[Bug tree-optimization/69042] [6 regression] Missed optimization in ivopts

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69042

--- Comment #3 from amker at gcc dot gnu.org ---
(In reply to amker from comment #2)
> For iv use:
> use 0
>   address
>   in statement _9 = indexes[i_23];
> 
>   at position indexes[i_23]
>   type const int *
>   base (const int *) (&indexes + 4)
>   step 4
>   base object (void *) &indexes
>   related candidates 
> 
> Before the change, two candidates are added:
> candidate 8
>   var_before ivtmp.12
>   var_after ivtmp.12
>   incremented before exit test
>   type unsigned int
>   base (unsigned int) (&indexes + 4)
>   step 4
>   base object (void *) &indexes
> candidate 10
>   var_before ivtmp.14
>   var_after ivtmp.14
>   incremented before exit test
>   type unsigned int
>   base (unsigned int) &indexes
>   step 4
>   base object (void *) &indexes
> After the change only candidate 8 is added.  I did this to minimize the
> number of candidates.  Maybe that's too aggressive.  Probably candidate like
> this (with offset stripped) should be added, I will check if it causes new
> regressions.
> 
> Another problem is candidate 8 could be chosen to decrease register
> pressure, but isn't.  I will check why the register pressure cost doesn't
> work in this case.  Though candidate 8 is still a little bit worse than
> candidate 10, because of one more setup instruction in loop pre-header block.

For this register pressure question.
GCC assumes that symbol "indexes" in memory reference "MEM[symbol: indexes,
index: _21, step: 4, offset: 0B]" doesn't introduce any register pressure
because we can use addressing mode like "indexes(,%eax,4)".  This is not true
for pic/pie code.  In such case, we need to move "indexes@GOT(%eax)" to a
register before using it in memory reference.  That's why the additional
register use not counted in register pressure computation in IVOPT.