Re: [PATCH v1] Mode-Switching: Add optional EMIT_AFTER hook

[Jeff Law <jeffreyalaw@gmail.com>]

On 8/25/23 06:44, Li, Pan2 wrote:
> Hi Jeff,
> 
>> You might also peek at the RTL gcse/pre code which is also LCM based and
>> has the same class of problems.
> 
> I found a similar approach to take care of this in gcse.cc/pre_edge_insert with some comments as below.
> 
>    /* We can't insert anything on an abnormal and
>         critical edge, so we insert the insn at the end of
>         the previous block. There are several alternatives
>         detailed in Morgans book P277 (sec 10.5) for
>         handling this situation.  This one is easiest for
>         now.  */
> 
> if (eg->flags & EDGE_ABNORMAL)
>    insert_insn_end_basic_block (index_map[j], bb);
> else
>    {
>        insn = process_insert_insn (index_map[j]);
>        insert_insn_on_edge (insn, eg);
>    }
> 
> It looks the insert_insn_end_basic_block is designed to handle the ABNORMAL edge by inserting at end of previous block from the comments.
That's probably dead code at this point.  IIRC rth did further work in 
this space because inserting in the end of the block with the abnormal 
edge isn't semantically correct.

It's been 20+ years, but IIRC he adjusted the PRE bitmaps so that we 
never would need to do an insertion on an abnormal edge.  Search for 
EDGE_ABNORMAL in gcse.cc.

Jeff