Re: Flow crash for conditional branches against a constant

[Daniel Jacobowitz <drow at false dot org>]

On Mon, Jun 14, 2004 at 10:52:17AM +0100, Richard Earnshaw wrote:
> No, I don't think so (unless I've misunderstood your patch).
> 
> The problem is that we only track the one register in the cond table, by
> removing the constraint that the second argument is against 0, you
> potentially allow
> 
> 	if (a < b)
> 		op1;
> 	else if (a >= c)
> 		op2;
> 
> to be transformed into 
> 
> 	if (a < b)
> 		op1;
> 	else
> 		op2;
> 
> which it might do because it only takes into account 2 of the 3
> constraints required (operand 1 is 'a', comparison is opposite of '<')
> and ignores the third (operand 2 is 'b').  This is clearly not the same.

Could you explain where you think this transformation might happen? 
The only things which examine the form of the comparisons in flow.c are
elim_reg_cond, and_reg_cond, not_reg_cond, and ior_reg_cond; they used
to ignore the second operand, but I changed that.

I guess this is the table you're referring to:
  /* Indexed by register number, holds a reg_cond_life_info for each
     register that is not unconditionally live or dead.  */
  splay_tree reg_cond_dead;

But that is indexed by the register we're tracking, which is not a, b,
or c; it's something set by op1.

-- 
Daniel Jacobowitz