[PATCH] D68816: [NFC] Replace a linked list in LiveDebugVariables pass with a DenseMap

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

Orlando created this revision.
Orlando added reviewers: aprantl, probinson, vsk, dblaikie.
Orlando added a project: debug-info.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
Orlando added a comment.

The numbers are much better than I expected but I haven't been able to prove
myself wrong after a bunch of testing. So, I've put this patch up for the
community's expert eyes.


Before going into details of the patch, I think it's important to point out
that it seems to reduces self-host build times of llvm on my Linux VM 
by up to 12%:

    ------------------------------------------------------------------

  Self-host build times with and without this patch
  ------------------------------------------------------------------
  With this patch applied | Build type           | CPU time (mins)
  ------------------------------------------------------------------
  No                      | RelWithDebInfo, asan | 226
  Yes                     | RelWithDebInfo, asan | 218 (-3.5%)
  No                      | RelWithDebInfo       | 183
  Yes                     | RelWithDebInfo       | 161 (-12%)

In LiveDebugValues.cpp:
Prior to this patch, UserValues were grouped into linked list chains. Each
chain was the union of two sets: { A: Matching Source variable } or 
{ B: Matching virtual register }. A ptr to the heads (or 'leaders')
of each of these chains were kept in a map with the { Source variable } used
as the key (set A predicate) and another with { Virtual register } as key
(set B predicate).

There was a search through the chains in the function getUserValue looking for
UserValues with matching { Source variable, Complex expression, Inlined-at
location }. Essentially searching for a subset of A through two interleaved
linked lists of set A and B. Importantly, by design, the subset will only
contain one or zero elements here. That is to say a UserValue can be uniquely
identified by the tuple { Source variable, Complex expression, Inlined-at
 location } if it exists.

This patch removes the linked list and instead uses a DenseMap to map
the tuple { Source variable, Complex expression, Inlined-at location }
to UserValue ptrs so that the getUserValue search predicate is this map key.
The virtual register map now maps a vreg to a SmallVector<UserVal *> so that
set B is still available for quick searches.


Repository:
  rL LLVM

https://reviews.llvm.org/D68816

Files:
  llvm/lib/CodeGen/LiveDebugVariables.cpp

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