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Re: [PATCH] Jump bypassing and improved cprop (take 2)

On Sun, Jun 02, 2002 at 11:01:10PM +0200, Jan Hubicka wrote:
> > Could you explain why the additional clobbers aren't valid when
> > copied to the redirected edge?  I must admit that this aspect
> > of the changes to my original patch is almost beyond me.  I'd
> > have thought that even if the values are no longer clobbered,
> > its still safe to claim that they are.
> The clobbers may kill value otherwise live on the edge, so change
> semantics of code in a way you don't want to.
> This is partly cared by my hoist_insn* bits that checks for such
> side effects and verifies wehther the move is valid.

No, actually he has a point, which I hadn't considered until now.

The original code is like

	  if (p) jump b


	  t = r op c
	  if (t) jump c


       / \ /
	 / \

The point of the excercise is to redirect the A->B edge to A->C
based on knowledge of R at the end of A.  Thus the revised flow is

       /|  /
	| B
	|/ \

Except that since we don't know the true lifetime of T, we have
to copy that operation onto the A->C edge.  Most of the time T is
actually dead, and thus the copied operation can be deleted later.
Otherwise the transformation isn't much of a win since we are in
effect duplicating block B.  I think that just about all of these
cases can be fixed by a later cross-jump pass, so I wouldn't
complicate the code here by worrying about it.

The point of contention just now is whether or not it is valid to
move the clobber that might be associated with OP onto that edge.

The answer is yes.  The reason is that the original control flow
*always* passed through that instruction via the A->B->C path.
Thus whatever value might be clobbered by OP, it for certain was
not previously live on entry to C.


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