It happens that COBOL has the COMPUTE statement. It takes the form of,
for example, COMPUTE DDD = AAA + BBB.
We implement that by creating a temporary variable, using that as the
target of an addition of AAA and BBB, and then doing an assignment to DDD.
(Recall that COBOL variables can be quite complex, so we are a long way
from being able to do this with an ADD_EXPR.)
We have determined that the way I've been producing GENERIC for that
results in N-squared computation time somewhere in the middle end.
I have been tearing my hair out trying to figure out what's causing that
N-squared behavior. I commented away the assignment, and I got rid of the
arithmetic. All that's left is the creation of the temporary, and some IF
statements that are generated to test for errors along the way of the
computation. (COBOL has a very rich error-detection and
exception-generating facility.
The remaining GIMPLE for a single iteration (as shown by
-fdump-tree-gimple) is shown below. The "phase opt and generate" times
for repetitions of that GIMPLE are shown here:
phase opt Factor
Repeats & generate
1,000 0.17
2,000 0.49 2.9
4,000 1.55 3.2
8,000 7.56 4.9
16,000 49.31 6.5
32,000 281.29 5.7
I have been struggling with this for days. Is there an explanation for
why the following GIMPLE is resulting in that N-squared behavior?
I agree that the conditionals are a bit convoluted, and Jim and I are
working to do COMPUTE in a much improved way. But I still feel a need to
understand what's going on!
Thanks so much for any insight into what might be happening here.
_intermediate__stack501_503.0.0.data = &_stack501_data_517.0;
_intermediate__stack501_503.0.0.capacity = 16;
_intermediate__stack501_503.0.0.allocated = 16;
_intermediate__stack501_503.0.0.offset = 0;
_intermediate__stack501_503.0.0.name = &"_stack501"[0];
_intermediate__stack501_503.0.0.picture = &""[0];
_intermediate__stack501_503.0.0.initial = 0B;
_intermediate__stack501_503.0.0.parent = 0B;
_intermediate__stack501_503.0.0.occurs_lower = 0;
_intermediate__stack501_503.0.0.occurs_upper = 0;
_intermediate__stack501_503.0.0.attr = 4160;
_intermediate__stack501_503.0.0.type = 6;
_intermediate__stack501_503.0.0.level = 0;
_intermediate__stack501_503.0.0.digits = 37;
_intermediate__stack501_503.0.0.rdigits = 0;
_intermediate__stack501_503.0.0.encoding = 1;
_intermediate__stack501_503.0.0.alphabet = 0;
D.2812 = 0;
D.2813 = 0;
_1013 = D.2812 & 18;
if (_1013 != 0) goto <D.9353>; else goto <D.9354>;
<D.9353>:
goto <D.9355>;
<D.9354>:
D.2814 = 0;
..pa_erf.1519_1014 = ..pa_erf;
D.2813 = D.2813 | ..pa_erf.1519_1014;
if (D.2813 != 0) goto <D.9357>; else goto <D.9358>;
<D.9357>:
goto <D.9359>;
<D.9358>:
<D.9359>:
<D.9355>:
