[llvm] r335675 - [JumpThreading] Don't try to rewrite a use if it's already valid.

Tue Jul 3 13:12:41 PDT 2018

Hi Mikael,

Sorry, got pulled away by other stuff, I’ll try to investigate this asap.

Michael

> On Jul 3, 2018, at 12:04 AM, Mikael Holmén <mikael.holmen at ericsson.com> wrote:
> 
> Hi,
> 
> On 06/29/2018 05:28 PM, Michael Zolotukhin wrote:
>> Thanks for the reproducer, I’ll take a look!
> 
> Sounds good! Did you get anywhere with this?
> 
> Regards,
> Mikael
> 
>> Michael
>>> On Jun 29, 2018, at 2:36 AM, Mikael Holmén <mikael.holmen at ericsson.com> wrote:
>>> 
>>> Aaaand here comes the reproducer as well.
>>> 
>>> /Mikael
>>> 
>>> On 06/29/2018 11:36 AM, Mikael Holmén wrote:
>>>> Hi Michael,
>>>> I get a miscompile from -jump-threading with this patch:
>>>> Reproduce with:
>>>>  opt -jump-threading -S -o - bbi-15872.ll
>>>> %check should be 0 first time we get to %if.end, then we should jump to %label, then %if.end, set %check to 1, and jump to %if.end2 where we should pass %check (which is now 1) to @g.
>>>> But after jump-threading the whole thing is replaced with just
>>>>   call void @g(i16 0)
>>>> If I revert this commit I instead get the expected
>>>>   call void @g(i16 1)
>>>> Regards,
>>>> Mikael
>>>> On 06/27/2018 12:19 AM, Michael Zolotukhin via llvm-commits wrote:
>>>>> Author: mzolotukhin
>>>>> Date: Tue Jun 26 15:19:48 2018
>>>>> New Revision: 335675
>>>>> 
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=335675&view=rev
>>>>> Log:
>>>>> [JumpThreading] Don't try to rewrite a use if it's already valid.
>>>>> 
>>>>> Summary:
>>>>> When recording uses we need to rewrite after cloning a loop we need to
>>>>> check if the use is not dominated by the original def. The initial
>>>>> assumption was that the cloned basic block will introduce a new path and
>>>>> thus the original def will only dominate the use if they are in the same
>>>>> BB, but as the reproducer from PR37745 shows it's not always the case.
>>>>> 
>>>>> This fixes PR37745.
>>>>> 
>>>>> Reviewers: haicheng, Ka-Ka
>>>>> 
>>>>> Subscribers: hiraditya, llvm-commits
>>>>> 
>>>>> Differential Revision: https://reviews.llvm.org/D48111
>>>>> 
>>>>> Added:
>>>>>      llvm/trunk/test/Transforms/JumpThreading/PR37745.ll
>>>>> Modified:
>>>>>      llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
>>>>> 
>>>>> Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
>>>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=335675&r1=335674&r2=335675&view=diff
>>>>> ==============================================================================
>>>>> --- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
>>>>> +++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Tue Jun 26 15:19:48 2018
>>>>> @@ -2009,6 +2009,14 @@ bool JumpThreadingPass::ThreadEdge(Basic
>>>>>         PredTerm->setSuccessor(i, NewBB);
>>>>>       }
>>>>> +  // Enqueue required DT updates.
>>>>> +  DDT->applyUpdates({{DominatorTree::Insert, NewBB, SuccBB},
>>>>> +                     {DominatorTree::Insert, PredBB, NewBB},
>>>>> +                     {DominatorTree::Delete, PredBB, BB}});
>>>>> +
>>>>> +  // Apply all updates we queued with DDT and get the updated Dominator Tree.
>>>>> +  DominatorTree *DT = &DDT->flush();
>>>>> +
>>>>>     // If there were values defined in BB that are used outside the block, then we
>>>>>     // now have to update all uses of the value to use either the original value,
>>>>>     // the cloned value, or some PHI derived value.  This can require arbitrary
>>>>> @@ -2018,16 +2026,16 @@ bool JumpThreadingPass::ThreadEdge(Basic
>>>>>     for (Instruction &I : *BB) {
>>>>>       SmallVector<Use*, 16> UsesToRename;
>>>>> -    // Scan all uses of this instruction to see if it is used outside of its
>>>>> -    // block, and if so, record them in UsesToRename. Also, skip phi operands
>>>>> -    // from PredBB - we'll remove them anyway.
>>>>> +    // Scan all uses of this instruction to see if their uses are no longer
>>>>> +    // dominated by the previous def and if so, record them in UsesToRename.
>>>>> +    // Also, skip phi operands from PredBB - we'll remove them anyway.
>>>>>       for (Use &U : I.uses()) {
>>>>>         Instruction *User = cast<Instruction>(U.getUser());
>>>>>         if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
>>>>>           if (UserPN->getIncomingBlock(U) == BB ||
>>>>>               UserPN->getIncomingBlock(U) == PredBB)
>>>>>             continue;
>>>>> -      } else if (User->getParent() == BB)
>>>>> +      } else if (DT->dominates(&I, U))
>>>>>           continue;
>>>>>         UsesToRename.push_back(&U);
>>>>> @@ -2046,12 +2054,6 @@ bool JumpThreadingPass::ThreadEdge(Basic
>>>>>         SSAUpdate.AddUse(VarNum, U);
>>>>>     }
>>>>> -  DDT->applyUpdates({{DominatorTree::Insert, NewBB, SuccBB},
>>>>> -                     {DominatorTree::Insert, PredBB, NewBB},
>>>>> -                     {DominatorTree::Delete, PredBB, BB}});
>>>>> -
>>>>> -  // Apply all updates we queued with DDT and get the updated Dominator Tree.
>>>>> -  DominatorTree *DT = &DDT->flush();
>>>>>     SSAUpdate.RewriteAllUses(DT);
>>>>>     // At this point, the IR is fully up to date and consistent.  Do a quick scan
>>>>> 
>>>>> Added: llvm/trunk/test/Transforms/JumpThreading/PR37745.ll
>>>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/PR37745.ll?rev=335675&view=auto
>>>>> ==============================================================================
>>>>> --- llvm/trunk/test/Transforms/JumpThreading/PR37745.ll (added)
>>>>> +++ llvm/trunk/test/Transforms/JumpThreading/PR37745.ll Tue Jun 26 15:19:48 2018
>>>>> @@ -0,0 +1,19 @@
>>>>> +; RUN: opt -jump-threading -verify-each -S -mtriple=x86_64-- -o - %s
>>>>> +
>>>>> +define void @foo() {
>>>>> +entry:
>>>>> +  br i1 false, label %A, label %B
>>>>> +
>>>>> +A:
>>>>> +  %x = phi i32 [ undef, %entry ], [ %z, %B ]
>>>>> +  br label %B
>>>>> +
>>>>> +B:
>>>>> +  %y = phi i32 [ undef, %entry ], [ %x, %A ]
>>>>> +  %z = add i32 %y, 1
>>>>> +  %cmp = icmp ne i32 %z, 0
>>>>> +  br i1 %cmp, label %exit, label %A
>>>>> +
>>>>> +exit:
>>>>> +  ret void
>>>>> +}
>>>>> 
>>>>> 
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>> 
>>> <bbi-15872.ll>
> 