[llvm] r265337 - Enable unroll for constant bound loops when TripCount is not modulo of unroll factor, reducing it to maximum power-of-2 that satisfies threshold limit.
via llvm-commits
llvm-commits at lists.llvm.org
Mon Apr 4 13:10:24 PDT 2016
What if we don’t want this behavior?
if (Count <= 1) {
// If there is no Count that is modulo of TripCount, set Count to
// largest power-of-two factor that satisfies the threshold limit.
Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
UnrolledSize = (LoopSize - 2) * Count + 2;
while (Count != 0 && UnrolledSize > UP.PartialThreshold) {
Count >>= 1;
UnrolledSize = (LoopSize - 2) * Count + 2;
}
}
Previously, partial unrolling was always modulo the trip count; this allows partial unrolling to duplicate the loop, which we don’t want. What should we do if we don’t want this behavior?
—escha
> On Apr 4, 2016, at 1:02 PM, Evgeny Stupachenko via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>
> When the a TripCount is not modulo of Count (unroll factor) we jump
> into the middle of a loop:
>
> i = 0;
> while(1) {
> a[i] = i;
> i++;
> if (i >= 15) break;
> }
>
> Unrolled by 2 into:
>
> i = 0;
> jump L1;
> while(1) {
> a[i] = i;
> i++;
> L1:
> a[i] = i;
> i++;
> if (i >= 15) break;
> }
>
> Here we check this type of unroll. It could be optimized by further
> passes, however unroll should do exactly this.
>
> Thanks,
> Evgeny
>
>
>
> On Mon, Apr 4, 2016 at 12:37 PM, Sean Silva via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>>
>>
>> On Mon, Apr 4, 2016 at 12:24 PM, Zia Ansari via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>>>
>>> Author: zansari
>>> Date: Mon Apr 4 14:24:46 2016
>>> New Revision: 265337
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=265337&view=rev
>>> Log:
>>> Enable unroll for constant bound loops when TripCount is not modulo of
>>> unroll factor, reducing it to maximum power-of-2 that satisfies threshold
>>> limit.
>>>
>>> Commit for Evgeny Stupachenko (evstupac at gmail.com)
>>>
>>> Differential Revision: http://reviews.llvm.org/D18290
>>>
>>>
>>> Added:
>>> llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll
>>> Modified:
>>> llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
>>>
>>> Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp?rev=265337&r1=265336&r2=265337&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp (original)
>>> +++ llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp Mon Apr 4
>>> 14:24:46 2016
>>> @@ -639,6 +639,16 @@ static bool tryToUnrollLoop(Loop *L, Dom
>>> Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
>>> while (Count != 0 && TripCount % Count != 0)
>>> Count--;
>>> + if (Count <= 1) {
>>> + // If there is no Count that is modulo of TripCount, set Count to
>>> + // largest power-of-two factor that satisfies the threshold
>>> limit.
>>> + Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
>>> + UnrolledSize = (LoopSize - 2) * Count + 2;
>>> + while (Count != 0 && UnrolledSize > UP.PartialThreshold) {
>>> + Count >>= 1;
>>> + UnrolledSize = (LoopSize - 2) * Count + 2;
>>> + }
>>> + }
>>> }
>>> } else if (Unrolling == Runtime) {
>>> if (!AllowRuntime && !CountSetExplicitly) {
>>>
>>> Added:
>>> llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll?rev=265337&view=auto
>>>
>>> ==============================================================================
>>> --- llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll
>>> (added)
>>> +++ llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll
>>> Mon Apr 4 14:24:46 2016
>>> @@ -0,0 +1,29 @@
>>> +; RUN: opt < %s -S -unroll-threshold=20 -loop-unroll
>>> -unroll-allow-partial | FileCheck %s
>>> +
>>> +; The Loop TripCount is 9. However unroll factors 3 or 9 exceed given
>>> threshold.
>>> +; The test checks that we choose a smaller, power-of-two, unroll count
>>> and do not give up on unrolling.
>>> +
>>> +; CHECK: for.body:
>>> +; CHECK: store
>>> +; CHECK: for.body.1:
>>> +; CHECK: store
>>
>>
>> Could these checks be tightened up? E.g. to verify that the bodies are
>> actually unrolled?
>>
>> -- Sean Silva
>>
>>>
>>> +
>>> +define void @foo(i32* nocapture %a, i32* nocapture readonly %b) nounwind
>>> uwtable {
>>> +entry:
>>> + br label %for.body
>>> +
>>> +for.body: ; preds = %for.body,
>>> %entry
>>> + %indvars.iv = phi i64 [ 1, %entry ], [ %indvars.iv.next, %for.body ]
>>> + %arrayidx = getelementptr inbounds i32, i32* %b, i64 %indvars.iv
>>> + %ld = load i32, i32* %arrayidx, align 4
>>> + %idxprom1 = sext i32 %ld to i64
>>> + %arrayidx2 = getelementptr inbounds i32, i32* %a, i64 %idxprom1
>>> + %st = trunc i64 %indvars.iv to i32
>>> + store i32 %st, i32* %arrayidx2, align 4
>>> + %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>>> + %exitcond = icmp eq i64 %indvars.iv.next, 10
>>> + br i1 %exitcond, label %for.end, label %for.body
>>> +
>>> +for.end: ; preds = %for.body
>>> + ret void
>>> +}
>>>
>>>
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>>
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