Hello,
I am trying to reorder certain basic blocks again after
rest_of_handle_reorder_blocks() (which in turn calls
reorder_basic_blocks). What I do is this:
cfg_layout_initialize (flags);
reorder_selected_blocks(); // sets bb->rbi->next on them
/* Leave the rest as it was. */
FOR_EACH_BB (bb)
if ((bb->next_bb != EXIT_BLOCK_PTR) && (!bb->rbi->next))
bb->rbi->next = bb->next_bb;
cfg_layout_finalize();
If I do that before rest_of_handle_partition_blocks, it works
just fine. (btw. I ensure that all blocks I force to follow
one another belong to the same partition) But then some of
them get repositioned, so I want to run the above again
after rest_of_handle_reorder_blocks() is done.
If I do that I run into an infinite loop in fixup_reorder_chain()
in its first for loop at:
for (bb = ENTRY_BLOCK_PTR->next_bb, index = 0;
bb != 0;
bb = bb->rbi->next, index++)
{
if (bb->rbi->header)
{
if (insn)
NEXT_INSN (insn) = bb->rbi->header;
else
set_first_insn (bb->rbi->header);
PREV_INSN (bb->rbi->header) = insn;
insn = bb->rbi->header;
while (NEXT_INSN (insn)) <<<<<< HERE
insn = NEXT_INSN (insn);
}
....
I tried to debug it. The insn's mode seems to be always VOIDmode,
and the u value seems to be increasing with each next insn.
There's not much else I can say about it, besides that it gets
effectively into an infinite loop.
I managed to isolate a case where moving a single BB would trigger
this behaviour. (This is as far as I can tell by looking at the
original code and matching BB's to source code positions. How do I get
a dump of insns of a BB into some textual form? Is there a utility
function for that?)
Original order and code:
BB1: // equiv. of: if (a == 5) && (b == 10) goto BB4
if (a != 5) goto BB2:
if (b == 10) goto BB4:
BB2: a = 0
BB3: // whatever follows
....
[ far away but still in the same partition ]
....
BB4: a = b/c
goto BB3;
Requested order:
BB1: // equiv. of: if (a == 5) && (b == 10) goto BB4
if (a != 5) goto BB2:
if (b == 10) goto BB4:
BB2: a = b/c
goto BB3;
BB4: a = 0
BB3: // whatever follows
It doesn't seem to be too tricky a reordering, but it does give me this
infinite loop, if I do the reordering after
rest_of_handle_reorder_blocks().
What can be the reason of it? Might I be violating some unwritten(?)
rules of what moves can be applied to bbs? By looking at cfg_layout_
initialize/finalize it seemed to me that I could make an arbitrary
reordering and it will be followed, sometimes at the cost of added jumps
and labels. [*]
Any help will be greatly appreciated,
Grzegorz B. Prokopski
[*] For the moment I ignore the issues like the max allowed distance to
a cond. jump target.
