On Tue, 2 Apr 2013, Jakub Jelinek wrote:
> Hi!
>
> On the (undefined behavior) testcase below, we end up with
> then_bb ending with __builtin_unreachable () at the tree level, therefore
> no successor at the RTL level, and else_bb being EXIT_BLOCK_PTR (i.e.
> conditional return before a bb with undefined behavior at the end).
> Trying to optimize that into a conditional execution of the then_bb insns
> doesn't work, we can't merge the else_bb with then_bb and test_bb in this
> case, plus it doesn't look like something that would be desirable to do
> (conditional return is surely better).
>
> Fixed thusly, ok for trunk/4.8?
I wonder if
/* Make sure IF, THEN, and ELSE, blocks are adjacent. Actually, we get
the
first condition for free, since we've already asserted that there's a
fallthru edge from IF to THEN. Likewise for the && and || blocks,
since
we checked the FALLTHRU flag, those are already adjacent to the last
IF
block. */
/* ??? As an enhancement, move the ELSE block. Have to deal with
BLOCK notes, if by no other means than backing out the merge if they
exist. Sticky enough I don't want to think about it now. */
next = then_bb;
if (else_bb && (next = next->next_bb) != else_bb)
return FALSE;
if ((next = next->next_bb) != join_bb && join_bb != EXIT_BLOCK_PTR)
{
if (else_bb)
join_bb = NULL;
else
return FALSE;
}
somehow tries to guard against join_bb == EXIT_BLOCK_PTR but fails.
Thus, why not do that explicitely here instead of just in the
single case you cover? (I can't see why join_bb could not be
set to EXIT_BLOCK_PTR in some weird case)
Richard.
> 2013-04-02 Jakub Jelinek <ja...@redhat.com>
>
> PR rtl-optimization/56745
> * ifcvt.c (cond_exec_find_if_block): Don't try to optimize
> if then_bb has no successors and else_bb is EXIT_BLOCK_PTR.
>
> * gcc.c-torture/compile/pr56745.c: New test.
>
> --- gcc/ifcvt.c.jj 2013-03-05 23:25:15.000000000 +0100
> +++ gcc/ifcvt.c 2013-04-02 12:31:19.476859094 +0200
> @@ -3473,7 +3473,7 @@ cond_exec_find_if_block (struct ce_if_bl
> code processing. ??? we should fix this in the future. */
> if (EDGE_COUNT (then_bb->succs) == 0)
> {
> - if (single_pred_p (else_bb))
> + if (single_pred_p (else_bb) && else_bb != EXIT_BLOCK_PTR)
> {
> rtx last_insn = BB_END (then_bb);
>
> --- gcc/testsuite/gcc.c-torture/compile/pr56745.c.jj 2013-04-02
> 12:38:54.718258527 +0200
> +++ gcc/testsuite/gcc.c-torture/compile/pr56745.c 2013-04-02
> 12:33:22.000000000 +0200
> @@ -0,0 +1,15 @@
> +/* PR rtl-optimization/56745 */
> +
> +unsigned char a[6];
> +
> +void
> +foo ()
> +{
> + int i;
> + for (i = 5; i >= 0; i++)
> + {
> + if (++a[i] != 0)
> + break;
> + ++a[i];
> + }
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE / SUSE Labs
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
GF: Jeff Hawn, Jennifer Guild, Felix Imend
