[Bug middle-end/19721] [meta-bug] optimizations that CSE still catches

[law at redhat dot com]

------- Additional Comments From law at redhat dot com  2005-08-17 19:31 -------
Subject: Re:  [meta-bug] optimizations that CSE still
	catches

On Wed, 2005-08-17 at 08:03 +0000, bonzini at gcc dot gnu dot org wrote:
> ------- Additional Comments From bonzini at gcc dot gnu dot org  2005-08-17 08:03 -------
> This small testcase is a typical case of the optimizations that CSE path
> following catches on PowerPC:
> 
>     unsigned outcnt;
>     extern void flush_outbuf(void);
> 
>     void
>     bi_windup(unsigned char *outbuf, unsigned char bi_buf)
>     {
>         outbuf[outcnt] = bi_buf;
>         if (outcnt == 16384)
>                 flush_outbuf();
>         outbuf[outcnt] = bi_buf;
>     }
> 
> Loading outcnt takes *three* insns: one to load the high part of the address,
> one to load the low part, one to load from memory.  CSE reduces them to two by
> combining the loading of the low part with the load from memory.  With CSE path
> following, in addition, CSE is able to factor the loads of the high part of the
> address, and do just one of them.
> 
> Now here comes GCSE.  If CSE path following is on, GCSE sees that the third
> occurrence of outcnt is the same as the second, and eliminates it.  If it is
> off, GCSE is wasted to factor the loading of the address high parts.
> 
> So, if we remove a pseudo-global cse pass by disabling path following, it would
> make sense to bump the default max-gcse-passes to 2.
Presumably the store into outbuf prevents the SSA optimizers from
commonizing the first two loads of outcnt and the call to flush_outbuf
prevents the SSA optimizers from commonizing the last load of outcnt on
the path which bypasses the call to flush_outbuf.  Right?

Jeff



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19721