[ThinLTO] Add lookup to ImportListsTy

[kazutakahirata]

This is primarily to unblock Rust, which could potentially use
ImportListsTy::operator[] on a module that's not in ListsImpl and
cause concurrency problems.

This patch fixes a regression in the sense that it restores
ImportListsTy::lookup, which was available when ImportListsTy was just
a plain DenseMap.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Kazu Hirata (kazutakahirata)

Changes

This is primarily to unblock Rust, which could potentially use
ImportListsTy::operator[] on a module that's not in ListsImpl and
cause concurrency problems.

This patch fixes a regression in the sense that it restores
ImportListsTy::lookup, which was available when ImportListsTy was just
a plain DenseMap.

---

Full diff: https://github.com/llvm/llvm-project/pull/109036.diff

1 Files Affected:

• (modified) llvm/include/llvm/Transforms/IPO/FunctionImport.h (+10-2)

diff --git a/llvm/include/llvm/Transforms/IPO/FunctionImport.h b/llvm/include/llvm/Transforms/IPO/FunctionImport.h
index 70739709a810ab..4b29d3f40ab7b5 100644
--- a/llvm/include/llvm/Transforms/IPO/FunctionImport.h
+++ b/llvm/include/llvm/Transforms/IPO/FunctionImport.h
@@ -270,13 +270,20 @@ class FunctionImporter {
   // A map from destination modules to lists of imports.
   class ImportListsTy {
   public:
-    ImportListsTy() = default;
-    ImportListsTy(size_t Size) : ListsImpl(Size) {}
+    ImportListsTy() : EmptyList(ImportIDs) {}
+    ImportListsTy(size_t Size) : EmptyList(ImportIDs), ListsImpl(Size) {}
 
     ImportMapTy &operator[](StringRef DestMod) {
       return ListsImpl.try_emplace(DestMod, ImportIDs).first->second;
     }
 
+    const ImportMapTy &lookup(StringRef DestMod) const {
+      auto It = ListsImpl.find(DestMod);
+      if (It != ListsImpl.end())
+        return It->second;
+      return EmptyList;
+    }
+
     size_t size() const { return ListsImpl.size(); }
 
     using const_iterator = DenseMap<StringRef, ImportMapTy>::const_iterator;
@@ -284,6 +291,7 @@ class FunctionImporter {
     const_iterator end() const { return ListsImpl.end(); }
 
   private:
+    ImportMapTy EmptyList;
     DenseMap<StringRef, ImportMapTy> ListsImpl;
     ImportIDTable ImportIDs;
   };

[kazutakahirata]

Just for reference, this PR is in response to #106427 (comment).

[nikic]

LGTM

[mstorsjo]

When compiling with GCC (13, in Ubuntu 24.04), this change added a rather noisy warning; as this issue is in a header, it gets printed a large number of times while building:

In file included from ../include/llvm/LTO/LTO.h:31,
                 from ../lib/LTO/LTO.cpp:13:
../include/llvm/Transforms/IPO/FunctionImport.h: In constructor ‘llvm::FunctionImporter::ImportListsTy::ImportListsTy()’:
../include/llvm/Transforms/IPO/FunctionImport.h:273:33: warning: member ‘llvm::FunctionImporter::ImportListsTy::ImportIDs’ is used uninitialized [-Wuninitialized]
  273 |     ImportListsTy() : EmptyList(ImportIDs) {}
      |                                 ^~~~~~~~~
../include/llvm/Transforms/IPO/FunctionImport.h: In constructor ‘llvm::FunctionImporter::ImportListsTy::ImportListsTy(size_t)’:
../include/llvm/Transforms/IPO/FunctionImport.h:274:44: warning: member ‘llvm::FunctionImporter::ImportListsTy::ImportIDs’ is used uninitialized [-Wuninitialized]
  274 |     ImportListsTy(size_t Size) : EmptyList(ImportIDs), ListsImpl(Size) {}
      |                                            ^~~~~~~~~