SIMD/intrinsincs questions

[Walter Bright]

Mike Farnsworth wrote:
> Walter Bright Wrote:
> 
>> Michael Farnsworth wrote:
>>> The ldc guys tell me that they didn't include the llvm vector
>>> intrinsics already because they were going to need either a
>>> custom type in the frontend, or else the D2 
>>> fixed-size-arrays-as-value-types functionality.  I might take a
>>> stab at some of that in ldc in the future to see if I can get it
>>> to work, but I'm not an expert in compilers by any stretch of the
>>> imagination.
>> I think there's a lot of potential in this. Most languages lack
>> array operations, forcing the compiler into the bizarre task of
>> trying to reconstruct high level operations from low level ones to
>> then convert to array ops.
> 
> Can you elaborate a bit on what you mean?

Sure. Consider the code:

     for (int i = 0; i < 100; i++)
          array[i] = 0;

It takes a fair amount of work for a compiler to deduce "aha!" this code 
is intended to clear the array! The compiler then replaces the loop with:

      memset(array, 0, 100 * sizeof(array[0]));

In D, you can specify the array operation at a high level:

     array[0..100] = 0;

In other words, a language is supposed to represent high level concepts 
and the compiler breaks it down into low level ones supported by the 
machine. With vector operations, etc., the language supports only the 
low level operations and the compiler must reconstruct the high level 
operations supported by the machine.

This inversion of roles is bizarre.