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.

This is with reference to my latest mail.

I found that while doing insertions of plus kinda expressions, the 
experssions inserted does not contain clobbering of CC, even if it is 
there in original instruction.
For example for the instruction
(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)

the instruction inserted is

(insn 72 64 36 2 (set (reg:SI 87)
        (plus:SI (reg:SI 86 [ a ])
            (reg:SI 85 [ b ]))) 134 {*lea_1} (nil)
    (nil))

That is it converts addsi_1 to lea_1. 
-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.