Re: gcse pass: expression hash table

[Tarun Kawatra <tarun at cse dot iitb dot ac dot in>]

On Thu, 24 Feb 2005, James E Wilson wrote:

> On Thu, 2005-02-24 at 03:15, Steven Bosscher wrote:
> > On Feb 24, 2005 11:13 AM, Tarun Kawatra <tarun@cse.iitb.ac.in> wrote:
> > Does GCSE look into stuff in PARALLELs at all?  From gcse.c:
>
> Shrug.  The code in hash_scan_set seems to be doing something
> reasonable.
>
> The problem I saw wasn't with finding expressions to gcse, it was with
> inserting them later.  The insertion would create a cc reg clobber, so
> we don't bother adding it to the hash table.  I didn't look any further,
> but it seemed reasonable that if it isn't in the hash table, then it
> isn't going to be optimized.
You are write here that if some expr doesn't get into hash table, it will 
not get optimized. But since plus expressions on x86 also clobber CC as 
shown below

(insn 40 61 42 2 (parallel [
            (set (reg/v:SI 74 [ c ])
                (plus:SI (reg:SI 86)
                    (reg:SI 85)))
            (clobber (reg:CC 17 flags))
        ]) 138 {*addsi_1} (nil)
    (nil))

then why the same reasoning does not apply to plus expressions. Why will 
there insertion later will not create any problems?

Actually I am trying to extend PRE implementation so that it performs 
strength reduction as well. it requires multiplication expressions to get 
into hash table.

I am debugging the code to find where the differences for the two kind of 
expressions occur. Will let you all know if I found anything interesting. 
If you know this already please share with me.

Thanks
-tarun

> It seems that switching the x86 backend from using cc0 to using a cc
> hard register has effectively crippled the RTL gcse pass for it.