[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.

[Evgeny Stupachenko via llvm-commits]

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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