#10626: Missed opportunity for SpecConstr
-------------------------------------+-------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.10.1
Keywords: | Operating System: Unknown/Multiple
Architecture: | Type of failure: None/Unknown
Unknown/Multiple | Blocked By:
Test Case: | Related Tickets:
Blocking: |
Differential Revisions: |
-------------------------------------+-------------------------------------
Look at `perf/should_run/T4830`. After `SpecConstr` and optimisation we
finally get
{{{
Rec {
Main.foo_$s$wfoo1 [Occ=LoopBreaker]
:: Int# -> Double -> Double -> Double#
[GblId, Arity=3, Caf=NoCafRefs, Str=DmdType ]
Main.foo_$s$wfoo1 =
\ (sc_s4eN :: Int#) (sc1_s4eO :: Double) (sc2_s4eP :: Double) ->
case sc_s4eN of ds_X1SR {
__DEFAULT ->
case sc1_s4eO of wild_a1Tf { D# x_a1Th ->
case sc2_s4eP of wild1_a1Tj { D# y_a1Tl ->
case tagToEnum# @ Bool (<=## x_a1Th y_a1Tl) of _ [Occ=Dead] {
False -> Main.foo_$s$wfoo1 (-# ds_X1SR 1#) wild1_a1Tj wild_a1Tf;
True -> Main.foo_$s$wfoo1 (-# ds_X1SR 1#) wild_a1Tf wild1_a1Tj
}
}
};
0# ->
case sc1_s4eO of _ [Occ=Dead] { D# x_a1UL ->
case sc2_s4eP of _ [Occ=Dead] { D# y_a1UP -> +## x_a1UL y_a1UP }
}
}
end Rec }
}}}
If we ran `SpecConstr` again we'd specialise this function, because the
recursive calls both have boxed arguments.
I looked into why `SpecConstr` didn't catch it, and it's because
`SpecConstr`'s input looks like this
{{{
case case tagToEnum# @ Bool (<=## x_a1Th y_a1Tl) of _
[Occ=Dead] {
False -> (wild1_a1Tj, wild_a1Tf);
True -> wild_X8
}
of r_s4e0 { (ipv_s4e1, ipv_s4e2) ->
$wfoo_s4db (I# (-# ds_X1SR 1#)) (Just @ (Double, Double)
r_s4e0)
}}}
Notice the case-of-case which doesn't expose the `Double` boxes of
arguments to `$wfoo`.
Why is that case-of-case still there? Because of `Note [Single-
alternative cases]` in Simplify. Which is clearly a delicate spot so I
don't want to meddle with it today. But it's intriguing. Maybe Sequent
Core will do better.
Note that late demand analysis also catches this case, yielding the
(rather good)
{{{
Rec {
Main.$w$s$wfoo [InlPrag=[0], Occ=LoopBreaker]
:: Int# -> Double# -> Double# -> Double#
[GblId, Arity=3, Caf=NoCafRefs, Str=DmdType ]
Main.$w$s$wfoo =
\ (w_s4gW :: Int#) (ww_s4h1 :: Double#) (ww1_s4h5 :: Double#) ->
case w_s4gW of ds_X1SW {
__DEFAULT ->
case tagToEnum# @ Bool (<=## ww_s4h1 ww1_s4h5) of _ [Occ=Dead] {
False -> Main.$w$s$wfoo (-# ds_X1SW 1#) ww1_s4h5 ww_s4h1;
True -> Main.$w$s$wfoo (-# ds_X1SW 1#) ww_s4h1 ww1_s4h5
};
0# -> +## ww_s4h1 ww1_s4h5
}
end Rec }
}}}
I'm pretty sure that late-demand-analysis should be on with `-O2` but
someone should do a nofib run to check.
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10626
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
