if-conversion/HOT-COLD partitioning

[Christophe Lyon]

Hi,

While debugging a GCC failure on ARM when compiling with
-fprofile-use, I came across a case where if_convert merges 2 blocks
which belong to different partitions (hot/cold).

In turn merge_blocks() (in cfghooks.c) merges the BB flags by ORing
them, resulting in a block belonging to both cold and hot partitions,
which seems to confuse the compiler: for instance in
bb-reorder.c:connect_traces(), the code expects only one partition
flag to be set.

I think merge_blocks() should be modified to handle such cases; I
experimented a little and forcing the merged BB's partition to be the
hot one in such a case is not sufficient: some edges have been marked
as crossing ones, and this would be no longer true. Is there a simple
way to clean that?

Thanks,

Christophe.