[LLVMdev] Failed to Unroll a Seemingly Simple Loop

[Jingyue Wu]

Nice find, Mark! Thanks a lot.

I filed http://llvm.org/bugs/show_bug.cgi?id=20100 to track the remark
issue.

Jingyue


On Sun, Jun 22, 2014 at 11:53 AM, Mark Heffernan <meheff at google.com> wrote:

> On Sun, Jun 22, 2014 at 11:05 AM, Jingyue Wu <jingyue at google.com> wrote:
> > I found LLVM cannot unroll the loop in the example below, while gcc can.
> > Before I dig more about this issue, is this behavior as designed?
>
> It doesn't unroll the loop because the trip count is not known at
> compile time (it could exit early because of the break), and by
> default loops with runtime loop counts are not unrolled.  However,
> even specifying -unroll-runtime still doesn't get the loop to unroll.
> Looking deeper it looks like runtime loop unrolling is only performed
> if the loop has a single exit.  See:
> Transforms/Util/LoopUnroll.cpp:211 and
> Transforms/Util/LoopUnrollRuntime.cpp:231.
>
> It'd be nice if an optimization missed remark was emitted.  Certainly
> a remark should be emitted in the case where you give a unroll pragma
> to specifically unroll the loop.  Can you file a PR about this remark
> issue and assign it to me?
>
> thanks,
> Mark
>
> >
> > bool bar(int i);
> >
> > void foo(int *a, int x, int y) {
> >   for (int i = 0; i < 4; ++i) {
> >     if (bar(i)) {
> >       break;
> >     }
> >     a[i] = i;
> >   }
> > }
> >
> > Btw, if s/break/continue, LLVM is able to unroll it.
> >
> > Thanks,
> > Jingyue
> >
>
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