[llvm-bugs] [Bug 42574] New: Non-deterministic result due to use of SmallPtrSet in llvm::removeUnreachableBlocks

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42574

            Bug ID: 42574
           Summary: Non-deterministic result due to use of SmallPtrSet in
                    llvm::removeUnreachableBlocks
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Transformation Utilities
          Assignee: unassignedbugs at nondot.org
          Reporter: mikael.holmen at ericsson.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 22222
  --> https://bugs.llvm.org/attachment.cgi?id=22222&action=edit
reproducer

Running

 opt -simplifycfg  -o foo.ll in3.ll -S --preserve-ll-uselistorder

a number of times with a clang 8 built opt we see that the produced output
differs now and then like:

 <   uselistorder i16 0, { 2, 3, 4, 1, 5, 0, 6 }
 ---
 >   uselistorder i16 0, { 3, 2, 4, 1, 5, 0, 6 }

I think it's the DeadBlockSet that is the cause:

-  // Loop over all of the basic blocks that are not reachable, dropping all of
-  // their internal references. Update DTU and LVI if available.
-  std::vector <DominatorTree::UpdateType> Updates;
+  SmallPtrSet<BasicBlock *, 16> DeadBlockSet;

As far as I can see, as long as a SmallPtrSet is "small", the iteration order
is deterministic (== insertion order), but I strongly suspect that when the set
grows to being "big", then it's not deterministic anymore.

DeadBlockSet as above was introduced in r339951:

    Update MemorySSA in Local utils removing blocks.

    Summary: Extend Local utils to update MemorySSA.

    Subscribers: sanjoy, jlebar, Prazek, george.burgess.iv, llvm-commits

    Differential Revision: https://reviews.llvm.org/D48790

    llvm-svn: 339951


Later this caused SCEV and LSR in llc to draw different conclusions and
produce different code.

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