[Bug tree-optimization/77902] Auto-vectorizes epilogue loops of manually vectorized functions

[linux at carewolf dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77902

Allan Jensen  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |FIXED

--- Comment #3 from Allan Jensen  ---
Since it appears to be optimized better in gcc 7, let's say this is resolved.

[Bug tree-optimization/77902] Auto-vectorizes epilogue loops of manually vectorized functions

[linux at carewolf dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77902

--- Comment #2 from Allan Jensen  ---
While this have been the case in both GCC 5 and GCC 6, it appears to both
failing cases previously meantioned already produced the best case result in
using a half recent GCC 7.
gcc version 7.0.0 20160923 (experimental) (GCC)

[Bug tree-optimization/77902] Auto-vectorizes epilogue loops of manually vectorized functions

[linux at carewolf dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77902

--- Comment #1 from Allan Jensen  ---
Further experimentation shows that GCC can sometimes reason about the remaining
range but does so inconsistenly.

For instance this examplse also fails:
int result = 0;
for (; count >= 4; count -= 4) {
// Manually vectorized or batched code
foobar_4x(result, vector);
vector += 4;
}
for (; count >= 0; --count) {   // Still autovectorized
result += *vector++;
}

But replacing the epilogue with a loop that counts up, and GCC
appears to figure out it is pointless to vectorize:

for (int i = 0; i < count; ++count) { // correctly not vectorized