On Mon, Aug 31, 2015 at 6:51 AM, shmeel gutl
<shmeelgutl@shmuelhome.mine.nu> wrote:
When dealing with an array with known values, pre will evaluate the first
iteration of a loop over the elements. The code generator with then jump
into the loop. This is at best increasing the size of the code. It also
creates inferior code when the hardware supports zero overhead loops. The
attached code demonstrates the difference between an unknown array and a
known array. The loop size has been picked large enough for cunrolli to not
fully unroll the loop. The problem did not exist in gcc 4.8.
I think you were just lucky with GCC 4.8 - the issue is present since forever.
Basically it's because we treat a constant as available. So PRE might end
up rotating the loop, inserting the 2nd iteration on the latch edge.
Unfortunately this transform sometimes improves code-gen, it would be
quite simple to disallow this kind of transform generally though.
Richard.