https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117008
--- Comment #9 from Matt Bentley <mattreecebentley at gmail dot com> ---
(In reply to Jonathan Wakely from comment #7)
> I'm certain it won't be, because (apart from vector<bool>) they access
> memory locations that are at least one byte, not repeating very similar
> bitwise ops for every bit in a word.
Have tested 4 additional scenarios: random access, sequential iteration with an
if statement, the vector equivalent, and sequential iteration with an if
statement and a more complicated action.
Can confirm that the issue only occurs in the second of those, ie.:
{
std::bitset<12800000> values;
for (unsigned int counter = 0; counter != 500; ++counter)
{
for (unsigned int index = 0; index != 12800000;
++index)
{
values.set(index, plf::rand() & 1);
}
for (unsigned int index = 0; index != 12800000;
++index)
{
if (values[index])
{
total += values[index] * 2;
}
}
}
}
While this statement is closer to what you might see in real world code ie. if
1, do this,
I'm assuming it gets optimized into a conditional move or something, as the
same 25% slowdown occurs.
Once you add in a more complicated action ie.:
for (unsigned int index = 0; index != 12800000;
++index)
{
if (values[index])
{
total += values[index] * 2;
total ^= values[index];
}
}
The problem goes away. So essentially Andrew's correct, unless the user's
chosen action can be trivially optimized.
Will generate the -v output for the second of the scenarios now.
