D71264 started using a context instruction in a computeKnownBits()
call. However, if aliasing between two GEPs is checked, then the
choice of context instruction will be different for alias(GEP1, GEP2)
and alias(GEP2, GEP1), which is not supposed to happen.
Resolve this by remembering which GEP a certain VarIndex belongs to,
and use that as the context instruction. This makes the choice of
context instruction predictable and symmetric.
It should be noted that this choice of context instruction is
non-optimal (just like the previous choice): The AA query result is
only valid at points that are reachable from *both* instructions.
Using either one of them is conservatively correct, but a larger
context may also be valid to use.
Differential Revision: https://reviews.llvm.org/D93183
Currently, BasicAA does not exploit information about value ranges of
indexes. For example, consider the 2 pointers %a = %base and
%b = %base + %stride below, assuming they are used to access 4 elements.
If we know that %stride >= 4, we know the accesses do not alias. If
%stride is a constant, BasicAA currently gets that. But if the >= 4
constraint is encoded using an assume, it misses the NoAlias.
This patch extends DecomposedGEP to include an additional MinOtherOffset
field, which tracks the constant offset similar to the existing
OtherOffset, which the difference that it also includes non-negative
lower bounds on the range of the index value. When checking if the
distance between 2 accesses exceeds the access size, we can use this
improved bound.
For now this is limited to using non-negative lower bounds for indices,
as this conveniently skips cases where we do not have a useful lower
bound (because it is not constrained). We potential miss out in cases
where the lower bound is constrained but negative, but that can be
exploited in the future.
Reviewers: sanjoy, hfinkel, reames, asbirlea
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D76194
In order to use assumptions, computeKnownBits needs a context
instruction. We can use the GEP, if it is an instruction. We already
pass the assumption cache, but it cannot be used without a context
instruction.
Reviewers: anemet, asbirlea, hfinkel, spatel
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D71264
Nikita Popov