[LLVMdev] Loop rotation and loop inversion in LLVM?

[Andrew Trick]

On May 16, 2013, at 5:07 PM, Paul Sokolovsky <pmiscml at gmail.com> wrote:

> Hello,
> 
> I'd be interested in knowing which pass performs loop inversion, i.e.
> transforms while loop into do/while wrapped with if. So, it's pretty
> easy to understand concept, http://en.wikipedia.org/wiki/Loop_inversion
> provides description of how its done and motivation, googling gives
> several relevant references, i.e. it's pretty settled term.
> 
> I also see this transform to be actually performed on trivial strlen
> function by clang -O2. However opt --help or grepping LLVM doesn't give
> any hints.
> 
> However, -loop-rotate calls attention, described as "A simple loop
> rotation transformation." However, Wikipedia doesn't gives hits for
> that related to compilation/optimization theory, nor google hits are
> relevant either - mostly LLVM-related hits just mentioning the term.
> 
> Trying -loop-rotate, I see loop being converted to post-condition, but
> don't see if wrapper around it.
> 
> So, can anyone suggest if LLVM loop rotation is related to loop
> inversion in Wikipedia terms, and if not, what it is.

On simple ‘for’ loops, rotation degenerates to inversion. Rotation is a more general transform that spans the range from inversion to loop peeling...

loop:
A
br X
B
br loop, Y

A’
br X
loop:
B
br Y
A
br loop, X

Sorry I don’t know of a text-book reference off-hand. I’ve seen it in practice before and assumed it was pretty standard. In LLVM it’s mostly used to put loops in a canonical form, but it’s also a cheap and dirty way to expose LICM. Another benefit is simplifying trip count expressions.

> And I hope that this feedback will allow maintainers to make
> documentation clearer and more user-friendly.

Me too :) Not sure if I’ll get around to it, but I’d be happy to review a patch.

-Andy

> Thanks,
> Paul                          mailto:pmiscml at gmail.com
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