#1498: Optimisation: eliminate unnecessary heap check in recursive function
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Reporter: simonmar | Owner:
Type: bug | Status: new
Priority: low | Milestone: 7.6.1
Component: Compiler | Version: 6.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Runtime performance
bug
Difficulty: Moderate (less than a day) | Testcase:
Blockedby: 4258 | Blocking:
Related: |
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Comment(by rl):
Very interesting (even if a bit disappointing). I'm somewhat suspicious of
your condition, though:
cases with primop scrutinees which have at least 2 alts *and* are which
will not take a heap check before scrutinee evaluation
The tight loops that LLVM can optimise well all look like this:
{{{
f x = case ... of
p1 -> ... f y ... -- recursive, doesn't allocate
...
pm -> ... f z ... -- recursive, doesn't allocate
q1 -> ... -- non-recursive, might allocate
...
qn -> ... -- non-recursive, might allocate
}}}
So we have some alternatives that don't allocate and perform recursive
tailcalls (the tight loop) and then a number of loop exits which might
allocate. It seems that pushing the heap check into the exits should
always be a win here. This seems quite different from the condition you
used so there might still be an easy fix for these cases.
In any case, thanks for looking into this!
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1498#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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