#4065: Inconsistent loop performance
---------------------------------+------------------------------------------
Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.14.1
Component: Compiler | Version: 6.13
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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Changes (by simonmar):
* milestone: => 6.14.1
Comment:
The problem is this:
{{{
Test.$wbar_entry()
{ []
}
cmr:
_slB::I64 = R2;
if (_slB::I64 != 0) goto cmv;
R1 = R3;
jump (I64[I64[Sp]]) ();
cmv:
R2 = _slB::I64 - 1;
R3 = R3 + 1;
jump Test.$wbar_entry ();
}
}}}
The assignment of `_slB` should really be inlined (or forward-propagated,
if you like). The current code generator isn't clever enough to do this
transformation when the use of the variable is in a different basic block
from its assignment, and it's not an easy extension to make. This very
example appears in a comment in `CmmOpt.hs`, in fact.
The new code generator will be able to do this using the dataflow
framework. I'm sure LLVM will do it too (and more). We can leave the
ticket open on the 6.14 branch to make sure the new backend fixes it.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4065#comment:1>
GHC <http://www.haskell.org/ghc/>
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