Re: Fix PR 53743 and other -freorder-blocks-and-partition failures (issue6823047)

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

On Sat, May 11, 2013 at 4:38 PM, Teresa Johnson <tejohnson@google.com> wrote:
>   /* If we are partitioning hot/cold basic blocks, we don't want to
>      mess up unconditional or indirect jumps that cross between hot
>      and cold sections.
>
>      Basic block partitioning may result in some jumps that appear to
>      be optimizable (or blocks that appear to be mergeable), but which really
>      must be left untouched (they are required to make it safely across
>      partition boundaries).  See the comments at the top of
>      bb-reorder.c:partition_hot_cold_basic_blocks for complete details.  */
>
> And at least a bunch of these are when we are in cfglayout mode.
>
> But let me locate a reproducer so we can make sure it isn't due to
> some issue with my patch.

It sounds like the issue here is that the partitioning code insists on
having an explicit jump for a section switch even for single_succ_p
blocks i.e. unconditional jump, while normally in cfglayout mode there
are no unconditional jumps.

If that is the problem, then the proper solution is to not have the
explicit jump. Just forward the jump, set the EDGE_CROSSING flag on
the crossing edge, and add a REG_CROSSING_JUMP note on the branching
insn. (FWIW, these REG_CROSSING_JUMP notes are also an aberration --
we have the CFG edges with all the information in them! But oh
well...) If the edge cannot fall through when going out of cfglayout
mode after bbro (and crossing edges can't fall through) then an extra
"forwarder jump" block will be inserted automatically. This may
require some hacking to invert branch conditions for branching insns
such that the branch goes to the other section and that the fallthru
path stays in the same section, but that's something that should be
relatively easy to do in cfgcleanup.

Ciao!
Steven