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

[Duncan P. N. Exon Smith]

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...

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)) {
     // 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}