Combine four insns

[Bernd Schmidt]

>> $ grep Trying.four log |wc -l
>> 307743
>> $ grep Trying.three log |wc -l
>> 592776
>> $ grep Trying.two log |wc -l
>> 1643112
>> $ grep Succeeded.two log |wc -l
>> 204808
>> $ grep Succeeded.three.into.two log |wc -l
>> 2976
>> $ grep Succeeded.three.into.one log |wc -l
>> 12473
>> $ grep Succeeded.four.into.two log |wc -l
>> 244
>> $ grep Succeeded.four.into.one log |wc -l
>> 140
> 
> No four into three?  So overall it's one order of magnitude less
> three than two and two order of magnitude less four than three.

I redid the numbers for Thumb-1, over the same set of input files.  I
left out the two-insn combinations because the greps take ages and the
comparison 3 vs 4 should provide enough information.

$ grep Trying.three log |wc -l
842213
$ grep Trying.four log |wc -l
488682
$ grep Succeeded.four.to.two log|wc -l
759
$ grep Succeeded.four.to.one log|wc -l
163
$ grep Succeeded.three.to.two log|wc -l
6230
$ grep Succeeded.three.to.one log|wc -l
3178

So the patch seems somewhat more effective for this target
(unsurprisingly as it has simpler instructions).

With the following heuristic in try_combine:

  if (i0)
    {
      int i;
      int ncst = 0;
      int nshift = 0;
      for (i = 0; i < 3; i++)
	{
	  rtx insn = i == 0 ? i0 : i == 1 ? i1 : i2;
	  rtx set = single_set (insn);

	  if (set && CONSTANT_P (SET_SRC (set)))
	    ncst++;
	  else if (set && (GET_CODE (SET_SRC (set)) == ASHIFT
			   || GET_CODE (SET_SRC (set)) == ASHIFTRT
			   || GET_CODE (SET_SRC (set)) == LSHIFTRT))
	    nshift++;
	}
      if (ncst == 0 && nshift < 2)
	return 0;
    }

$ grep Trying.four log2 |wc -l
187120
$ grep Succeeded.four log2|wc -l
828
$ grep Succeeded.three.to.one log2|wc -l
3161
$ grep Succeeded.three.to.two log2|wc -l
6218

With the heuristic, we still catch the majority of interesting cases on
Thumb-1, with a reduced number of attempts, but we also miss some
optimizations like these:

-       add     r2, r2, #1
        lsl     r2, r2, #5
-       add     r3, r3, r2
-       sub     r3, r3, #32
+       add     r3, r2, r3
====
-       mvn     r3, r3
        lsr     r3, r3, #16
-       mvn     r3, r3
====

In summary, should I check in the patch with the above heuristic?


Bernd
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