[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]

Wed Jan 27 04:14:43 PST 2016

----- Original Message -----

> From: "Larisse Voufo" <lvoufo at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Chandler Carruth" <chandlerc at gmail.com>,
> reviews+D15124+public+ddba332c127856cb at reviews.llvm.org,
> "llvm-commits" <llvm-commits at lists.llvm.org>
> Sent: Monday, January 25, 2016 5:47:42 PM
> 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]

> I have put together a short document to (hopefully) help keep this
> conversation concise:

> https://docs.google.com/document/d/1R-gINRdpxzLy82EZK_ymnVNyOPO-tzW5ksNcHuRvxBs/edit?usp=sharing

> Let's see what you think of it. Comments (on the doc) welcome.

> I'll reply to all the other comments (not addressed in the doc) below

> Thanks,
> -- Larisse.

> On Fri, Jan 15, 2016 at 9:55 AM, Larisse Voufo < lvoufo at gmail.com >
> wrote:

> > 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).
> > > > 
> > > 
> > 
> 
> Correct.
> > > > > Regarding invariant_end, we need to make sure that no paths
> > > > > from
> > > > > the
> > > > > invariant_start to the load pass through an invariant_end.
> > > > 
> > > 
> > 
> 
> This is a bit tricky (see attached doc).

> > > > > For this, it seems conservatively correct to check that the
> > > > > load
> > > > > dominates the invariant_end (or all invariant_ends, should
> > > > > there
> > > > > be
> > > > > multiple),
> > > > 
> > > 
> > 
> 
> This would not handle nested invariant regions consistently (see
> doc).
> > > > > 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.
> > > > 
> > > 
> > 
> 

> Post-dom is the only way to handle general cases consistently, and it
> does not cost anything more than using dominance analysis (if done
> right).
I agree that post-dominance is not more expensive than dominance if done right. It would be really nice to make that true for us in practice. However, we need to be careful here because this is not really a post-dominance problem. As I mentioned in the other reply, just using post-dominance here will cause us to miss the early-exit case. 

> > > > 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.
> > > 
> > 
> 
> If we are conservative about where we allow invariant ranges to be
> open (cf. doc), Neither load #1, nor load #2 should be considered
> invariant here. Load #1 is dominated by invariant_start, but not
> post-dominated by the invariant_end. Load#2, in turn is dominated by
> invariant_start, but also not post-dominated by invariant_end. There
> is no guarantee of consistent optimizations here (as inner blocks
> are merged into outer blocks etc...) So, we should not be optimizing
> these.

> If however, there were no invariant_end at all, then we can safely
> assume the range open and thus consider the loads invariant.
> > > > > Maybe we need a dedicated CFG walk to collect the necessary
> > > > > information?
> > > > 
> > > 
> > 
> 

> I'm curious. What would this do, and how cheaply, that we wouldn't do
> with postdom or the dom alternatives?
It would check the necessary condition directly. It would walk all paths from the load to the exit, stopping when finding blocks with invariant_exit, previously-visited blocks, exits, or when some search limit is reached. This is obviously not the ideal solution, solving the data-flow problem would be better. 

> > > > 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?
> > > 
> > 
> 
> I think a lot of this is already handled in InvariantInfo.

> > > 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
> > 
> 

> These inner invariant_start should have no effect on the outer load.
> Invariant_ends are defined in terms of invariant_starts, and
> Invariant_ranges do not extend beyond the scope on invariant_starts.
> So, I think this case is not relevant (at least for now).

> > > 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.
> > 
> 
> This sounds like is a case for nested invariant regions in the doc
> (?)...
> > > > 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...
> > > 
> > 
> 
> I am not exactly how this makes a difference, performance-wise. But
> then again, any sound implementation of queriedFromInvariantRange()
> would do...

> > > 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).
> > 
> 

> The invariant-range-analysis pass and other (InvariantInfo-related
> passes) are doing this.

> > > 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.
> > 
> 
> I don't see how this is possible. Invariant intrinsics are ignored by
> basic AA except for when invariant range analysis is computed.
You don't see how what is possible? The fact that the intrinsics are ignored by BasicAA is precisely why we'd want to PRE the intrinsics along with the loads. 

-Hal 

> > > -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
> > 
> 

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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