[LLVMdev] Failed to Unroll a Seemingly Simple Loop

[Jingyue Wu]

The FIXME at ScalarEvolution.cpp:4084 explains LoopUnroll's behavior on my
example. I'll try to fix it.

Jingyue


On Sun, Jun 22, 2014 at 1:42 PM, Jingyue Wu <jingyue at google.com> wrote:

> 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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