[llvm] r279007 - [Docs] Update MemorySSA doc to address more feedback.

Thu Aug 18 15:28:54 PDT 2016

Hi!

This commit is just a doc update in LLVM, so I don't think it would break
an OpenMP test in clang. :)

Looking at the builds since then (specifically #119 and #120), it seems
that the OpenMP test failure has vanished, but other tests are now
consistently failing; all of the failing tests say "llvm-dis: Could not
open [...]: no such file or directory,"

I'm not sure what commit could have caused this. :/

Thanks,
George

On Thu, Aug 18, 2016 at 2:35 PM, Galina Kistanova <gkistanova at gmail.com>
wrote:

> Hello George,
>
> It looks like this commit broke one of our new builders on staging master:
>
> http://lab.llvm.org:8014/builders/clang-with-lto-ubuntu/builds/117
>
> The previous revision r279002 id green
> http://lab.llvm.org:8014/builders/clang-with-lto-ubuntu/builds/116
>
> Please have a look at this?
> I am going to move the builder from staging to main master today.
>
> Thanks
>
> Galina
>
> On Wed, Aug 17, 2016 at 4:21 PM, George Burgess IV via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: gbiv
>> Date: Wed Aug 17 18:21:56 2016
>> New Revision: 279007
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=279007&view=rev
>> Log:
>> [Docs] Update MemorySSA doc to address more feedback.
>>
>> Primarily, this clarifies wording in a few places, and adds "\ "s to
>> make the formatting of things like "``Foo`` s" better.
>>
>> Thanks to Michael Kuperstein for the comments.
>>
>> Modified:
>>     llvm/trunk/docs/MemorySSA.rst
>>
>> Modified: llvm/trunk/docs/MemorySSA.rst
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/MemorySS
>> A.rst?rev=279007&r1=279006&r2=279007&view=diff
>> ============================================================
>> ==================
>> --- llvm/trunk/docs/MemorySSA.rst (original)
>> +++ llvm/trunk/docs/MemorySSA.rst Wed Aug 17 18:21:56 2016
>> @@ -36,8 +36,8 @@ MemorySSA Structure
>>  ===================
>>
>>  MemorySSA is a virtual IR. After it's built, ``MemorySSA`` will contain a
>> -structure that maps ``Instruction`` s to ``MemoryAccess`` es, which are
>> -``MemorySSA``'s parallel to LLVM ``Instruction`` s.
>> +structure that maps ``Instruction``\ s to ``MemoryAccess``\ es, which are
>> +``MemorySSA``'s parallel to LLVM ``Instruction``\ s.
>>
>>  Each ``MemoryAccess`` can be one of three types:
>>
>> @@ -45,25 +45,25 @@ Each ``MemoryAccess`` can be one of thre
>>  - ``MemoryUse``
>>  - ``MemoryDef``
>>
>> -``MemoryPhi`` s are ``PhiNode`` , but for memory operations. If at any
>> -point we have two (or more) ``MemoryDef`` s that could flow into a
>> +``MemoryPhi``\ s are ``PhiNode``\ s, but for memory operations. If at any
>> +point we have two (or more) ``MemoryDef``\ s that could flow into a
>>  ``BasicBlock``, the block's top ``MemoryAccess`` will be a
>> -``MemoryPhi``. As in LLVM IR, ``MemoryPhi`` s don't correspond to any
>> -concrete operation. As such, you can't look up a ``MemoryPhi`` with an
>> -``Instruction`` (though we do allow you to do so with a
>> -``BasicBlock``).
>> +``MemoryPhi``. As in LLVM IR, ``MemoryPhi``\ s don't correspond to any
>> +concrete operation. As such, ``BasicBlock``\ s are mapped to
>> ``MemoryPhi``\ s
>> +inside ``MemorySSA``, whereas ``Instruction``\ s are mapped to
>> ``MemoryUse``\ s
>> +and ``MemoryDef``\ s.
>>
>>  Note also that in SSA, Phi nodes merge must-reach definitions (that
>>  is, definite new versions of variables).  In MemorySSA, PHI nodes merge
>>  may-reach definitions (that is, until disambiguated, the versions that
>>  reach a phi node may or may not clobber a given variable)
>>
>> -``MemoryUse`` s are operations which use but don't modify memory. An
>> example of
>> +``MemoryUse``\ s are operations which use but don't modify memory. An
>> example of
>>  a ``MemoryUse`` is a ``load``, or a ``readonly`` function call.
>>
>> -``MemoryDef`` s are operations which may either modify memory, or which
>> -otherwise clobber memory in unquantifiable ways. Examples of
>> ``MemoryDef`` s
>> -include ``store`` s, function calls, ``load`` s with ``acquire`` (or
>> higher)
>> +``MemoryDef``\ s are operations which may either modify memory, or which
>> +otherwise clobber memory in unquantifiable ways. Examples of
