[FRIAM] deployable and open source ABMs

[Marcus G. Daniels]

Here is one weakness of native code ahead-of-time compilers relative to 
just-in-time compilers embedded in runtimes.  I think it is relevant to 
HPC applications.

Consider a program made up of some source code which references a third 
party library providing a wide variety of features.  The third party and 
the program author are at different organizations and are not sharing 
source code.

Few non-local dynamics of a program can be deduced by an ahead-of-time 
compiler, just because it is hard to do that.  Without tolerating huge 
code bloat and stressing caches, the compiler can't just inline 
everything.  Worse, it can't even see everything because the libraries 
are separately compiled and then linked.  (Most compiled object code 
just advertises a vector of opaque entry points to the linker, which 
itself is dumb.)

Now imagine that certain callers in the program result in a predictable 
complement of internal routines in the library always being called, and 
that there is an public interface intermediate between the two (e.g. a 
message dispatch).   The compiler would need to see all of that context 
to ensure that the native code would be generated such that branch 
prediction on the CPU would never get confused, but it cannot due to the 
organization of the typical HPC toolchain.

A CLR or JVM runtime, however, can watch the dynamics of the program and 
make experiments on code generation strategies as the code runs.  The 
ones that pay off can be cached & reused.   Here, cross-module inlining 
is the desired optimization. 

Given the other disadvantages of Java for HPC, I'm not arguing that this 
makes up for them, but it's possibly worth thinking about how to learn 
from Java approaches in HPC applications.

One way to try that is using a package like Dynamo.  What Dynamo does is 
treat the non-branching sequences of real instructions as chunks to 
thread together into native code.  These cached native code chunks can 
be selected-for as in a JVM. 

http://www.cag.lcs.mit.edu/dynamorio

(Dynamo worked for me in Fedora 5 x86, no luck in Windows, maybe due to 
data execution prevention.)