[PATCH] D15124: Use @llvm.invariant.start/end intrinsics to extend the meaning of basic AA's pointsToConstantMemory(), for GVN-based load elimination purposes [Local objects only]

Fri Jan 15 09:55:17 PST 2016

Disclaimer: It's a bit hard for me to follow the different "legs" in this
conversation. So I'm likely to miss some important background info. But
I'll try my best, starting with this one question below.

On Wed, Jan 13, 2016 at 3:17 AM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> ------------------------------
>
> *From: *"Chandler Carruth" <chandlerc at gmail.com>
> *To: *"Hal Finkel" <hfinkel at anl.gov>, "Chandler Carruth" <
> chandlerc at gmail.com>
> *Cc: *reviews+D15124+public+ddba332c127856cb at reviews.llvm.org, "Larisse
> Voufo" <lvoufo at gmail.com>, "llvm-commits" <llvm-commits at lists.llvm.org>
> *Sent: *Wednesday, January 13, 2016 4:27:13 AM
> *Subject: *Re: [PATCH] D15124: Use @llvm.invariant.start/end intrinsics
> to extend the meaning of basic AA's pointsToConstantMemory(), for GVN-based
> load elimination purposes [Local objects only]
>
> On Wed, Jan 13, 2016 at 2:00 AM Hal Finkel via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Actually, can you please remind me why we want post-dominance information
>> here at all?
>
>
> FWIW, I also would like to see a clear explanation of this -- I haven't
> yet.
>
> It all boils down to solving the following problem:

*Given all the tracked invariant_start and invariant_end calls in a given
InvariantInfo instance, implement the following function:*

  /// \brief Performs invariant range analysis for some given instruction,
  /// based on some allocated memory address:
  //  If the instruction accesses the given address, then this checks
whether
  /// there is an invariant range (over the address) that the instruction
  /// belongs in.
  /// NOTE: Range analysis must be enabled for this computation to go
through.
*  bool queriedFromInvariantRange(const Instruction *I, const Value *Addr);*

The test cases in the patch show different combinations of load
instructions and (nested) invariant ranges where some loads are merged
together while other can't.

When all instructions share the same block, that's easy. One can simply
check that:
* the invariant_start dominates the instruction and that
* the instruction dominates the invariant_end.

The real challenge is when one considers all instructions being nested in
complex ways, some of which have been highlighted here, and to try and
generalize the solution to all possible case scenari. Unless we limit the
cases that we care about, I find it hard to conceive a way of getting
around computing (once) and reusing both dominance and post-dominance info.
In practice, I do not see it getting all that expensive if we keep the
effects localized like they are on this patch.

>
>> For the invariant_start -> load, it seems like dominance information is
>> what we want (all paths to the load need to pass through the
>> invariant_start (or some invariant_start, should there be multiple ones for
>> the same pointer). Regarding invariant_end, we need to make sure that no
>> paths from the invariant_start to the load pass through an invariant_end.
>> For this, it seems conservatively correct to check that the load dominates
>> the invariant_end (or all invariant_ends, should there be multiple), and
>> that any containing loop has backedges that branch to blocks dominating the
>> invariant_start(s). We could use post-dom information for this check, but
>> that's just a different approximation.
>>
>
> Sure.
>
> I think that your statement is really hitting on the critical point here
> though: the issue is whether the invariant ends between the start and the
> load, nothing else. As you demonstrate, postdom actually gets the wrong
> answer:
>
>
>>  invariant_start
>>      /     \
>>      |    load #1
>>      |     |
>>      \    /
>>       \  /
>>        |
>>       load #2
>>      /   \
>>     /     \
>>    ret     \
>>            invariant_end
>>              \
>>               \
>>               ret
>>
>> So in this case (assuming my attempt at ASCII-art is successful), both
>> loads here can be considered to be invariant (both are dominated by the
>> invariant_start, and there are no paths from the invariant_start to either
>> load passing through an invariant_end). However, while it is true that load
>> #2 is post-dominated by the invariant_end w.r.t. one of the exits, it is
>> not w.r.t. all exits. load #1 here does not even post-dominate the
>> invariant_start, and that's fine. However, load #1 does not dominate the
>> invariant_end either.
>>
>
> This is a great example. Larisse, this should really be a good test case
> for this patch.
>
>
>>
>> Maybe we need a dedicated CFG walk to collect the necessary information?
>>
>
> Yea. We check for the same kinds of things in a number of other places
> without post dominators. It might be useful to mirror the logic on them.
>
> However, maybe there are easier ways. Thinking about this, for GVN in
> particular, I think I see really easy ways to make this fast and reasonably
> precise... Check my reasoning:
>
> 1) The start must dominate the load.
> 2) Therefore, if an end exists on a path from the start to the load, the
> start must dominate the end.
> 3) GVN never introduces ends or changes the dominance relationship of
> starts and ends.
> 4) We scan the function finding all of these intrinsics ahead of time
> already.
> 5) We can build a map from domtree node (or basic block essentially) to
> the list of invariant end intrinsics in that block.
> 6) We can walk the dom tree from the start to the load, and check each end
> in each block
>
> I feel like I'm missing a case... do you see the hole in this plan?
>
> This works if there's only one start. If there are multiple starts, then
> we need only the set to dominate, not any individual one.
>
> invariant_start         invariant_start
>           |                             |
>           |________________ |
>                          |
>                        load
>                          |
>                         ret
>
> At least in theory, this is fine too. The "right" way to solve this might
> be to set it up as a lattice/dataflow problem on the CFG, and then iterate
> until convergence (which should happen in two iterations, aside perhaps
> from pathological cases). The tricky part here, I suspect, is actually the
> representation (you can't actually build a large map covering (all pointers
> x all loads), and then prune it). Aside from that, you mark all live-in
> pointers to all blocks as invariant, except for the entry block, and at the
> instructions that define the pointer value, and then iterate the system,
> accounting for the invariant_start/end intrinsics, and you should find the
> maximal fixed point. The result would be a map of which pointers are
> invariant at which loads (which, as mentioned, you'd need to represent
> intelligently). Doing this might actually be faster than locally walking
> the CFG many times anyway.
>
> Here's another test case:
>
> invariant_start
> ____|
> |      |
> |   load
> |      |
> |  invariant_end
> |____|
>        |
>      ret
>
> Here, we can't treat the load as invariant, because there's a path from
> the invariant_start passing through the invariant_end via the loop backedge.
>
>
> It'll be a bit expensive because of the linear scan if there are a very
> large number of invariants. Maybe that's OK? Hard for me to tell...
>
> GVN already scans the entire (reachable) function body (and, by an order
> of magnitude, is dominated by the cost of AA queries). I doubt we'll
> notice. That having been said, we can avoid the linear scan if we use a
> registration approach for these intrinsics, just as we did with
> @llvm.assume (the AssumptionCache).
>
> Another issue worth considering:
>
>                        entry
>       ___________|_____
>       |                            |
> invariant_start            invariant_start
>       |                            |
>     load                        load
>       |                             |
>      ...                           ...
>
> Generally, we'd PRE this load (adding it to the entry block). Either the
> invariant_start will block this (which is bad), or we'll want to also PRE
> the invariant_starts to avoid PRE killing the invariant info.
>
>  -Hal
>
>
>
>>  -Hal
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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