Re: [GCC 3.x] Performance testing for QA

[Dale Johannesen <dalej@apple.com>]

On Tuesday, September 3, 2002, at 01:55 PM, Robert Dewar wrote:

> <<The type of operations that encoder decoders use (lots of arithmetic 
> and
> data movement) should somehow be incorporate as a benchmark test gcc
> should optimize for.  The ideal goal would be to make the code fast 
> enough
> that hand written assembly would not be necessary.  Thus using an 
> encoder as
> a benchmark would be beneficial.  Even if it concentrates on tight 
> loops.
>>>
>
> You need to remember that it is trivial to make a compiler make any ONE
> simple program as fast as optimal assembly language.
>
> How? Simply by recognizing the program, and saying "ah ha, this is 
> program
> number 234c, so replace it by the super efficient asm that Joe wrote".
>
> Now of course doing things this blatantly would be considered 
> cheating, but
> in practice you can do things almost this outrageous in a much more
> reserved and non-obvious way.

One of the SPEC92 benchmarks, eqntott, was dominated by a single 
function
that looped through a linked data structure.  Unrolling the loop the 
right
number of times was critical, but you couldn't figure this out 
legitimately
because the right number of times depended on the data in the structure,
and that was read in from a file.  By the end-of-life of Spec92 every
commercial compiler I know of had special code to recognize this 
function
and unroll it the right number of times (in some cases, by emitting 
canned
assembly as you suggest).  The benchmark was removed in later SPEC's...