Commit d5c4d46a authored by Michal Terepeta's avatar Michal Terepeta Committed by Ben Gamari

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's avatarMichal Terepeta <>

Test Plan: ./validate

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #12915, #14226

Differential Revision:
parent 8cfd2e4e
......@@ -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)
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
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE TypeFamilies #-}
module CmmPipeline (
-- | Converts C-- with an implicit stack and native C-- calls into
......@@ -28,6 +29,8 @@ import Control.Monad
import Outputable
import Platform
import Data.Maybe
-- | Top level driver for C-- pipeline
......@@ -67,9 +70,9 @@ cpsTop hsc_env proc =
, do_layout = do_layout }} = h
----------- Eliminate common blocks -------------------------------------
g <- {-# SCC "elimCommonBlocks" #-}
condPass Opt_CmmElimCommonBlocks elimCommonBlocks g
Opt_D_dump_cmm_cbe "Post common block elimination"
(g, _) <- {-# SCC "elimCommonBlocks" #-}
condPass2 Opt_CmmElimCommonBlocks elimCommonBlocks g mapEmpty
Opt_D_dump_cmm_cbe "Post common block elimination"
-- Any work storing block Labels must be performed _after_
-- elimCommonBlocks
......@@ -104,6 +107,32 @@ cpsTop hsc_env proc =
condPass Opt_CmmSink (cmmSink dflags) g
Opt_D_dump_cmm_sink "Sink assignments"
(g, call_pps, proc_points) <- do
-- Only do the second CBE if we did the sinking pass. Otherwise,
-- it's unlikely we'll have any new opportunities to find redundant
-- blocks.
if not (gopt Opt_CmmSink dflags)
then pure (g, call_pps, proc_points)
else do
(g, cbe_subst) <- {-# SCC "elimCommonBlocks2" #-}
Opt_CmmElimCommonBlocks elimCommonBlocks g mapEmpty
Opt_D_dump_cmm_cbe "Post common block elimination 2"
-- CBE might invalidate the results of proc-point analysis (by
-- removing labels). So we need to fix it. Instead of re-doing
-- the whole analysis, we use the final substitution env from
-- CBE to update existing results.
let cbe_fix set bid =
setInsert (fromMaybe bid (mapLookup bid cbe_subst)) set
let !new_call_pps = setFoldl cbe_fix setEmpty call_pps
let !new_proc_points
| splitting_proc_points =
setFoldl cbe_fix setEmpty proc_points
| otherwise = new_call_pps
return (g, new_call_pps, new_proc_points)
------------- CAF analysis ----------------------------------------------
let cafEnv = {-# SCC "cafAnal" #-} cafAnal g
dumpWith dflags Opt_D_dump_cmm_caf "CAFEnv" (ppr cafEnv)
......@@ -155,6 +184,13 @@ cpsTop hsc_env proc =
return g
else return g
condPass2 flag pass g a dumpflag dumpname =
if gopt flag dflags
then do
(g, a) <- return $ pass g
dump dumpflag dumpname g
return (g, a)
else return (g, a)
-- we don't need to split proc points for the NCG, unless
-- tablesNextToCode is off. The latter is because we have no
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