>> ``MemoryDef``\ s
>> +include ``store``\ s, function calls, ``load``\ s with ``acquire`` (or
>> higher)
>>  ordering, volatile operations, memory fences, etc.
>>
>>  Every function that exists has a special ``MemoryDef`` called
>> ``liveOnEntry``.
>> @@ -73,12 +73,12 @@ run on, and implies that we've hit the t
>>  ``liveOnEntry`` implies that the memory being used is either undefined or
>>  defined before the function begins.
>>
>> -An example of all of this overlayed on LLVM IR (obtained by running ``opt
>> +An example of all of this overlaid on LLVM IR (obtained by running ``opt
>>  -passes='print<memoryssa>' -disable-output`` on an ``.ll`` file) is
>> below. When
>>  viewing this example, it may be helpful to view it in terms of clobbers.
>> The
>>  operands of a given ``MemoryAccess`` are all (potential) clobbers of said
>>  MemoryAccess, and the value produced by a ``MemoryAccess`` can act as a
>> clobber
>> -for other ``MemoryAccess`` es. Another useful way of looking at it is in
>> +for other ``MemoryAccess``\ es. Another useful way of looking at it is in
>>  terms of heap versions.  In that view, operands of of a given
>>  ``MemoryAccess`` are the version of the heap before the operation, and
>>  if the access produces a value, the value is the new version of the heap
>> @@ -125,7 +125,7 @@ to (if such an instruction exists). For
>>  is a ``MemoryAccess`` (specifically, a ``MemoryDef``), and it describes
>> the LLVM
>>  instruction ``store i8 0, i8* %p3``. Other places in ``MemorySSA`` refer
>> to this
>>  particular ``MemoryDef`` as ``1`` (much like how one can refer to ``load
>> i8, i8*
>> -%p1`` in LLVM with ``%1``). Again, ``MemoryPhi`` s don't correspond to
>> any LLVM
>> +%p1`` in LLVM with ``%1``). Again, ``MemoryPhi``\ s don't correspond to
>> any LLVM
>>  Instruction, so the line directly below a ``MemoryPhi`` isn't special.
>>
>>  Going from the top down:
>> @@ -135,7 +135,9 @@ Going from the top down:
>>    ``MemoryPhi`` is referred to in the textual IR by the number ``6``.
>>  - ``2 = MemoryDef(6)`` notes that ``store i8 0, i8* %p1`` is a
>> definition,
>>    and its reaching definition before it is ``6``, or the ``MemoryPhi``
>> after
>> -  ``while.cond``.
>> +  ``while.cond``. (See the `Build-time use optimization`_ and
>> `Precision`_
>> +  sections below for why this ``MemoryDef`` isn't linked to a seperate,
>> +  disambiguated ``MemoryPhi``.)
>>  - ``3 = MemoryDef(6)`` notes that ``store i8 0, i8* %p2`` is a
>> definition; its
>>    reaching definition is also ``6``.
>>  - ``5 = MemoryPhi({if.then,2},{if.else,3})`` notes that the clobber
>> before
>> @@ -146,9 +148,9 @@ Going from the top down:
>>    reaching definition is ``5``.
>>  - ``MemoryUse(1)`` notes that ``load i8, i8* %p3`` is just a user of
>> memory,
>>    and the last thing that could clobber this use is above ``while.cond``
>> (e.g.
>> -  the store to ``%p3``).  In heap versioning parlance, it really
>> -  only depends on the heap version 1, and is unaffected by the new
>> -  heap versions generated since then.
>> +  the store to ``%p3``). In heap versioning parlance, it really only
>> depends on
>> +  the heap version 1, and is unaffected by the new heap versions
>> generated since
>> +  then.
>>
>>  As an aside, ``MemoryAccess`` is a ``Value`` mostly for convenience;
>> it's not
>>  meant to interact with LLVM IR.
>> @@ -158,9 +160,9 @@ Design of MemorySSA
>>
>>  ``MemorySSA`` is an analysis that can be built for any arbitrary
>> function. When
>>  it's built, it does a pass over the function's IR in order to build up
>> its
>> -mapping of ``MemoryAccess`` es. You can then query ``MemorySSA`` for
>> things like
>> -the dominance relation between ``MemoryAccess`` es, and get the
>> ``MemoryAccess``
>> -for any given ``Instruction`` .
>> +mapping of ``MemoryAccess``\ es. You can then query ``MemorySSA`` for
>> things
>> +like the dominance relation between ``MemoryAccess``\ es, and get the
>> +``MemoryAccess`` for any given ``Instruction`` .
>>
>>  When ``MemorySSA`` is done building, it also hands you a
>> ``MemorySSAWalker``
>>  that you can use (see below).
>> @@ -171,7 +173,7 @@ The walker
>>
>>  A structure that helps ``MemorySSA`` do its job is the
>> ``MemorySSAWalker``, or
