Hi,
That’s indeed a large difference that looks more like a bug than anything else.
To (try) reproduce the bug, can you please provide us with the exact commit,
build command and run command you are using?
A few possible causes of divergence are:
1. The random number generator, as suggested in the previous answer. I don’t
think it is the case as the main gem5 random number generator is based on
std::mt19937_64 which is deterministic for a given seed and the seed is picked
deterministically in gem5 by default (it has been serving me just fine for a
couple of years).
2. Relying on addresses to define order. This can occur, for instance, when
storing pointers in a std::\[unordered_\]{map,set} with default comparison or
hash functions that rely on the pointer value that itself depends on the memory
allocator that gets its memory ranges from the OS.
3. Undefined behavior. This is the cursed benediction of unsafe languages like
C and C++. A program with undefined behavior in it ***can*** lead to, well…
undefined result. Most hidden undefined behaviors have benign consequences
(that’s why they remain hidden) but one of the consequences is slightly
different behavior of a program based on, among other things, the compiler and
the optimization level. Compilers can do the assumption that undefined behavior
never happens to perform clever deductions on the state of the program and
perform killer optimizations. Depending on the smartness of the compiler you
use (the one on the recent computer likely being smarter than the one on the
older computer), you will get odd results.
Beug 2 can be found by running the same program with the same input several
times with different optimization levels, compilers or host. It is also a good
idea to load the host while running the program under test to increase system
entropy and reduce the odds of always having the same addresses provided by the
system.
For beug 3, it will likely suffice to run the program with and without
optimization enabled (i.e., gem5.debug and gem5.opt).
In all cases, you should run the simulation with as many debug flags enabled as
possible and compare the outputs. I recommend using
> colordiff -U 1000 <(simulation command 1) <(simulation command 2) | less -R
Best,
Gabriel
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