> -----Original Message-----
> From: gcc-ow...@gcc.gnu.org [mailto:gcc-ow...@gcc.gnu.org] On Behalf Of
> Jiangning Liu
> Sent: Tuesday, December 27, 2011 5:10 PM
> To: 'Richard Guenther'
> Cc: Michael Matz; gcc@gcc.gnu.org
> Subject: RE: A case exposing code sink issue
>
> >
> > The job to do this is final value replacement, not sinking (we do not
> > sink non-invariant expressions - you'd have to translate them through
> > the loop-closed SSA exit PHI node, certainly doable, patches
> > welcome ;)).
> >
>
> Richard,
>
> In final value replacement, expression "&a + D.xxxx" can be figured out,
> while "&a[i_xxx]" failed to be CHRECed, so I'm wondering if we should
> lower
> &a[i_xxx] to "&a + unitsize(a) * i_xxx" first? It seems GCC intends to
> keep
> &a[i_xxx] until cfgexpand pass. Or we have to directly modify CHREC
> algorithm to get it calculated?
>
> Appreciate your kindly help in advance!
>
Richard,
Now I have a patch working for the case of step "i++", by directly modifying
scalar evolution algorithm. the following code would be generated after
SCCP,
l
# i_13 = PHI <i_6(7), k_2(D)(4)>
a_p.0_4 = &a[i_13];
MEM[(int *)&a][i_13] = 100;
i_6 = i_13 + 1;
if (i_6 <= 999)
goto <bb 7>;
else
goto <bb 6>;
<bb 6>:
a_p_lsm.5_11 = &MEM[(void *)&a + 3996B];
a_p = a_p_lsm.5_11;
goto <bb 3>;
It looks good, but I still have problem when the case has step "i+=k".
For this case the value of variable i exiting loop isn't invariant, the
algorithm below in scalar evolution doesn't work on it,
compute_overall_effect_of_inner_loop()
{
...
tree nb_iter = number_of_latch_executions (inner_loop);
if (nb_iter == chrec_dont_know)
return chrec_dont_know;
else
{
tree res;
/* evolution_fn is the evolution function in LOOP. Get
its value in the nb_iter-th iteration. */
res = chrec_apply (inner_loop->num, evolution_fn, nb_iter);
if (chrec_contains_symbols_defined_in_loop (res, loop->num))
res = instantiate_parameters (loop, res);
/* Continue the computation until ending on a parent of LOOP.
*/
return compute_overall_effect_of_inner_loop (loop, res);
}
}
In theory, we can still have the transformation like below even if the step
is "i+=k",
# i_13 = PHI <i_6(7), k_2(D)(4)>
i_14 = i_13,
a_p.0_4 = &a[i_13];
MEM[(int *)&a][i_13] = 100;
i_6 = i_13 + k_2(D); // i+=k
if (i_6 <= 999)
goto <bb 7>;
else
goto <bb 6>;
<bb 6>:
a_p_lsm.5_11 = &a[i_14];
a_p = a_p_lsm.5_11;
goto <bb 3>;
But I realize this is not a loop closed SSA form at all, because i_14 is
being used out of the loop. Where could we extend the liverange of variable
i in GCC infrastructure and finally solve this problem?
> Thanks,
> -Jiangning
>
>
>
