[PATCH] Fix PR18754: add early loop pass, 2nd try

[Giovanni Bajo]

Richard Guenther <rguenth@tat.physik.uni-tuebingen.de> wrote:

>> This patch adds an early loop pass (just after ch) just
>> containing complete unrolling.  This allows the following
>> SRA pass to scalarize small arrays that were manipulated
>> in constant rolling loops, as it often happens in C++ code.
>
> Merely the same as before, but guarded with -ftree-early-loop-optimize
> and proper documentation.  Disabled by default.

I'm actually against adding "secret" tree optimizations that only a few people
know about and use, it gives us some maintenance burden. Can you prove this
optimization is always useful? How does it affect SPEC? How does it affect
bootstrap times? How does it affect PR 8361 and POOMA, in both compile-time and
run-time?

For instance, if it is a little too expensive we could activate it only at -O3,
but I can't see numbers (again) supporting this patch.

Giovanni Bajo