[tree-ssa] Insert on edge comment

[Andrew MacLeod]

On Tue, 2003-06-03 at 01:01, Daniel Berlin wrote:
> 
> >> Like so:
> >>
> >> # BLOCK 0.  PRED: -1.  SUCC: 1.
> >>    pretmp.4_24 = in_9 + 1B; <<<<<<<<<< THIS
> >>    while (1)
> >>      {
> >>>>>>>>>>> SHOULD REALLY BE HERE
> >>        # BLOCK 1
> >> (/compilerstuff/gcc-rw-ssa/gcc/gcc/testsuite/gcc.c-torture/execute/
> >> 20011126-2.c:25).  PRED: 12 1 0.  SUCC: 1 2.
> >>
> > What I dont understand is why where it is inserted doesn't work for 
> > you?
> Because it causes an infinite loop in this testcase?
> :)
> 
> > It'll be executed just before entering the loop, which is what the edge
> > (0,1) represents. Executed only upon entry to the loop.  You are trying
> > to make LOOP_EXPR more complicated than it is.
> >
> > BB0
> > LOOP_EXPR
> >    |
> > BB1
> >   BODY
> >
> > is funcationally identical to
> >
> > BB0
> > goto loop_body_label;
> > BB1
> > loop_body_label:
> > BODY
> > goto loop_body_label;
> 
> 
> The difference is that the second will cause PRE to insert in the right 
> place, and the first will not.
> I tried it exactly as described, and get the correct result using the 
> latter form explicitly in the code, and with a while loop, it infinite 
> loops because of the placement you describe.
> PRE doesn't know it's a LOOP_EXPR, as i mentioned.
> 

Send me your testcase.

So in the second case you insert it before the goto in BB0 and that
provides a different result than inserting it in front of the LOOP_EXPR?

Andrew