[llvm] r272224 - [LoopSimplify] Preserve LCSSA when merging exit blocks.

Wed Jun 22 23:09:24 PDT 2016

Hi,

On 06/23/2016 01:54 AM, Michael Zolotukhin wrote:
>
>> On Jun 22, 2016, at 1:25 AM, Mikael Holmén <mikael.holmen at ericsson.com> wrote:
>>
>> Hi Michael,
>>
>> The checks introduced in this commit fail on:
>>
>> opt -S -lcssa -loop-unroll red.ll
>>
>> I don't know if it's the checks or the optimization that is actually wrong.
> Hi Mikael,
>
> The checks are correct, they just exposed a bug that has been there for a long time. It seems that a routine separateNestedLoop in LoopSimplify breaks LCSSA - the test you provided shows exactly that. I’ll look into how we can fix it.

Great!

> Does it block you in any way?

Well we see the checks failing quite alot in our testing now, so I 
simply turned off the preservation of LCSSA in LoopSimplify as a 
workaround that I hope is ok until the bug is fixed for real.

   // Temporarily turn off preservation of LCSSA to prevent #10729 from
   // triggering all the time.
#if 0
   AU.addPreservedID(LCSSAID);
#endif

I suppose this should be ok?

Thanks,
Mikael

>
> Michael
>>
>> Regards,
>> Mikael
>>
>> On 06/09/2016 01:13 AM, Michael Zolotukhin via llvm-commits wrote:
>>> Author: mzolotukhin
>>> Date: Wed Jun  8 18:13:21 2016
>>> New Revision: 272224
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=272224&view=rev
>>> Log:
>>> [LoopSimplify] Preserve LCSSA when merging exit blocks.
>>>
>>> Summary:
>>> This fixes PR26682. Also add LCSSA as a preserved pass to LoopSimplify,
>>> that looks correct to me and allows to write a test for the issue.
>>>
>>> Reviewers: chandlerc, bogner, sanjoy
>>>
>>> Subscribers: llvm-commits
>>>
>>> Differential Revision: http://reviews.llvm.org/D21112
>>>
>>> Added:
>>>      llvm/trunk/test/Transforms/LoopSimplify/pr26682.ll
>>> Modified:
>>>      llvm/trunk/lib/Transforms/Utils/LoopSimplify.cpp
>>>
>>> Modified: llvm/trunk/lib/Transforms/Utils/LoopSimplify.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopSimplify.cpp?rev=272224&r1=272223&r2=272224&view=diff
>>> ==============================================================================
>>> --- llvm/trunk/lib/Transforms/Utils/LoopSimplify.cpp (original)
>>> +++ llvm/trunk/lib/Transforms/Utils/LoopSimplify.cpp Wed Jun  8 18:13:21 2016
>>> @@ -686,8 +686,10 @@ ReprocessLoop:
>>>         }
>>>         DT->eraseNode(ExitingBlock);
>>>
>>> -      BI->getSuccessor(0)->removePredecessor(ExitingBlock);
>>> -      BI->getSuccessor(1)->removePredecessor(ExitingBlock);
>>> +      BI->getSuccessor(0)->removePredecessor(
>>> +          ExitingBlock, /* DontDeleteUselessPHIs */ PreserveLCSSA);
>>> +      BI->getSuccessor(1)->removePredecessor(
>>> +          ExitingBlock, /* DontDeleteUselessPHIs */ PreserveLCSSA);
>>>         ExitingBlock->eraseFromParent();
>>>       }
>>>     }
>>> @@ -748,6 +750,7 @@ namespace {
>>>         AU.addPreserved<GlobalsAAWrapperPass>();
>>>         AU.addPreserved<ScalarEvolutionWrapperPass>();
>>>         AU.addPreserved<SCEVAAWrapperPass>();
>>> +      AU.addPreservedID(LCSSAID);
>>>         AU.addPreserved<DependenceAnalysisWrapperPass>();
>>>         AU.addPreservedID(BreakCriticalEdgesID);  // No critical edges added.
>>>       }
>>> @@ -781,11 +784,27 @@ bool LoopSimplify::runOnFunction(Functio
>>>     SE = SEWP ? &SEWP->getSE() : nullptr;
>>>     AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
>>>     bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
>>> +#ifndef NDEBUG
>>> +  if (PreserveLCSSA) {
>>> +    assert(DT && "DT not available.");
>>> +    assert(LI && "LI not available.");
>>> +    bool InLCSSA =
>>> +        all_of(*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT); });
>>> +    assert(InLCSSA && "Requested to preserve LCSSA, but it's already broken.");
>>> +  }
>>> +#endif
>>>
>>>     // Simplify each loop nest in the function.
>>>     for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
>>>       Changed |= simplifyLoop(*I, DT, LI, SE, AC, PreserveLCSSA);
>>>
>>> +#ifndef NDEBUG
>>> +  if (PreserveLCSSA) {
>>> +    bool InLCSSA =
>>> +        all_of(*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT); });
>>> +    assert(InLCSSA && "LCSSA is broken after loop-simplify.");
>>> +  }
>>> +#endif
>>>     return Changed;
>>>   }
>>>
>>>
>>> Added: llvm/trunk/test/Transforms/LoopSimplify/pr26682.ll
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopSimplify/pr26682.ll?rev=272224&view=auto
>>> ==============================================================================
>>> --- llvm/trunk/test/Transforms/LoopSimplify/pr26682.ll (added)
>>> +++ llvm/trunk/test/Transforms/LoopSimplify/pr26682.ll Wed Jun  8 18:13:21 2016
>>> @@ -0,0 +1,32 @@
>>> +; RUN: opt < %s -lcssa -loop-simplify -indvars -S | FileCheck %s
>>> +target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
>>> +target triple = "x86_64-unknown-unknown"
>>> +
>>> + at a = external global i32, align 4
>>> +
>>> +; Check that loop-simplify merges two loop exits, but preserves LCSSA form.
>>> +; CHECK-LABEL: @foo
>>> +; CHECK: for:
>>> +; CHECK: %or.cond = and i1 %cmp1, %cmp2
>>> +; CHECK-NOT: for.cond:
>>> +; CHECK: for.end:
>>> +; CHECK: %a.lcssa = phi i32 [ %a, %for ]
>>> +define i32 @foo(i32 %x) {
>>> +entry:
>>> +  br label %for
>>> +
>>> +for:
>>> +  %iv = phi i32 [ 0, %entry ], [ %iv.next, %for.cond ]
>>> +  %cmp1 = icmp eq i32 %x, 0
>>> +  %iv.next = add nuw nsw i32 %iv, 1
>>> +  %a = load i32, i32* @a
>>> +  br i1 %cmp1, label %for.cond, label %for.end
>>> +
>>> +for.cond:
>>> +  %cmp2 = icmp slt i32 %iv.next, 4
>>> +  br i1 %cmp2, label %for, label %for.end
>>> +
>>> +for.end:
>>> +  %a.lcssa = phi i32 [ %a, %for ], [ %a, %for.cond ]
>>> +  ret i32 %a.lcssa
>>> +}
>>>
>>>
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>>>
>> <red.ll>
>