https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68953
--- Comment #2 from vries at gcc dot gnu.org ---
(In reply to vries from comment #1)
> Created attachment 38141 [details]
> updated test-case
>
> Fails with -O1, passes with -O2.
>
> The problem is that this loop nest:
> ...
> for (zh = 0; zh < 2; ++zh)
> for (ro = 0; ro < 3; ++ro)
> yu[ro][0] = yu[zh + 1][0];
> ...
Unrolled, that looks like:
/* yu == { { 1 }, { 2 }, { 3 }, { 4 } }. */
yu[0][0] = yu[1][0]; /* zh == 0, ro == 0. */
/* yu == { { 2 }, { 2 }, { 3 }, { 4 } }. */
yu[1][0] = yu[1][0]; /* zh == 0, ro == 1. */
/* yu == { { 2 }, { 2 }, { 3 }, { 4 } }. */
yu[2][0] = yu[1][0]; /* zh == 0, ro == 2. */
/* yu == { { 2 }, { 2 }, { 2 }, { 4 } }. */
yu[0][0] = yu[2][0]; /* zh == 1, ro == 0. */
/* yu == { { 2 }, { 2 }, { 2 }, { 4 } }. */
yu[1][0] = yu[2][0]; /* zh == 1, ro == 1. */
/* yu == { { 2 }, { 2 }, { 2 }, { 4 } }. */
yu[2][0] = yu[2][0]; /* zh == 1, ro == 2. */
/* yu == { { 2 }, { 2 }, { 2 }, { 4 } }. */
> is rewritten into this loop nest, which has different semantics:
> ...
> for (ro = 0; ro < 3; ++ro)
> for (zh = 0; zh < 2; ++zh)
> yu[ro][0] = yu[zh + 1][0];
> ...
and unrolled, this looks like:
/* yu == { { 1 }, { 2 }, { 3 }, { 4 } }. */
yu[0][0] = yu[1][0]; /* zh == 0, ro == 0. */
/* yu == { { 2 }, { 2 }, { 3 }, { 4 } }. */
yu[0][0] = yu[2][0]; /* zh == 1, ro == 0. */
/* yu == { { 3 }, { 2 }, { 3 }, { 4 } }. */
yu[1][0] = yu[1][0]; /* zh == 0, ro == 1. */
/* yu == { { 3 }, { 2 }, { 3 }, { 4 } }. */
yu[1][0] = yu[2][0]; /* zh == 1, ro == 1. */
/* yu == { { 3 }, { 3 }, { 3 }, { 4 } }. */
yu[2][0] = yu[1][0]; /* zh == 0, ro == 2. */
/* yu == { { 3 }, { 3 }, { 3 }, { 4 } }. */
yu[2][0] = yu[2][0]; /* zh == 1, ro == 2. */
/* yu == { { 3 }, { 3 }, { 3 }, { 4 } }. */
