BasicAA knows how to analyze phis, but to control compile time, we're fairly limited in doing so. This patch loosens that restriction just slightly when there is exactly one phi input (after discounting induction variable increments). The result of this is that we can handle more cases around nested and sibling loops with pointer induction variables.
A few points to note.
* This is deliberately extremely restrictive about recursing through at most one input of the phi. There's a known general problem with BasicAA sometimes hitting exponential compile time already, and this patch makes every effort not to compound the problem. Once the root issue is fixed, we can probably loosen the restrictions here a bit.
* As seen in the test file, we're still missing cases which aren't *directly* based on phis (e.g. using the indvar increment). I believe this to be a separate problem and am going to explore this in another patch once this one lands.
* As seen in the test file, this results in the unfortunate fact that using phivalues sometimes results in worse quality results. I believe this comes down to an oversight in how recursive phi detection was implemented for phivalues. I'm happy to tackle this in a follow up change.
Differential Revision: https://reviews.llvm.org/D97401
This reverts commit 43a569faeb.
Unhelpfully, the tool just added the header and didn't actually update any of the tests. I didn't notice until after pushing.
Temporarily revert commit 8b1c4e310c.
After 8b1c4e310c the compile-time for `MultiSource/Benchmarks/MiBench/consumer-lame`
dramatically increases with -O3 & LTO, causing issues for builders with
that configuration.
I filed PR48553 with a smallish reproducer that shows a 10-100x compile
time increase.
If we have two unknown sizes and one GEP operand and one non-GEP
operand, then we currently simply return MayAlias. The comment says
we can't do anything useful ... but we can! We can still check that
the underlying objects are different (and do so for the GEP-GEP case).
To reduce the compile-time impact, this a) checks this early, before
doing the relatively expensive GEP decomposition that will not be
used and b) doesn't do the check if the other operand is a phi or
select. In that case, the phi/select will already recurse, so this
would just do two slightly different recursive walks that arrive at
the same roots.
Compile-time is still a bit of a mixed bag: https://llvm-compile-time-tracker.com/compare.php?from=624af932a808b363a888139beca49f57313d9a3b&to=845356e14adbe651a553ed11318ddb5e79a24bcd&stat=instructions
On average this is a small improvement, but sqlite with ThinLTO has
a 0.5% regression (lencod has a 1% improvement).
The BasicAA test case checks this by using two memsets with unknown
size. However, the more interesting case where this is useful is
the LoopVectorize test case, as analysis of accesses in loops tends
to always us unknown sizes.
Differential Revision: https://reviews.llvm.org/D92401
For recursive phis, we skip the recursive operands and check that
the remaining operands are NoAlias with an unknown size. Currently,
this is limited to inbounds GEPs with positive offsets, to
guarantee that the recursion only ever increases the pointer.
Make this more general by only requiring that the underlying object
of the phi operand is the phi itself, i.e. it it based on itself in
some way. To compensate, we need to use a beforeOrAfterPointer()
location size, as we no longer have the guarantee that the pointer
is strictly increasing.
This allows us to handle some additional cases like negative geps,
geps with dynamic offsets or geps that aren't inbounds.
Differential Revision: https://reviews.llvm.org/D91914
This option was added a while back, to help improve AA around pointer
phi loops. It looks for phi(gep(phi, const), x) loops, checking if x can
then prove more precise aliasing info.
Differential Revision: https://reviews.llvm.org/D82998
As shown in D82998, the basic-aa-recphi option can cause miscompiles for
gep's with negative constants. The option checks for recursive phi, that
recurse through a contant gep. If it finds one, it performs aliasing
calculations using the other phi operands with an unknown size, to
specify that an unknown number of elements after the initial value are
potentially accessed. This works fine expect where the constant is
negative, as the size is still considered to be positive. So this patch
expands the check to make sure that the constant is also positive.
Differential Revision: https://reviews.llvm.org/D83576
This option was added a while back, to help improve AA around pointer
phi loops. It looks for phi(gep(phi, const), x) loops, checking if x can
then prove more precise aliasing info.
Differential Revision: https://reviews.llvm.org/D82998
With the option -basic-aa-recphi we can detect recursive phis that loop
through constant geps, which allows us to detect more no-alias case for
pointer IV's. If the other phi operand and the other alias value are
MustAlias though, we cannot presume that every element in the loop is
also MustAlias. We need to instead be conservative and return MayAlias.
Differential Revision: https://reviews.llvm.org/D82987
dfukalov