#5615: ghc produces poor code for `div` with constant powers of 2.
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Reporter: Lennart | Owner: daniel.is.fischer
Type: bug | Status: new
Priority: normal | Milestone: 7.6.1
Component: Compiler | Version: 7.2.1
Keywords: | Os: Unknown/Multiple
Architecture: x86 | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonmar):
Well, that's a good point. Integer right shift is equivalent to division
truncated to negative infinity (i.e. `div`), so the test on the numerator
would not be necessary. That's a good argument for having a built in
rule.
For `quot` some additional checks would be necessary in the rule. I just
checked what gcc does, and for `x / 2` it generates the clever sequence
{{{
movl %edi, %eax
shrl $31, %eax
addl %edi, %eax
sarl %eax
}}}
which is basically the same as `x < 0 ? (x+1) >> 1 : x >> 1`. Dividing by
4 instead gives this:
{{{
leal 3(%rdi), %eax
testl %edi, %edi
cmovns %edi, %eax
sarl $2, %eax
}}}
which is `x < 0 ? (x + 3) >> 2 : x >> 2`.
We could do the same tricks with a built-in rule for `quot`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5615#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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