Bizarrely Poor Code from Bizarre Machine-Generated C Sources

[Rask Ingemann Lambertsen]

On Sun, May 27, 2007 at 03:05:38PM +0100, Barak A. Pearlmutter wrote:

> In particular, it defines gobs of new
> structure types and gobs of very very short functions, and there are
> no pointers used.  It should be possible, using the optimization
> techniques already present in GCC, for very tense machine code to be
> generated from this admittedly strange FORTRAN-style C source code.
> But instead, the assembly code GCC generates is full of unnecessary
> data shuffling.

   The way you are using structures forces GCC to copy data around. Unless
you somehow manage to inline the whole program into main(), I don't see how
it can be any different.

>  - Some small change we could make to the generated C sources that
>    would cause it to be optimized well.  (Add some magic __attribute__
>    somewhere.)

   Change the structures into scalar variables for a start. GCC has more
freedom to place scalar variables than structures. Also, try to arrange
function parameters such that sibling call optimization has a chance of
working.

BAD:

int g (int c, int b, int a)
{ ... }

int f (int a, int b, int c)
{
  return g (c, b, a);
}

GOOD:

int g (int a, int b, int c)
{ ... }

int f (int a, int b, int c)
{
  return g (a, b, c);
}

> Below are notes that include detailed version information on the
> compilers used.  In the notes below we used
>  -O2 -freg-struct-return -fomit-frame-pointer -mfpmath=sse -msse3
> but the results don't seem to improve by changing them.

   You will definitely want a lot of inlining for this sort of code, so at
least use -O3, but perhaps play with the inlining parameters too. On a side
note, consider using using -march to tell GCC which model of CPU you intend
to run the code on.

> $ wc --lines *.s
>   163922 particle1-gcc295.s
>   343012 particle1-gcc33.s
>   353057 particle1-gcc34.s
>   100697 particle1-gcc41.s
>    47030 particle1-gcc42.s

   I imagine you'll be enlightened by running

$ for i in *.s; do echo -n "${i}: "; grep -F -e memcpy ${i} | wc --lines; done

-- 
Rask Ingemann Lambertsen