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. Cheers! -Tyson