GHC wants to box up strict values when returning from tight inner loops, even when they're immediately taken apart. This leads to redundant instructions in the bodies of tight loops, and more code.

It affects, in particular, loops that result from fusion, which need to be tight, but often return multiple values via unlifted pairs.

Consider this program:

{-# OPTIONS -fexcess-precision #-}
{-# LANGUAGE TypeOperators #-}

import System.Environment
import Text.Printf
import Data.Array.Vector

mean :: UArr Double -> Double
mean arr = s / fromIntegral l
  where
    s :*: l = foldlU k (0 :*: 0) arr :: (Double :*: Int)
    k (s :*: n) x = s+x :*: n+1

main = do
    [d] <- map read `fmap` getArgs
    printf "%f\n" (mean (enumFromToFracU 1 d))

It generates this rather good Core (ghc 6.8.2):

$s$wfold_s1rB :: Double#
               -> Int#
               -> Double#
               -> (# Double, Int #)

$s$wfold_s1rB =
\ (sc_s1rr :: Double#)
  (sc1_s1rs :: Int#)
  (sc2_s1rt :: Double#) ->
  case >## sc_s1rr y_a1pr of wild4_X1no {
    False ->
      $s$wfold_s1rB
        (+## sc_s1rr 1.0)
        (+# sc1_s1rs 1)
        (+## sc2_s1rt sc_s1rr);
    True -> (# D# sc2_s1rt, I# sc1_s1rs #)
  };
} in 
case $s$wfold_s1rB 2.0 1 1.0 of ww_s1qg { (# ww1_s1qi, ww2_s1qj #) ->
case ww1_s1qi of wild4_a1mC { D# x_a1mE ->
case ww2_s1qj of wild5_aP6 { I# x1_aP8 ->
case /## x_a1mE (int2Double# x1_aP8)
of wild21_a1mK { __DEFAULT ->
D# wild21_a1mK

But note, what's this?

    True -> (# D# sc2_s1rt, I# sc1_s1rs #)
  };
} in 
case $s$wfold_s1rB 2.0 1 1.0 of ww_s1qg { (# ww1_s1qi, ww2_s1qj #) ->
case ww1_s1qi of wild4_a1mC { D# x_a1mE ->
case ww2_s1qj of wild5_aP6 { I# x1_aP8 ->
case /## x_a1mE (int2Double# x1_aP8)

The return values of what was a strict pair are boxed, placed in an unboxed tuple, and then immediately unboxed and the division takes place.

Ok, let's isolate this. Here, the boxed return, from the inner loop:

mean_s19V :: Double#
           -> Int#
           -> Double#
           -> (# Double, Int #)

mean_s19V =
\ (ds1_dD3 :: Double#)
  (ds2_dD4 :: Int#)
  (ds3_dD5 :: Double#) ->
  case >## ds1_dD3 d#_aoG of wild4_Xw {
    False ->
      mean_s19V
        (+## ds1_dD3 1.0)
        (+# ds2_dD4 1)
        (+## ds3_dD5 ds1_dD3);
    True -> (# D# ds3_dD5, I# ds2_dD4 #)
  };
} in 
case mean_s19V 2.0 1 1.0 of wild4_Xr { (# ds1_dCV, ds2_dCW #) ->
case ds1_dCV of wild5_Xv { D# x_aoR ->
case ds2_dCW of wild6_Xy { I# y_aoS ->
case /## x_aoR (int2Double# y_aoS) of wild7_XB { __DEFAULT ->
D# wild7_XB

And the inner loop and exit:

s1bd_info:

  -- what's this stuff?
  leaq        32(%r12), %rax
  cmpq        %r15, %rax
  movq        %rax, %r12
  ja  .L17

  -- ok, to business:
  ucomisd     5(%rbx), %xmm5
  ja  .L19
  movapd      %xmm6, %xmm0
  leaq        -32(%rax), %r12
  incq        %rsi
  addsd       %xmm5, %xmm0
  addsd       .LC1(%rip), %xmm5
  movapd      %xmm0, %xmm6
  jmp s1bd_info


.L19:
  movq        %rsi, -16(%rax)
  movq        $base_GHCziBase_Izh_con_info, -24(%rax)
  movq        $base_GHCziFloat_Dzh_con_info, -8(%rax)
  movsd       %xmm6, (%rax)
  leaq        -7(%rax), %rbx
  leaq        -23(%rax), %rsi
  jmp *(%rbp)

Now, I can avoid the reboxing manually:

mean_s19R :: Double#
           -> Int#
           -> Double#
           -> (# Double#, Int# #)

mean_s19R =
\ (ds1_dCZ :: Double#)
  (ds2_dD0 :: Int#)
  (ds3_dD1 :: Double#) ->
  case >## ds1_dCZ d#_aoG of wild4_Xw {
    False ->
      mean_s19R
        (+## ds1_dCZ 1.0)
        (+# ds2_dD0 1)
        (+## ds3_dD1 ds1_dCZ);
    True -> (# ds3_dD1, ds2_dD0 #)
  };
} in 
case mean_s19R 2.0 1 1.0 of wild4_Xr { (# x_aoR, y_aoS #) ->
case /## x_aoR (int2Double# y_aoS) of wild5_Xv { __DEFAULT ->
D# wild5_Xv

And we get:

s1b9_info:
  -- hey , our junk is gone!

  ucomisd     5(%rbx), %xmm5
  ja  .L17
  movapd      %xmm6, %xmm0
  incq        %rsi
  addsd       %xmm5, %xmm0
  addsd       .LC1(%rip), %xmm5
  movapd      %xmm0, %xmm6
  jmp s1b9_info

-- cool, that was it, let's go home:
.L17:
  movapd      %xmm6, %xmm5
  movq        %rsi, %rbx
  jmp *(%rbp)

Which is a much better result. The loop is tighter.

What can be done here?

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	dons@galois.com
Operating system	Unknown
Architecture	Unknown