Skip to content
Snippets Groups Projects
Commit 8ff32124 authored by Sebastian Graf's avatar Sebastian Graf Committed by Marge Bot
Browse files

DmdAnal: Don't unbox recursive data types (#11545)

As `Note [Demand analysis for recursive data constructors]` describes, we now
refrain from unboxing recursive data type arguments, for two reasons:

 1. Relating to run/alloc perf: Similar to
    `Note [CPR for recursive data constructors]`, it seldomly improves run/alloc
    performance if we just unbox a finite number of layers of a potentially huge
    data structure.
 2. Relating to ghc/alloc perf: Inductive definitions on single-product
    recursive data types like the one in T11545 will (diverge, and) have very
    deep demand signatures before any other abortion mechanism in Demand
    analysis is triggered. That leads to great and unnecessary churn on Demand
    analysis when ultimately we will never make use of any nested strictness
    information anyway.

Conclusion: Discard nested demand and boxity information on such recursive types
with the help of `Note [Detecting recursive data constructors]`.

I also implemented `GHC.Types.Unique.MemoFun.memoiseUniqueFun` in order to avoid
the overhead of repeated calls to `GHC.Core.Opt.WorkWrap.Utils.isRecDataCon`.
It's nice and simple and guards against some smaller regressions in T9233 and
T16577.

ghc/alloc performance-wise, this patch is a very clear win:

                               Test    Metric          value      New value Change
---------------------------------------------------------------------------------------
                LargeRecord(normal) ghc/alloc  6,141,071,720  6,099,871,216  -0.7%
MultiLayerModulesTH_OneShot(normal) ghc/alloc  2,740,973,040  2,705,146,640  -1.3%
                     T11545(normal) ghc/alloc    945,475,492     85,768,928 -90.9% GOOD
                     T13056(optasm) ghc/alloc    370,245,880    326,980,632 -11.7% GOOD
                     T18304(normal) ghc/alloc     90,933,944     76,998,064 -15.3% GOOD
                     T9872a(normal) ghc/alloc  1,800,576,840  1,792,348,760  -0.5%
                     T9872b(normal) ghc/alloc  2,086,492,432  2,073,991,848  -0.6%
                     T9872c(normal) ghc/alloc  1,750,491,240  1,737,797,832  -0.7%
       TcPlugin_RewritePerf(normal) ghc/alloc  2,286,813,400  2,270,957,896  -0.7%

                          geo. mean                                          -2.9%

No noteworthy change in run/alloc either.

NoFib results show slight wins, too:

--------------------------------------------------------------------------------
        Program         Allocs    Instrs
--------------------------------------------------------------------------------
    constraints          -1.9%     -1.4%
          fasta          -3.6%     -2.7%
reverse-complem          -0.3%     -0.9%
       treejoin          -0.0%     -0.3%
--------------------------------------------------------------------------------
            Min          -3.6%     -2.7%
            Max          +0.1%     +0.1%
 Geometric Mean          -0.1%     -0.1%

Metric Decrease:
    T11545
    T13056
    T18304
parent 8eadea67
No related merge requests found
Loading
0% Loading or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment