SIMD/intrinsincs questions

[Walter Bright]

Mike Farnsworth wrote:
> Was it actually rewriting the executable code to call the alternate
> functions (e.g. a exe load time decision, patch the code in memory,
> and then run)?  I thought that sort of thing would run into all sorts
> of runtime linker issues (ro code pages in memory, shared libs that
> also need the rewriting, etc.), but then again, they do that with JIT
> compiling all the time.


It's much simpler than that. Some C:

=========================================
void foo_with_FPU();
void foo_without_FPU();

void (*foo)();

void main()
{
     has_fp = doesCPUhaveFPU();
     if (has_fp)
         foo = &foo_with_FPU();
     else
         foo = &foo_without_FPU();
     ... execute app ...
     (*foo)();
     ... execute more app ...
}
=========================================

#if WITH_FPU
#define FOO foo_with_FPU
#else
#define FOO foo_without_FPU
#endif

void FOO()
{
     ... do some floating point calculations ...
}
==========================================
dmc -DWITH_FPU -c foo.c -f -ofoo_with_fpu.obj
dmc -c foo.c -ofoo_without_fpu.obj
dmc app.obj foo_with_fpu.obj foo_without_fpu.obj
===========================================

Hope that makes it clearer. No runtime linking, no runtime compiling, no 
self-modifying code, etc.

A better way to do it is to put your FP code behind a class interface, 
then have derived classes implement them, compiled with different 
instruction set options. At runtime, decide which derived class to use.