses the limit (i.e. the maximum value END can be without resulting in UB) to restrict the bound computation. As a result, it returns 0 which is also correct.
WARNING: The logic above only holds for a single exit loop. The current logic for max trip count would be incorrect for multiple exit loops, except that we never call computeMaxBECountForLT except when we can prove either a) no overflow occurs in this IV before exit, or b) this is the sole exit.
An alternate approach here would be to add the limit logic to the symbolic path. I haven't played with this extensively, but I'm hesitant because a) the term is optional and b) I'm not sure it'll reliably simplify away. As such, the resulting code quality from expansion might actually get worse.
This was noticed while trying to figure out why D108848 wasn't NFC, but is otherwise standalone.
Differential Revision: https://reviews.llvm.org/D108921
ExposePointerBase() in SCEVExpander implements basically the same
functionality as removePointerBase() in SCEV, so reuse it.
The SCEVExpander code assumes that the pointer operand on adds is
the last one -- I'm not sure that always holds. As such this might
not be strictly NFC.
This was previously committed in 914836b, and reverted due to confusion on the status of the review.
Differential Revision: https://reviews.llvm.org/D108601
If we no an addrec doesn't self-wrap, the increment is strictly positive, and the start value is the smallest representable value, then we know that the corresponding wrap type can not occur.
Differential Revision: https://reviews.llvm.org/D108601
Since then, the SCEV pointer handling as been improved,
so the assertion should now hold.
This reverts commit b96114c1e1,
relanding the assertion from commit 141e845da5.
This adjusts mayHaveSideEffect() to return true for !willReturn()
instructions. Just like other side-effects, non-willreturn calls
(aka "divergence") cannot be removed and cannot be reordered relative
to other side effects. This fixes a number of bugs where
non-willreturn calls are either incorrectly dropped or moved. In
particular, it also fixes the last open problem in
https://bugs.llvm.org/show_bug.cgi?id=50511.
I performed a cursory review of all current mayHaveSideEffect()
uses, which convinced me that these are indeed the desired default
semantics. Places that do not want to consider non-willreturn as a
sideeffect generally do not want mayHaveSideEffect() semantics at
all. I identified two such cases, which are addressed by D106591
and D106742. Finally, there is a use in SCEV for which we don't
really have an appropriate API right now -- what it wants is
basically "would this be considered forward progress". I've just
spelled out the previous semantics there.
Differential Revision: https://reviews.llvm.org/D106749
Eli pointed out the issue when reviewing D104140. The max trip count logic makes an assumption that the value of IV changes. When the step is zero, the nowrap fact becomes trivial, and thus there's nothing preventing the loop from being nearly infinite. (The "nearly" part is because mustprogress may disallow an infinite loop while still allowing 999999999 iterations before RHS happens to allow an exit.)
This is very difficult to see in practice. You need a means to produce a loop varying RHS in a mustprogress loop which doesn't allow the loop to be infinite. In most cases, LICM or SCEV are smart enough to remove the loop varying expressions.
Differential Revision: https://reviews.llvm.org/D106327
Allow arbitrary strides, and make sure we return the correct result when
the backedge-taken count is zero.
Differential Revision: https://reviews.llvm.org/D106197
Wrap semantics are subtle when combined with multiple exits. This has caused several rounds of confusion during recent reviews, so try to document the subtly distinction between when wrap flags provide <u and <=u facts.
The current implementation of computeBECount doesn't account for the
possibility that adding "Stride - 1" to Delta might overflow. For almost
all loops, it doesn't, but it's not actually proven anywhere.
To deal with this, use a variety of tricks to try to prove that the
addition doesn't overflow. If the proof is impossible, use an alternate
sequence which never overflows.
Differential Revision: https://reviews.llvm.org/D105216
This is split from D105216, it handles only a subset of the cases in that patch.
Specifically, the issue being fixed is that the code incorrectly assumed that (Start-Stide) < End implied that the backedge was taken at least once. This is not true when e.g. Start = 4, Stride = 2, and End = 3. Note that we often do produce the right backedge taken count despite the flawed reasoning.
The fix chosen here is to use an alternate form of uceil (ceiling of unsigned divide) lowering which is safe when max(RHS,Start) > Start - Stride. (Note that signedness of both max expression and comparison depend on the signedness of the comparison being analyzed, and that overflow in the Start - Stride expression is allowed.) Note that this is weaker than proving the backedge is taken because it allows start - stride < end < start. Some cases which can't be proven safe are sent down the generic path, and we do end up generating less optimal expressions in a few cases.
Credit for coming up with the approach goes entirely to Eli. I just split it off, tweaked the comments a bit, and did some additional testing.
Differential Revision: https://reviews.llvm.org/D105942
This is split from D105216, but the code is hoisted much earlier into
the path where we can actually get a zero stride flowing through. Some
fairly simple proofs handle the cases which show up in practice. The
only test changes are the cases where we really do need a non-zero
divider to produce the right result.
Recommitting with isLoopInvariant() check.
Differential Revision: https://reviews.llvm.org/D105921
This is split from D105216, but the code is hoisted much earlier into the path where we can actually get a zero stride flowing through. Some fairly simple proofs handle the cases which show up in practice. The only test changes are the cases where we really do need a non-zero divider to produce the right result.
Differential Revision: https://reviews.llvm.org/D105921
Split off from D105216 to simplify review. Rewritten with a lambda to be easier to follow. Comments clarified.
Sorry for no test case, this is tricky to exercise with the current structure of the code. It's about to be hit more frequently in a follow up patch, and the change itself is simple.
This is split from D105216 to reduce patch complexity. Original code by Eli with very minor modification by me.
The primary point of this patch is to add the getUDivCeilSCEV routine. I included the two callers with constant arguments as we know those must constant fold even without any of the fancy inference logic.
Rules:
1. SCEVUnknown is a pointer if and only if the LLVM IR value is a
pointer.
2. SCEVPtrToInt is never a pointer.
3. If any other SCEV expression has no pointer operands, the result is
an integer.
4. If a SCEVAddExpr has exactly one pointer operand, the result is a
pointer.
5. If a SCEVAddRecExpr's first operand is a pointer, and it has no other
pointer operands, the result is a pointer.
6. If every operand of a SCEVMinMaxExpr is a pointer, the result is a
pointer.
7. Otherwise, the SCEV expression is invalid.
I'm not sure how useful rule 6 is in practice. If we exclude it, we can
guarantee that ScalarEvolution::getPointerBase always returns a
SCEVUnknown, which might be a helpful property. Anyway, I'll leave that
for a followup.
This is basically mop-up at this point; all the changes with significant
functional effects have landed. Some of the remaining changes could be
split off, but I don't see much point.
Differential Revision: https://reviews.llvm.org/D105510
Philip Reames