[llvm-dev] [RFC] NewGVN

Wed Nov 16 02:03:20 PST 2016

This is really great to see, as I’ve spent far too much of my life over the past two years fighting with undocumented assumptions made by GVN.  A couple of quick questions about the new GVN, based on problems I’ve had with the old one:

Does it assume that it’s always safe to widen a load (or store) to a power of two?  For our target, this is only sound if you can show that the pointer was used to read all of the bytes that you are loading (we have byte-granularity memory safety).  Old GVN has no hooks for targets to specify whether this is safe and so is implicitly assuming a page-based MMU.  This optimisation is also unsound for M-profile ARM cores, though will fail occasionally there, whereas it fails deterministically for us.

Does it make any assumptions about the layout of memory in pointers?  Old GVN treats pointers as integers and assumes that it’s safe to do partial stores to them.  As a prerequisite for memory safety, we must be able to guarantee atomic updates to pointers and we had to hack GVN to disable a bunch of these things.  In LLVM IR, pointers are opaque and there is no guarantee that their representation is the same as a same-sized integer, but old GVN makes this assumption.

David

> On 16 Nov 2016, at 06:49, Davide Italiano via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> we would like to propose  a new Global Value Numbering pass in LLVM.
> The ideas/code are from Daniel Berlin (with a minor overhaul/splitting
> into submittable patches from me). The code has been around for a
> while (2012 or before), and we think it's getting ready to be
> committed upstream.
> 
> ### Motivation
> 
> To put things into context: my personal motivation for having a new
> GVN/PRE algorithm is LTO.
> It's not a secret that LLVM is getting slower and slower release after
> release, as Rafael discovered/pointed out in March [1] (and probably
> many others found out). I personally took a shot at profiling LTO on
> many internal/opensource applications (including clang itself) and
> noticed that GVN always show in the top-3 passes (and it's generally
> the pass where we spend most of the time in the middle-end). There are
> cases (extreme) where 90% of the compile time goes in GVN.
> 
> Example:
> ===-------------------------------------------------------------------------===
>                      ... Pass execution timing report ...
> ===-------------------------------------------------------------------------===
>  Total Execution Time: 684.7622 seconds (683.7141 wall clock)
> 
>   ---User Time---   --System Time--   --User+System--   ---Wall
> Time---  --- Name ---
>  130.2558 ( 20.0%)   6.3128 ( 18.7%)  136.5686 ( 19.9%)  137.6145 (
> 20.1%)  X86 DAG->DAG Instruction Selection
>  55.4695 (  8.5%)   0.5501 (  1.6%)  56.0196 (  8.2%)  56.1049 (
> 8.2%)  Function Integration/Inlining
>  42.3492 (  6.5%)   0.0364 (  0.1%)  42.3856 (  6.2%)  42.8676 (
> 6.3%)  Global Value Numbering
> 
> ### Problems in the current GVN
> 
> There are some issues in the current GVN infrastructure. A
> non-exhaustive list (I'm pretty sure people have their list of
> complains, these are the ones I care about the most).
> * GVN is very slow for large test cases
> * GVN doesn't do a real analysis, instead it eliminates as it goes, so
> it's hard to reason about it.
> * GVN doesn't perform any phi predication, i.e. doesn't know about phi
> nodes, so later passes have to do some extra work to clean up
> * There are bugs, e.g. [2] which would require a rewrite of PRE to be fixed.
> 
> ### NewGVN
> 
> The new algorithm implements the ideas described in [3] with some
> engineering optimizations by Dan (for example the set of touched
> instructions is represented using a Bitvector instead of a set because
> it's not uncommon for large functions where a predicate change that
> thousands of instructions need to be changed, and we both measured 30%
> of the whole pass time spent just modifying the set).
> The code pretty much maps what the paper describe so I won't try to
> repeat it here =)
> 
> Some advantages of NewGVN:
> * GVN performs a real analysis (which is now part of the pass itself
> but potentially could be split and used as an utility by other
> passes). For example, Quentin/Dan pointed out that outlining at the IR
> level is hard without a proper value numbering analysis.
> * On large testcases, It's faster than current GVN, even though didn't
> go through a lot of profiling/optimization lately (I found it up to
> 50% faster in compile time on some internal benchmarks). There are
> some places were we can improve. For example, we spend a lot of time
> inside Simplify* functions. Another considerable chunk of the time is
> spent inside MemorySSA, but this is going to change once we preserve
> MemSSA more aggressively.
> * The code is easier to understand (at least to me)
> [...]
> 
> Some current limitations of NewGVN:
> * NewGVN doesn't do everything that the current GVN does. There are
> plans to implement missing features and more.
> * On small testcases NewGVN may be slower as it does some work upfront
> that the current GVN doesn't do. NewGVN is probably less lazy than it
> should be, but it didn't matter for us so we didn't consider it a
> priority. If somebody cares and finds a case where NewGVN
> substantially regresses, speak up.
> 
> The initial code can be found here https://reviews.llvm.org/D26224
> The current patch includes only the "core" GVN algorithm, i.e. the
> expression framework/the logic to perform congruence finding. Other
> pieces (e.g. PRE will build on top of that).
> 
> My rough plan is:
> * Try to get this reviewed/tested and in tree.
> * Fix miscompiles/bugs (and performance problems as/if they arise).
> * Build other pieces of the algorithm on top of what we have. My
> immediate concern will be implementing support for llvm.assume and
> load coercion. Dan said he will try to find the time to work on the
> predicate handling (actually, there's already a branch
> https://github.com/dberlin/llvm-gvn-rewrite/tree/newgvn-predicates).
> 
> Please let us know what you think. Any feedback/review comment/testing
> is very appreciated!
> 
> Thanks!
> 
> [1] http://lists.llvm.org/pipermail/llvm-dev/2016-March/096488.html
> [2] https://llvm.org/bugs/show_bug.cgi?id=30692#c11
> [3] http://dl.acm.org/citation.cfm?id=512536
> 
> -- 
> Davide
> 
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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