[analyzer] Fix symbolic element index lifetime.

SymbolReaper was destroying the symbol too early when it was referenced only
from an index SVal of a live ElementRegion.

In order to test certain aspects of this patch, extend the debug.ExprInspection
checker to allow testing SymbolReaper in a direct manner.

Differential Revision: http://reviews.llvm.org/D12726

llvm-svn: 255236

1 files changed, 23 insertions, 0 deletions

diff --git a/clang/docs/analyzer/DebugChecks.rst b/clang/docs/analyzer/DebugChecks.rst
index 14d6ae4c4c9..771e39fc439 100644
--- a/clang/docs/analyzer/DebugChecks.rst
+++ b/clang/docs/analyzer/DebugChecks.rst
@@ -138,6 +138,29 @@ ExprInspection checks
       clang_analyzer_warnIfReached();  // no-warning
     }
 
+- void clang_analyzer_warnOnDeadSymbol(int);
+
+  Subscribe for a delayed warning when the symbol that represents the value of
+  the argument is garbage-collected by the analyzer.
+
+  When calling 'clang_analyzer_warnOnDeadSymbol(x)', if value of 'x' is a
+  symbol, then this symbol is marked by the ExprInspection checker. Then,
+  during each garbage collection run, the checker sees if the marked symbol is
+  being collected and issues the 'SYMBOL DEAD' warning if it does.
+  This way you know where exactly, up to the line of code, the symbol dies.
+
+  It is unlikely that you call this function after the symbol is already dead,
+  because the very reference to it as the function argument prevents it from
+  dying. However, if the argument is not a symbol but a concrete value,
+  no warning would be issued.
+
+  Example usage::
+
+    do {
+      int x = generate_some_integer();
+      clang_analyzer_warnOnDeadSymbol(x);
+    } while(0);  // expected-warning{{SYMBOL DEAD}}
+
 
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