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Re: Eliminate write-only variables


On 05/18/2014 08:45 PM, Sandra Loosemore wrote:
On 05/18/2014 02:59 PM, Jan Hubicka wrote:
For cases like local-statics-7 your approach can be "saved" by adding
simple IPA analysis
to look for static vars that are used only by one function and keeping
your DSE code active
for them, so we can still get rid of this special case assignments
during late compilation.
I am however not quite convinced it is worth the effort - do you have
some real world
cases where it helps?

Um, the well-known benchmark.  ;-)

I looked at the generated code for this benchmark and see that your patch is indeed not getting rid of all the pointless static variables, while ours is, so this is quite disappointing. I'm thinking now that we will still need to retain our patch at least locally, although we'd really like to get it on trunk if possible.

Here is another testcase vaguely based on the same kind of signal-processing algorithm as the benchmark, but coded without reference to it. I have arm-none-eabi builds around for both 4.9.0 with our remove_local_statics patch applied, and trunk with your patch. With -O2, our optimization produces 23 instructions and gets rid of all 3 statics, yours produces 33 and only gets rid of 1 of them.

Of course it's lame to use static variables in this way, but OTOH it's lame if GCC can't recognize them and optimize them away, too.

-Sandra

void
fir (int *coeffs, int coeff_length, int *input, int input_length, int *output)
{
  static int *coeffp;
  static int *inputp;
  static int *outputp;
  int i, c, acc;

  for (i = 0; i < input_length; i++)
    {
      coeffp = coeffs;
      inputp = input + i;
      outputp = output + i;
      acc = 0;
      for (c = 0; c < coeff_length; c++)
	{
	  acc += *coeffp * *inputp;
	  coeffp++;
	  inputp--;
	}
      *outputp = acc;
    }
}

  

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