https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104543
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |matz at gcc dot gnu.org
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Hmm, no. The issue seems to be that the outer loop is head-controlled which
means unrolling duplicates the exit check in the middle of the fused loop.
I think we miss a check for an empty header of the outer loop, the
perfect nest check is too simplistic here. We have
b = 0;
do
{
if (b > 2)
break;
c = 0;
do
{
a[c] ^= 1;
++c
}
while (c <= 2);
++b;
}
while (1);
and fusing ends up with something like
b = 0;
do
{
c = 0;
do
{
a[c] ^= 1;
if (b > 2)
break;
a[c] ^= 1;
}
while (c <= 2);
b+=2;
}
while (1);
in particular the comment in bb_prevents_fusion_p saying
/* BB is duplicated by outer unrolling and then all N-1 first copies
move into the body of the fused inner loop. If BB exits the outer loop
the last copy still does so, and the first N-1 copies are cancelled
by loop unrolling, so also after fusion it's the exit block.
looks wrong. Yes, the first is cancelled but the remaining is not the
exit block.
