Problems using cfg_layout_finalize()

[James E Wilson]

Grzegorz B. Prokopski wrote:
> If I do that I run into an infinite loop in fixup_reorder_chain()
> in its first for loop at:
> 	  while (NEXT_INSN (insn))  <<<<<< HERE
> 	    insn = NEXT_INSN (insn);

It looks like there is an undocumented assumption about insn-chains and 
the rbi->header and rbi->footer fields.  Normally, all of the insns for 
a function are part of a single linked list.  Blocks are numbered 
sequentially, so it is always true that the last insn of block N is 
followed by the first insn of block N+1.  If blocks N and N+1 are 
separated, then you are supposed to split the insn chain, and set 
rbi->footer for block N, and rbi->header for block N+1.  These 
header/footer fields are used to reconnect the insn chain when the block 
movement is done.  That is from a quick look, and may not be entirely 
accurate.  I haven't done extensive work with this code.

> 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?)

There are macros in the .gdbinit file that is automatically loaded by 
gdb.  You can use pr to print a single instruction.  E.g.
(gdb) print insn
(gdb) pr
You can use prl to print a list of N instructions.  E.g.
(gdb) print debug_rtx_count = 1000
(gdb) print get_insns ()
(gdb) prl
will print 1000 instructions from the beginning of the function.  This 
assumes that the insn-chain is intact, which isn't true during block 
reorg.  You probably want to use something like BB_HEAD here.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com