V4: Fix issues with very small loops5.8

Loops consisting of just a single basic block (e.g. a do-while loop with
no nested loops or if statements) have a back-edge to themselves. There
were 2 problems:
- loop detection would create LoopInfo for any loop header referred to
  by blocks inside the loop (and a 1 block loop doesn't have body
  blocks), nor would it mark the loop header as such
- when splitting critical edges, the newly inserted block would not be
  marked as part of the loop

This is a problem specifically for 1 block loops: the block ends with
a CJUMP, so the back-edge is a critical edge. So the new block inserted
by edge splitting wouldn't be marked as belonging to the loop.

The end result was that the life-time intervals for temporaries that
are defined before the loop, but that are used inside the loop, and not
after the loop, would have their life-time ended before the loop ends
(instead of spanning the whole loop, *including* the back-edge). This
in turns could lead to the stack/register allocator re-using the storage
for that temporary, resulting in strange things happening.

Task-number: QTBUG-59012
Change-Id: Ic946c73913711272efea2151cb85350412ca2fde
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

0 files changed, 0 insertions, 0 deletions