[PATCH] D28593: Update loop branch_weight metadata after loop rotation.

[Xin Tong via Phabricator via llvm-commits]

trentxintong added a comment.

After we rotate the loop, we duplicate the comparison the old header into the guard block and we move the header to the end of the loop. So basically we have 2 branches that carry the branch_weight metadata they need to be adjusted.  With this patch, we only do the adjustment for loop with only 1 exit block, if the loop has early exits, its harder to adjust the branch weight properly. e.g. if we have an early exit in the loop, we will not be able to tell whether the latch block will ever be reached after rotation, not to mention to adjust its branch weight, (we need to look at the branch weights of the early exits to do this properly).

There are 2 conditions that are constant after rotation, (1) # of times the loop body is executed and (2) the # of times the exit block is executed.  (these are the values we can get by extracting the prof metadata from the header branch before rotation). The backedge weight should simply be the # of times the loop body is executed - # of times the loop exit is executed. With the loop exit count, loop body weight and loop backedge weight, we can compute the branch weight for the guard block.

This is true iff the loop executes at least once every time. If the loop body execute seldom, they we can not do it (and we will get a negative backedge weight num with # of times the loop body is executed - # of times the loop exit is executed.) In this case, we cap the backedge # to 1 (should be 0). And with this information we can compute loop exit weight and all the other weights.

Thats roughly what the patch is doing and how the numbers in the test case is computed.


https://reviews.llvm.org/D28593