[llvm] r231100 - Fix PR22750: non-determinism causes assertion failure in DWARF generation

Dario Domizioli dario.domizioli at gmail.com
Tue Mar 3 10:40:53 PST 2015

Author: ddomizioli
Date: Tue Mar  3 12:40:53 2015
New Revision: 231100

URL: http://llvm.org/viewvc/llvm-project?rev=231100&view=rev
Fix PR22750: non-determinism causes assertion failure in DWARF generation

The cause of the issue is the interaction of two factors:
1) When generating a DW_TAG_imported_declaration DIE which imports another 
   imported declaration, the code in AsmPrinter/DwarfCompileUnit.cpp 
   asserts that the second imported declaration must already have a DIE.
2) There is a non-determinism in the order in which imported declarations
   within the same scope are processed.
Because of the non-determinism (2), it is possible that an imported 
declaration is processed before another one it depends on, breaking the 
assumption in (1).

The source of the non-determinism is that the imported declaration 
DIDescriptors are sorted by scope in DwarfDebug::beginModule(); however that
sort is not a stable_sort, therefore the order of the declarations within 
the same scope is not preserved. The attached patch changes the std::sort to
a std::stable_sort and it fixes the problem.

Test omitted due to it being non-deterministic and depending on the
implementation of std::sort.


Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp?rev=231100&r1=231099&r2=231100&view=diff
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp Tue Mar  3 12:40:53 2015
@@ -477,8 +477,11 @@ void DwarfDebug::beginModule() {
-    std::sort(ScopesWithImportedEntities.begin(),
-              ScopesWithImportedEntities.end(), less_first());
+    // Stable sort to preserve the order of appearance of imported entities.
+    // This is to avoid out-of-order processing of interdependent declarations
+    // within the same scope, e.g. { namespace A = base; namespace B = A; }
+    std::stable_sort(ScopesWithImportedEntities.begin(),
+                     ScopesWithImportedEntities.end(), less_first());
     DIArray GVs = CUNode.getGlobalVariables();
     for (unsigned i = 0, e = GVs.getNumElements(); i != e; ++i)

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