[PATCH, Loop optimizer]: Add logic to disable certain loop optimizations on pre-/post-loops

[Fang, Changpeng]

My initial intention is Not to unroll prologue and epilogue loops. An estimated trip count
may not be that useful for the unrolling decision. To me, unrolling a loop that has at most 
3 (or 7) iterations does not make sense. RTL unrolling does not use the estimated trip 
count to determine the unroll factor, and thus it may still unroll the loop 4 or 8 times if
the loop is small ( #insns). To make things simple, we just don't unroll such loops.

However, a prologue or epilogue loop may still be a hot loop, depending on the outer 
loops. It may still be beneficial to perform other optimizations on such loops, if the code
size is not expanded multiple times.

For prefetching of prologue or epilogue loops, we have two choices (1) prefetching not 
not unrolling, (2) not prefetching.  Which one do you prefer?

Thanks,

Changpeng



________________________________________
From: Zdenek Dvorak [rakdver@kam.mff.cuni.cz]
Sent: Thursday, December 16, 2010 6:09 AM
To: Richard Guenther
Cc: Xinliang David Li; Fang, Changpeng; gcc-patches@gcc.gnu.org
Subject: Re: [PATCH, Loop optimizer]: Add logic to disable certain loop optimizations on pre-/post-loops

Hi,

> Btw, any reason why we do not use static profiles for number of iteration
> estimates?  We after all _do_ use the static profile to guide the
> maybe_hot/cold_bb tests.

for loops for that we cannot determine the # of iterations statically,
basically the only important predictors are PRED_LOOP_BRANCH and
PRED_LOOP_EXIT, which predict that the loop will iterate about 10 times.  So,
by using static profile, we would just learn that every such loop is expected
to iterate 10 times, which is kind of useless,

Zdenek