stl::bitset does these type of optimizations underneath and it's
portable.
Ali
On Wed, 6 Apr 2011 15:57:37 -0500 (CDT), Nilay Vaish
<ni...@cs.wisc.edu> wrote:
I would prefer we make use of GCC builtin __builtin_popcount() for
counting the number of 1's in an int or related data type.
Nilay
On Wed, 6 Apr 2011, Ali Saidi wrote:
And actually, couldn't you use an stl bitset for this?
Thanks,
Ali
On Wed, 06 Apr 2011 15:34:01 -0500, Ali Saidi <sa...@umich.edu>
wrote:
Jumping in somewhat randomly here, uint64_t even on a 32bit machine
is reasonably fast. It's not going to be as fast, but it will be
correct. My vote would be to just switch all that Set code that
uses
long to explicitly use uint64_t and if it's slower on a 32bit
machine
so be it. At least it's correct.
Ali
On Wed, 6 Apr 2011 15:24:24 -0500, "Beckmann, Brad"
<brad.beckm...@amd.com> wrote:
Hi Korey,
Yes, let's move this conversation back to m5-dev, since I think
others may be interested and could help.
I don't know what the problem is exactly, but at some point of
time
(probably back in the early GEMS days) I seem to remember the Set
code
included an assertion check about the 31st bit in 32-bit mode.
Therefore, I think we knew about this problem and made sure that
never
happened. I believe that is why we used to have a restriction
that
Ruby could only support 16 processors. I'm really fuzzy on the
details...maybe someone else can elaborate.
In the end, I just want to make sure we add something in the code
that makes sure we don't encounter this problem again. This is
one of
those bugs that can take a while to track down, if you don't catch
it
right when it happens with an assertion.
Brad
From: koreylsew...@gmail.com [mailto:koreylsew...@gmail.com] On
Behalf Of Korey Sewell
Sent: Tuesday, April 05, 2011 7:14 AM
To: Beckmann, Brad
Subject: Re: [m5-dev] Running Ruby w/32 Cores
Hi again Brad,
I looked this over again and although my 32-bit patch "fixes"
things,
now that I look at it again, I'm not convinced that I actually
fixed
the symptom of the bug but rather the cause of the bug.
Do you happen to know what are the problems with the 32-bit Set
counts?
Sorry for prolonging the issue, but I thought I had put this to
bed
but maybe not. Finally, it may not matter that this works on
32-bit
machines but it'd be nice if it did. (Let me know if I should move
this convo to the m5-dev list)
I end up checking the last bit in the count function manually (the
code as follows):
int
Set::count() const
{
int counter = 0;
long mask;
for (int i = 0; i < m_nArrayLen; i++) {
mask = (long)0x01;
for (int j = 0; j < LONG_BITS; j++) {
// FIXME - significant performance loss when array
// population << LONG_BITS
if ((m_p_nArray[i] & mask) != 0) {
counter++;
}
mask = mask << 1;
}
#ifndef _LP64
long msb_mask = 0x80000000;
if ((m_p_nArray[i] & msb_mask) != 0) {
counter++;
}
#endif
}
return counter;
}
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