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

[Michael Zolotukhin via llvm-commits]

Thanks for the reproducer, I’ll take a look!

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
>>> +}
>>> 
>>> 
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>>> 
> <bbi-15872.ll>