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

[bonzini at gcc dot gnu dot org]

------- 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.

Paolo

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