performance issues in simple arithmetic code
twhitehead at gmail.com
Thu Apr 28 06:30:11 CEST 2011
On April 27, 2011 23:01:50 Denys Rtveliashvili wrote:
> Question 1: what is the meaning of those magic numbers
> -9223372036854775808, -6677706553, -7418931981405, -8257271295304186?
> Question 2: under which circumstances those strange branches of
> execution will be used and what those results would mean?
> Question 3: why is the Core for 'foo' so different to 'bar'?
The largest representable 64bit negative number in twos complement is
-9223372036854775808. It doesn't have a positive counter part as 0 eats one
out of the range of positive numbers.
The code for "quot x y" therefore checks for the special case "x==minBound"
and "y==-1" in order to be able to thrown an overFlowError. I suspect what
you are seeing is a specialization for this branch.
That is, the start of the overflow case check introduced a branch for
x==-9223372036854775808. The compiler then notices that it know both x and y
here, so it just puts in the answer, resulting in a permanent special case.
Your foo and bar code is so different as direct use of the quotInt# primitive
avoids this check and hence the resulting specialization for it.
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