Re: Questions about code generation

[richardcavell at mail dot com]

Thanks, David.  Some follow up questions:

> It is typically possible, but it depends on the compile-time flags you
> use and the exact formulation of the source code.  Don't expect that 
the
> compiler will be able to read your mind here - there are all sorts of
> subtleties that come into play with automatic vectorisation.  gcc will
> err on the side of generating definitely correct code, rather than
> generating fast code that might be wrong due to things like aliasing 
issues.

Is the best solution to use variable types that are 
architecture-specific?  For example, defining a series of variable 
types that reflect every possible interpretation of the x86 XMM 
register file?  Or should we be more general about it and allow for 
arrays of type int/float and hope that the compiler will vectorise 
them?  There are certain groups that are commonly used - for example, a 
matrix of 2x2 floating point numbers, or an array of 3 or 4 pairs of 
floats.  Should they have their own variable type?

> gcc does not knowingly produce buggy code - therefore it cannot 
produce
> code that is "less buggy".  But I believe it can produce alternate
> pathways that are chosen at runtime according to the processor being
> used - though I haven't worked with such code myself.

Well, some code that is perfectly legal for both the 68000 and 68060 
chips will not work correctly on the 68060 due to errata.  Certain 
instruction sequences are not executed correctly even though they 
should be - and one would only know if one reads the errata documents.  
I wonder whether gcc avoids such instruction sequences.

Richard