The pointsToConstantMemory() method returns true only if the memory pointed to
by the memory location is globally invariant. However, the LLVM memory model
also has the semantic notion of *locally-invariant*: memory that is known to be
invariant for the life of the SSA value representing that pointer. The most
common example of this is a pointer argument that is marked readonly noalias,
which the Rust compiler frequently emits.
It'd be desirable for LLVM to treat locally-invariant memory the same way as
globally-invariant memory when it's safe to do so. This patch implements that,
by introducing the concept of a *ModRefInfo mask*. A ModRefInfo mask is a bound
on the Mod/Ref behavior of an instruction that writes to a memory location,
based on the knowledge that the memory is globally-constant memory (in which
case the mask is NoModRef) or locally-constant memory (in which case the mask
is Ref). ModRefInfo values for an instruction can be combined with the
ModRefInfo mask by simply using the & operator. Where appropriate, this patch
has modified uses of pointsToConstantMemory() to instead examine the mask.
The most notable optimization change I noticed with this patch is that now
redundant loads from readonly noalias pointers can be eliminated across calls,
even when the pointer is captured. Internally, before this patch,
AliasAnalysis was assigning Ref to reads from constant memory; now AA can
assign NoModRef, which is a tighter bound.
Differential Revision: https://reviews.llvm.org/D136659
Summary:
This patch removes references to AliasAnalysisCounter pass from the AliasAnalysis documentation. That pass have been eliminated in 2015, at revision trunk@247167.
Reviewed By: hfinkel
Differential Revision: https://reviews.llvm.org/D45876
llvm-svn: 330590
objects to start at the same address
As discussed on the ML, there's consensus that this is what the implementations
do and it seems sensible.
llvm-svn: 308090
Summary:
Alias analysis would like to know that
invariant.group.barrier returns pointer that mustalias,
but this can't imply that we can replace one pointer with another
Reviewers: dberlin, sanjoy
Subscribers: llvm-commits, chandlerc, hfinkel, nlewycky, amharc
Differential Revision: https://reviews.llvm.org/D31758
llvm-svn: 300033
This debugger was designed to catch places where the old update API was
failing to be used correctly. As I've removed the update API, it no
longer serves any purpose. We can introduce new debugging aid passes
around any future work w.r.t. updating AAs.
Note that I've updated the documentation here, but really I need to
rewrite the documentation to carefully spell out the ideas around
stateful AA and how things are changing in the AA world. However, I'm
hoping to do that as a follow-up to the refactoring of the AA
infrastructure to work in both old and new pass managers so that I can
write the documentation specific to that world.
Differential Revision: http://reviews.llvm.org/D11984
llvm-svn: 244825
This will allow classes to implement the AA interface without deriving
from the class or referencing an internal enum of some other class as
their return types.
Also, to a pretty fundamental extent, concepts such as 'NoAlias',
'MayAlias', and 'MustAlias' are first class concepts in LLVM and we
aren't saving anything by scoping them heavily.
My mild preference would have been to use a scoped enum, but that
feature is essentially completely broken AFAICT. I'm extremely
disappointed. For example, we cannot through any reasonable[1] means
construct an enum class (or analog) which has scoped names but converts
to a boolean in order to test for the possibility of aliasing.
[1]: Richard Smith came up with a "solution", but it requires class
templates, and lots of boilerplate setting up the enumeration multiple
times. Something like Boost.PP could potentially bundle this up, but
even that would be quite painful and it doesn't seem realistically worth
it. The enum class solution would probably work without the need for
a bool conversion.
Differential Revision: http://reviews.llvm.org/D10495
llvm-svn: 240255
:doc:`...` and :ref:`...` links help Sphinx keep track the dependencies
between documents and ensure that they are not pointing to nowhere.
Raw HTML links work just fine and are easier for people less familiar
with reST/Sphinx. They are easy to change over to the :doc:/:ref: style
after the fact so this is not a problem.
This commit doesn't fix all of them.
llvm-svn: 205792
Before we learned about :doc:, we used :ref: and put a dummy link at the
top of each page. Don't do that anymore.
This fixes PR14891 as a special case.
llvm-svn: 172162
Patrick Walton