CmmPipeline: add a second pass of CmmCommonBlockElim

The sinking pass often gets rid of unnecessary registers
registers/assignements exposing more opportunities for CBE, so this
commit adds a second round of CBE after the sinking pass and should
fix #12915 (and some examples in #14226).

Nofib results:
* Binary size:         0.9% reduction on average
* Compile allocations: 0.7% increase on average
* Runtime:             noisy, two separate runs of nofib showed a tiny
                       reduction on average, (~0.2-0.3%), but I think
                       this is mostly noise
* Compile time:        very noisy, but generally within +/- 0.5% (one
                       run faster, one slower)

One interesting part of this change is that running CBE invalidates
results of proc-point analysis. But instead of re-doing the whole
analysis, we can use the map that CBE creates for replacing/comparing
block labels (maps a redundant label to a useful one) to update the
results of proc-point analysis. This lowers the overhead compared to the
previous experiment in #12915.

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: ./validate

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #12915, #14226

Differential Revision: https://phabricator.haskell.org/D4417

1 files changed, 4 insertions, 4 deletions

diff --git a/compiler/cmm/CmmCommonBlockElim.hs b/compiler/cmm/CmmCommonBlockElim.hs
index c91d553c47..ad10511c1d 100644
--- a/compiler/cmm/CmmCommonBlockElim.hs
+++ b/compiler/cmm/CmmCommonBlockElim.hs
@@ -59,9 +59,10 @@ import Data.List (foldl')
 -- hashes, and at most once otherwise. Previously, we were slower, and people
 -- rightfully complained: #10397
 
--- TODO: Use optimization fuel
-elimCommonBlocks :: CmmGraph -> CmmGraph
-elimCommonBlocks g = replaceLabels env $ copyTicks env g
+type Subst = LabelMap BlockId
+
+elimCommonBlocks :: CmmGraph -> (CmmGraph, Subst)
+elimCommonBlocks g = (replaceLabels env $ copyTicks env g, env)
   where
      env = iterate mapEmpty blocks_with_key
      -- The order of blocks doesn't matter here, but revPostorder also drops any
@@ -73,7 +74,6 @@ elimCommonBlocks g = replaceLabels env $ copyTicks env g
 -- (so avoid comparing them again)
 type DistinctBlocks = [CmmBlock]
 type Key = [Label]
-type Subst = LabelMap BlockId
 
 -- The outer list groups by hash. We retain this grouping throughout.
 iterate :: Subst -> [[(Key, DistinctBlocks)]] -> Subst