On Sat, Jul 15, 2017 at 9:47 PM, Alexander Monakov <amona...@ispras.ru> wrote:
> The reload_pseudo_compare_func comparator, when used from assign_by_spills,
> can be non-transitive, indicating A < B < C < A if both A and C satisfy
> !bitmap_bit_p (&non_reload_pseudos, rAC), but B does not.
>
> This function was originally a proper comparator, and the problematic
> clause was added to fix PR 57878:
> https://gcc.gnu.org/ml/gcc-patches/2013-07/msg00732.html
>
> That the comparator is invalid implies that that PR, if it still exists,
> can reappear (but probably under more complicated circumstances).
>
> This looks like a sensitive area, so disabling checking is the only
> obvious approach.
May make sense to add PR rtl-optimization/68988 annotation to changelog.
> * lra-assigns.c (reload_pseudo_compare_func): Add a FIXME.
> (assign_by_spills): Use non-checking qsort.
> ---
> gcc/lra-assigns.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/lra-assigns.c b/gcc/lra-assigns.c
> index 2aadeef..a67d1a6 100644
> --- a/gcc/lra-assigns.c
> +++ b/gcc/lra-assigns.c
> @@ -217,6 +217,7 @@ reload_pseudo_compare_func (const void *v1p, const void
> *v2p)
> /* The code below executes rarely as nregs == 1 in most cases.
> So we should not worry about using faster data structures to
> check reload pseudos. */
> + /* FIXME this makes comparator non-transitive and thus invalid. */
> && ! bitmap_bit_p (&non_reload_pseudos, r1)
> && ! bitmap_bit_p (&non_reload_pseudos, r2))
> return diff;
> @@ -1384,7 +1385,7 @@ assign_by_spills (void)
> bitmap_ior_into (&non_reload_pseudos, &lra_optional_reload_pseudos);
> for (iter = 0; iter <= 1; iter++)
> {
> - qsort (sorted_pseudos, n, sizeof (int), reload_pseudo_compare_func);
> + qsort_nochk (sorted_pseudos, n, sizeof (int),
> reload_pseudo_compare_func);
> nfails = 0;
> for (i = 0; i < n; i++)
> {
> --
> 1.8.3.1
>
