Fix nasty bug in reg-stack.c

[Eric Botcazou]

The bug was introduced 6 years ago but its occurrences apparently are quite 
rare since we detected it only very recently in our 4.3-based Ada compiler.

The end of convert_regs reads:

  inserted |= compensate_edges ();

  clear_aux_for_blocks ();

  fixup_abnormal_edges ();
  if (inserted)
    commit_edge_insertions ();

There is a nasty ordering problem in these lines.  fixup_abnormal_edges is 
intended to fix up the CFG by moving insns that were inserted past the last 
insn in a BB before compensate_edges is run onto the fallthrough edge.  And
compensate_edges also inserts insns onto (fallthrough) edges.  Now inserting 
insns on edges is a FIFO process so, if an insn was inserted past the last 
insn in a BB and a compensation insn is inserted afterward on the fallthrough 
edge, then they end up being swapped after the call to fixup_abnormal_edges.

A possible fix would be to use in the earlier phases the same mechanism used in 
compensate_edges: inserting insns on edges directly (except for the case of a 
single successor).  This seems inappropriate at this point of the lifetime of 
the reg-stack.c code.  The attached fix moves the call to fixup_abnormal_edges 
up to before the call to compensate_edges instead; this in turn requires some 
changes to fixup_abnormal_edges itself, mostly the uncoupling of the fixup and 
the finalization codes.

Tested on i586-suse-linux, applied on the mainline.


2011-03-26  Eric Botcazou  <ebotcazou@adacore.com>

	* basic-block.h (fixup_abnormal_edges): Adjust prototype.
	* reload1.c (reload): Adjust call to fixup_abnormal_edges.  Rediscover
	basic blocks and call commit_edge_insertions directly.
	(fixup_abnormal_edges): Move from here to...
	* cfgrtl.c (fixup_abnormal_edges): ...here.  Only insert instructions
	on the edges and return whether some have actually been inserted.
	* reg-stack.c (convert_regs): Fix up abnormal edges before inserting
	compensation code.


-- 
Eric Botcazou
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