[llvm] r321653 - [BasicBlockUtils] Check for unreachable preds before updating LI in UpdateAnalysisInformation

[Volkan Keles via llvm-commits]

Hi Anna,

Yes, LI is computed using DT, but this doesn’t mean DT is required. Here is what llc with the option `-debug-pass=Executions` produces:

[2018-01-03 11:24:29.528119000] 0x7f90546035a0     Executing Pass 'Dominator Tree Construction' on Function ‘foo’…
…
[2018-01-03 11:24:29.528212000] 0x7f90546035a0     Executing Pass 'Natural Loop Information' on Function ‘foo’…
…
[2018-01-03 11:24:29.528834000] 0x7f90546035a0      Freeing Pass 'Dominator Tree Construction' on Function ‘foo’…

So, DominatorTree may not be available at this point. I think DT shouldn’t be optional in this function if it’s required.

Volkan

> On Jan 3, 2018, at 6:09 AM, Anna Thomas <anna at azul.com> wrote:
> 
>> 
>> On Jan 2, 2018, at 10:06 PM, Friedman, Eli <efriedma at codeaurora.org <mailto:efriedma at codeaurora.org>> wrote:
>> 
>> On 1/2/2018 3:45 PM, Volkan Keles via llvm-commits wrote:
>>> +llvm-commits
>>> 
>>>> On Jan 2, 2018, at 8:25 AM, Anna Thomas via llvm-commits <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
>>>> 
>>>> Author: annat
>>>> Date: Tue Jan  2 08:25:50 2018
>>>> New Revision: 321653
>>>> 
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=321653&view=rev <http://llvm.org/viewvc/llvm-project?rev=321653&view=rev>
>>>> Log:
>>>> [BasicBlockUtils] Check for unreachable preds before updating LI in UpdateAnalysisInformation
>>>> 
>>>> Summary:
>>>> We are incorrectly updating the LI when loop-simplify generates
>>>> dedicated exit blocks for a loop. The issue is that there's an implicit
>>>> assumption that the Preds passed into UpdateAnalysisInformation are
>>>> reachable. However, this is not true and breaks LI by incorrectly
>>>> updating the header of a loop.
>>>> 
>>>> One such case is when we generate dedicated exits when the exit block is
>>>> a landing pad (through SplitLandingPadPredecessors). There maybe other
>>>> cases as well, since we do not guarantee that Preds passed in are
>>>> reachable basic blocks.
>>>> 
>>>> The added test case shows how loop-simplify breaks LI for the outer loop (and DT in turn)
>>>> after we try to generate the LoopSimplifyForm.
>>>> 
>>>> Reviewers: davide, chandlerc, sanjoy
>>>> 
>>>> Reviewed By: davide
>>>> 
>>>> Subscribers: llvm-commits
>>>> 
>>>> Differential Revision: https://reviews.llvm.org/D41519 <https://reviews.llvm.org/D41519>
>>>> 
>>>> Modified:
>>>>    llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp
>>>>    llvm/trunk/test/Transforms/LoopSimplify/unreachable-loop-pred.ll
>>>> 
>>>> Modified: llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp
>>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp?rev=321653&r1=321652&r2=321653&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp?rev=321653&r1=321652&r2=321653&view=diff>
>>>> ==============================================================================
>>>> --- llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp (original)
>>>> +++ llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp Tue Jan  2 08:25:50 2018
>>>> @@ -323,6 +323,12 @@ static void UpdateAnalysisInformation(Ba
>>>>   bool IsLoopEntry = !!L;
>>>>   bool SplitMakesNewLoopHeader = false;
>>>>   for (BasicBlock *Pred : Preds) {
>>>> +    // Preds that are not reachable from entry should not be used to identify if
>>>> +    // OldBB is a loop entry or if SplitMakesNewLoopHeader. Unreachable blocks
>>>> +    // are not within any loops, so we incorrectly mark SplitMakesNewLoopHeader
>>>> +    // as true and make the NewBB the header of some loop. This breaks LI.
>>>> +    if (!DT->isReachableFromEntry(Pred))
>>> Hi Anna,
>>> 
>>> This change breaks our internal bots because DT might be nullptr as it is optional. Is there another way to check this?
>> 
>> This code only executes if the LoopInfo pointer is non-null, and LoopInfo requires a DominatorTree to compute, so you probably have a domtree somewhere even if your pass doesn't explicitly require it.
> Agree with Eli here. If LI is being passed in to this method, it makes sense to pass in the DT as well (which exists because LI was computed using DT). 
> Reachability analysis without DT would be from first principles because LI does not contain that information. 
> 
> 
> Anna
>> 
>> -Eli
>> 
>> -- 
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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