[Bug target/68690] New: PowerPC64: TOC save in PHP core loop results in load hit store

["anton at samba dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68690

            Bug ID: 68690
           Summary: PowerPC64: TOC save in PHP core loop results in load
                    hit store
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: anton at samba dot org
  Target Milestone: ---

We see a load hit store issue in the core loop of the PHP interpreter. A simple
test case:

$ cat test.c

void (*fn)(void);

void do_nothing(void) { }

int main(void)
{
        unsigned long i;

        fn = do_nothing;
        for (i = 0; i < 1000000000; i++)
                fn();

        return 0;
}

$ gcc -O2 -o test test.c

We continually save the TOC on each call:

    100003f0:   00 00 3e e9     ld      r9,0(r30)
    100003f4:   a6 03 29 7d     mtctr   r9
    100003f8:   78 4b 2c 7d     mr      r12,r9
    100003fc:   18 00 41 f8     std     r2,24(r1) <---------- save it again!
    10000400:   21 04 80 4e     bctrl
    10000404:   18 00 41 e8     ld      r2,24(r1)
    10000408:   01 00 3f 2c     cmpdi   r31,1
    1000040c:   ff ff ff 3b     addi    r31,r31,-1
    10000410:   e0 ff 82 40     bne     100003f0 <main+0x40>

This should be moved out of the loop. One way to force that is via the
-msave-toc-indirect option:

gcc -msave-toc-indirect -O2 -o test test.c

Which is over 2x faster on this (admittedly worst case) test. For something
more real world, we also see a 20% speedup on PHP7 on various microbenchmarks.