>>  the walker, for short. The goal of the walker is to provide answers to
>> clobber
>> -queries beyond what's represented directly by ``MemoryAccess`` es. For
>> example,
>> +queries beyond what's represented directly by ``MemoryAccess``\ es. For
>> example,
>>  given:
>>
>>  .. code-block:: llvm
>> @@ -190,10 +192,11 @@ The store to ``%a`` is clearly not a clo
>>  be the walker's goal to figure this out, and return ``liveOnEntry`` when
>> queried
>>  for the clobber of ``MemoryAccess`` ``2``.
>>
>> -By default, ``MemorySSA`` provides a walker that can optimize
>> ``MemoryDef`` s
>> -and ``MemoryUse`` s by consulting alias analysis. Walkers were built to
>> be
>> -flexible, though, so it's entirely reasonable (and expected) to create
>> more
>> -specialized walkers (e.g. one that queries ``GlobalsAA``).
>> +By default, ``MemorySSA`` provides a walker that can optimize
>> ``MemoryDef``\ s
>> +and ``MemoryUse``\ s by consulting whatever alias analysis stack you
>> happen to
>> +be using. Walkers were built to be flexible, though, so it's entirely
>> reasonable
>> +(and expected) to create more specialized walkers (e.g. one that
>> specifically
>> +queries ``GlobalsAA``, one that always stops at ``MemoryPhi`` nodes,
>> etc).
>>
>>
>>  Locating clobbers yourself
>> @@ -201,22 +204,23 @@ Locating clobbers yourself
>>
>>  If you choose to make your own walker, you can find the clobber for a
>>  ``MemoryAccess`` by walking every ``MemoryDef`` that dominates said
>> -``MemoryAccess``. The structure of ``MemoryDef`` s makes this relatively
>> simple;
>> +``MemoryAccess``. The structure of ``MemoryDef``\ s makes this
>> relatively simple;
>>  they ultimately form a linked list of every clobber that dominates the
>>  ``MemoryAccess`` that you're trying to optimize. In other words, the
>>  ``definingAccess`` of a ``MemoryDef`` is always the nearest dominating
>>  ``MemoryDef`` or ``MemoryPhi`` of said ``MemoryDef``.
>>
>>
>> -Use optimization
>> -----------------
>> +Build-time use optimization
>> +---------------------------
>>
>> -``MemorySSA`` will optimize some ``MemoryAccess`` es at build-time.
>> +``MemorySSA`` will optimize some ``MemoryAccess``\ es at build-time.
>>  Specifically, we optimize the operand of every ``MemoryUse`` to point to
>> the
>>  actual clobber of said ``MemoryUse``. This can be seen in the above
>> example; the
>>  second ``MemoryUse`` in ``if.end`` has an operand of ``1``, which is a
>>  ``MemoryDef`` from the entry block.  This is done to make walking,
>>  value numbering, etc, faster and easier.
>> +
>>  It is not possible to optimize ``MemoryDef`` in the same way, as we
>>  restrict ``MemorySSA`` to one heap variable and, thus, one Phi node
>>  per block.
>> @@ -228,13 +232,13 @@ Invalidation and updating
>>  Because ``MemorySSA`` keeps track of LLVM IR, it needs to be updated
>> whenever
>>  the IR is updated. "Update", in this case, includes the addition,
>> deletion, and
>>  motion of ``Instructions``. The update API is being made on an as-needed
>> basis.
>> -If you'd like examples, ``GVNHoist`` is a user of ``MemorySSA`` s update
>> API.
>> +If you'd like examples, ``GVNHoist`` is a user of ``MemorySSA``\ s
>> update API.
>>
>>
>>  Phi placement
>>  ^^^^^^^^^^^^^
>>
>> -``MemorySSA`` only places ``MemoryPhi`` s where they're actually
>> +``MemorySSA`` only places ``MemoryPhi``\ s where they're actually
>>  needed. That is, it is a pruned SSA form, like LLVM's SSA form.  For
>>  example, consider:
>>
>> @@ -274,7 +278,7 @@ for ``if.end`` would be pointless, so we
>>  place a ``MemoryDef`` in ``if.then`` or ``if.else``, you'll need to also
>> create
>>  a ``MemoryPhi`` for ``if.end``.
>>
>> -If it turns out that this is a large burden, we can just place
>> ``MemoryPhi`` s
>> +If it turns out that this is a large burden, we can just place
>> ``MemoryPhi``\ s
>>  everywhere. Because we have Walkers that are capable of optimizing above
>> said
>>  phis, doing so shouldn't prohibit optimizations.
>>
>>
>>
>> _______________________________________________
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>> llvm-commits at lists.llvm.org
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>>
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