[llvm] r243961 - Linker: Fix references to uniqued nodes after r243883

[Justin Bogner]

"Duncan P. N. Exon Smith" <dexonsmith at apple.com> writes:
> Author: dexonsmith
> Date: Tue Aug  4 01:42:31 2015
> New Revision: 243961
>
> URL: http://llvm.org/viewvc/llvm-project?rev=243961&view=rev
> Log:
> Linker: Fix references to uniqued nodes after r243883
>
> r243883 started moving 'distinct' nodes instead of duplicated them in
> lib/Linker.  This had the side-effect of sometimes not cloning uniqued
> nodes that reference them.  I missed a corner case:
>
>     !named = !{!0}
>     !0 = !{!1}
>     !1 = distinct !{!0}
>
> !0 is the entry point for "remapping", and a temporary clone (say,
> !0-temp) is created and mapped in case we need to model a uniquing
> cycle.
>
>     Recursive descent into !1.  !1 is distinct, so we leave it alone,
>     but update its operand to !0-temp.
>
> Pop back out to !0.  Its only operand, !1, hasn't changed, so we don't
> need to use !0-temp.  !0-temp goes out of scope, and we're finished
> remapping, but we're left with:
>
>     !named = !{!0}
>     !0 = !{!1}
>     !1 = distinct !{null} ; uh oh...
>
> Previously, if !0 and !0-temp ended up with identical operands, then
> !0-temp couldn't have been referenced at all.  Now that distinct nodes
> don't get duplicated, that assumption is invalid.  We need to
> !0-temp->replaceAllUsesWith(!0) before freeing !0-temp.
>
> I found this while running an internal `-flto -g` bootstrap.  Strangely,
> there was no case of this in the open source bootstrap I'd done before
> commit...

You probably got a mail from the bot, but if not it looks like msan is
unhappy about this change:

  http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/6041/steps/check-llvm%20msan/logs/stdio

>
> Added:
>     llvm/trunk/test/Linker/uniqued-distinct-cycles.ll
> Modified:
>     llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp
>
> Modified: llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp?rev=243961&r1=243960&r2=243961&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp (original)
> +++ llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp Tue Aug  4 01:42:31 2015
> @@ -258,9 +258,11 @@ static Metadata *mapUniquedNode(const MD
>  
>    // Create a temporary node upfront in case we have a metadata cycle.
>    auto ClonedMD = Node->clone();
> -  if (!remap(Node, ClonedMD.get(), Cycles, VM, Flags, TypeMapper, Materializer))
> +  if (!remap(Node, ClonedMD.get(), Cycles, VM, Flags, TypeMapper, Materializer)) {

80-col?

>      // No operands changed, so use the identity mapping.
> +    ClonedMD->replaceAllUsesWith(const_cast<MDNode *>(Node));
>      return mapToSelf(VM, Node);
> +  }
>  
>    // At least one operand has changed, so uniquify the cloned node.
>    return mapToMetadata(VM, Node,
>
> Added: llvm/trunk/test/Linker/uniqued-distinct-cycles.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Linker/uniqued-distinct-cycles.ll?rev=243961&view=auto
> ==============================================================================
> --- llvm/trunk/test/Linker/uniqued-distinct-cycles.ll (added)
> +++ llvm/trunk/test/Linker/uniqued-distinct-cycles.ll Tue Aug  4 01:42:31 2015
> @@ -0,0 +1,14 @@
> +; RUN: llvm-link -o - %s | llvm-dis | FileCheck %s
> +
> +; CHECK: !named = !{!0, !2}
> +!named = !{!0, !2}
> +
> +; CHECK:      !0 = !{!1}
> +; CHECK-NEXT: !1 = distinct !{!0}
> +!0 = !{!1}
> +!1 = distinct !{!0}
> +
> +; CHECK-NEXT: !2 = distinct !{!3}
> +; CHECK-NEXT: !3 = !{!2}
> +!2 = distinct !{!3}
> +!3 = !{!2}
>
>
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