[llvm-dev] [RFC] NewGVN

[Davide Italiano via llvm-dev]

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