H. S. Teoh:
I've seen gcc/gdc unroll loops with unknown number of
iterations, esp. when you're using -O3. It just unrolls into
something
like:
loop_start:
if (!loopCondition) goto end;
loopBody();
if (!loopCondition) goto end;
loopBody();
if (!loopCondition) goto end;
loopBody();
if (!loopCondition) goto end;
loopBody();
goto loop_start;
end: ...
I'm pretty sure I've seen gcc/gdc do this before.
deadalnix:
LLVM is also able to generate Duff's device on the fly for
loops where it make sense.
I have not seen this optimization done on my code (both ldc2 and
gcc), but I am glad to be wrong on this.
The OracleVM uses a very different unrolling strategy: it splits
the loop in two loops, the first loop has 2, 4 (or sometimes 8
times) unrolling and it doesn't contain tests beside one at the
start and end, followed by a second normal (not unrolled) loop of
the remaining n % 8 times.
I was able to reach the same performance as Java using this
strategy manually in D using ldc2.
Bye,
bearophile
