Simon

I reverted your commit

https://phabricator.haskell.org/rGHC546f24e4f8a7c086b1e5afcdda624176610cbcf8

because it made every single binary that GHC builds seg-fault on windows.  Even

                main = return ()

seg-faults.

You may want to review and re-commit it.

Simon

commit 24864ba5587c1a0447beabae90529e8bb4fa117a

Author: Simon Marlow <marlowsd@gmail.com>

Date:   Sat Apr 23 22:14:43 2016 +0100

    Use __builtin_clz() to implement log_2()

        A microoptimisation in the block allocator.