On 7/11/2023 9:13 PM, Nick F via gem5-users wrote:
Good afternoon,
I have been trying to use Gem5 to research and study the performance of several different computer
architectures. However, I have been noticing that I may be unable to accurately model the
differences in cycle length for computer programs.
Take for example these two programs:
#include <stdint.h>
int main(void)
{
for (uint32_t i = 0; i < 1000; i++) {
uint32_t x = 5 * 6;
if (x != 30) {
return 1;
}
}
return 0;
}
#include <stdint.h>
int main(void)
{
for (uint32_t i = 0; i < 1000; i++) {
uint32_t x = 5 + 6;
if (x != 11) {
return 1;
}
}
return 0;
}
Compiling and running both individually on a basic RISC-V CPU config, they both exit at exactly
1,297,721,000. However, in a real system, each multiply operation would take longer and I'd suspect
doing 1000 multiplications would have even a tiny difference in performance. My own research would
also have difficulties analyzing relative performance unless I'm missing something.
Even custom instructions seem to execute in a single CPU cycle regardless of how the hardware would
be implemented.
Is there a good way to define cycle delays in my Gem5 environment? I can implement a "multiply"
function inserts a bunch of no-ops, but that would make it more complicated when the program
complexity grows.
I've written a small blog post
<https://fleker.medium.com/modeling-memristors-to-execute-physically-accurate-imply-operations-in-gem5-ef888b7dc49b> exploring some of what I've tried in the past week. If anyone here has any suggestions I'd be interested to hear them.
Unless you work really hard to defeat it, any compiler will do 5 + 6 or 5 * 6 at
compile time, producing a small constant. The if test will go away, and the
loop
probably will, too. Did you look at the actual machine code of the executable?
Even if you turned all optimizations off, etc., many cores can do adds and
multiplies in about the same amount of time, and given pipelines, other work,
etc., it might well come out to the same number of cycles even if the operations
are there.
So, I'd first check the machine code, and would also want to know the specific
gem5 model being used (in order, out of order, etc.) and other parameters ...
Best - EM
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