Make iteration over the DeclContext::lookup_result safe.

The idiom:
```
DeclContext::lookup_result R = DeclContext::lookup(Name);
for (auto *D : R) {...}
```

is not safe when in the loop body we trigger deserialization from an AST file.
The deserialization can insert new declarations in the StoredDeclsList whose
underlying type is a vector. When the vector decides to reallocate its storage
the pointer we hold becomes invalid.

This patch replaces a SmallVector with an singly-linked list. The current
approach stores a SmallVector<NamedDecl*, 4> which is around 8 pointers.
The linked list is 3, 5, or 7. We do better in terms of memory usage for small
cases (and worse in terms of locality -- the linked list entries won't be near
each other, but will be near their corresponding declarations, and we were going
to fetch those memory pages anyway). For larger cases: the vector uses a
doubling strategy for reallocation, so will generally be between half-full and
full. Let's say it's 75% full on average, so there's N * 4/3 + 4 pointers' worth
of space allocated currently and will be 2N pointers with the linked list. So we
break even when there are N=6 entries and slightly lose in terms of memory usage
after that. We suspect that's still a win on average.

Thanks to @rsmith!

Differential revision: https://reviews.llvm.org/D91524

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
index 38a9d4b..c8b2e3d 100644
--- a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
@@ -842,7 +842,7 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
     llvm::Optional<QualType> operator()(StringRef Name) {
       IdentifierInfo &II = ACtx.Idents.get(Name);
       auto LookupRes = ACtx.getTranslationUnitDecl()->lookup(&II);
-      if (LookupRes.size() == 0)
+      if (LookupRes.empty())
         return None;
 
       // Prioritze typedef declarations.
@@ -994,7 +994,7 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
         return false;
       IdentifierInfo &II = ACtx.Idents.get(Name);
       auto LookupRes = ACtx.getTranslationUnitDecl()->lookup(&II);
-      if (LookupRes.size() == 0)
+      if (LookupRes.empty())
         return false;
       for (Decl *D : LookupRes) {
         if (auto *FD = dyn_cast<FunctionDecl>(D)) {