function code order changes from gcc-3.3 to 3.4

function code order changes from gcc-3.3 to 3.4

[Alan Lehotsky]

We have a tool that generates C++ code as a series of functions inside 
different classes.

Because we also generate the "driver function" that calls these class 
member functions in a particular order,
we generate the C++ functions in a linear order that is identical to the 
order in which the functions are called.

e.g. if we had

         class a {
            static void fx();
            static void fy();
         };
         class b {
             static void fx();
             static void fy();
         }

and the order that the functions were called was

    a::fy();
    b::fy();
    b::fx()
    a::fx();

Then we would generate a C++ source file containing

    void a::fy() {...}
    void b::fy() {...]
    void b::fx() {...}
    void a::fx() {...}
 
This is a win for paging and cache-line prefetching - essentially it's 
compile-time profile-guided optimization.

With 3.3, the functions are in fact emitted as assembly language in the 
order in which they were declared.

With 3.4, this is not true - functions are not emitted in the order that 
they were defined in the source file.

We have measured a 5-9% degradation in execution time and suspect that 
this is the source of the problem.

I've tried -fno-unit-at-a-time (and that leads to worse performance - 
perhaps a 10% SLOWER gcc 3.4.3).  Is there any other way to control 
function ordering (short of compiling each function to a separate 
section and writing linker language that orders the sections in the 
order we want)?

I can't see any other compiler switches that would affect this other 
than perhaps -fno-reorder-functions, and that seems to hurt performance 
as well.