#3065: Reorder tests in quot to improve code
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Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: libraries/base | Version: 6.10.1
Resolution: | Keywords:
Difficulty: Unknown | Os: Unknown/Multiple
Testcase: | Architecture: Unknown/Multiple
Failure: Runtime performance bug |
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Comment (by rl):
Doh. Your program prints -2147483648 with -O2 (which is what I tried) but
that's only because of gcc's constant folding. You are right that it traps
if it doesn't see the constants. I thought it handled this differently,
sorry for the confusion.
However, that doesn't really change my opinion. If x86 doesn't have a
64-bit div instruction, then I think that having the NCG insert the
necessary test when generating code for `quotInt#` or a calling a 64-bit
div subroutine would result in much better code in the vast majority of
cases. Exposing the test to GHC really tends to break loop performance. It
also imposes a performance penalty on signed types smaller than `Int`
which would work just fine without the test.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3065#comment:8>
GHC <http://www.haskell.org/ghc/>
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