TransformUtils: Fix metadata handling in CloneModule (and improve CloneFunctionInto)

This commit fixes how metadata is handled in CloneModule to be sound,
and improves how it's handled in CloneFunctionInto (although the latter
is still awkward when called within a module).

Ruiling Song pointed out in PR48841 that CloneModule was changed to
unsoundly use the RF_ReuseAndMutateDistinctMDs flag (renamed in
fa35c1f80f0ea080a7cbc581416929b0a654f25c for clarity). This flag papered
over a crash caused by other various changes made to CloneFunctionInto
over the past few years that made it unsound to use cloning between
different modules.

(This commit partially addresses PR48841, fixing the repro from
preprocessed source but not textual IR. MDNodeMapper::mapDistinctNode
became unsound in df763188c9a1ecb1e7e5c4d4ea53a99fbb755903 and this
commit does not address that regression.)

RF_ReuseAndMutateDistinctMDs is designed for the IRMover to use,
avoiding unnecessary clones of all referenced metadata when linking
between modules (with IRMover, the source module is discarded after
linking). It never makes sense to use when you're not discarding the
source. This commit drops its incorrect use in CloneModule.

Sadly, the right thing to do with metadata when cloning a function is
complicated, and this patch doesn't totally fix it.

The first problem is that there are two different types of referenceable
metadata and it's not obvious what to with one of them when remapping.

- `!0 = !{!1}` is metadata's version of a constant. Programatically it's
  called "uniqued" (probably a better term would be "constant") because,
  like `ConstantArray`, it's stored in uniquing tables. Once it's
  constructed, it's illegal to change its arguments.
- `!0 = distinct !{!1}` is a bit closer to a global variable. It's legal
  to change the operands after construction.

What should be done with distinct metadata when cloning functions within
the same module?

- Should new, cloned nodes be created?
- Should all references point to the same, old nodes?

The answer depends on whether that metadata is effectively owned by a
function.

And that's the second problem. Referenceable metadata's ownership model
is not clear or explicit. Technically, it's all stored on an
LLVMContext. However, any metadata that is `distinct`, that transitively
references a `distinct` node, or that transitively references a
GlobalValue is specific to a Module and is effectively owned by it. More
specifically, some metadata is effectively owned by a specific Function
within a module.

Effectively function-local metadata was introduced somewhere around
c10d0e5ccd12f049bddb24dcf8bbb7fbbc6c68f2, which made it illegal for two
functions to share a DISubprogram attachment.

When cloning a function within a module, you need to clone the
function-local debug info and suppress cloning of global debug info (the
status quo suppresses cloning some global debug info but not all). When
cloning a function to a new/different module, you need to clone all of
the debug info.

Here's what I think we should do (eventually? soon? not this patch
though):
- Distinguish explicitly (somehow) between pure constant metadata owned
  by the LLVMContext, global metadata owned by the Module, and local
  metadata owned by a GlobalValue (such as a function).
- Update CloneFunctionInto to trigger cloning of all "local" metadata
  (only), perhaps by adding a bit to RemapFlag. Alternatively, split
  out a separate function CloneFunctionMetadataInto to prime the
  metadata map that callers are updated to call ahead of time as
  appropriate.

Here's the somewhat more isolated fix in this patch:
- Converted the `ModuleLevelChanges` parameter to `CloneFunctionInto` to
  an enum called `CloneFunctionChangeType` that is one of
  LocalChangesOnly, GlobalChanges, DifferentModule, and ClonedModule.
- The code maintaining the "functions uniquely own subprograms"
  invariant is now only active in the first two cases, where a function
  is being cloned within a single module. That's necessary because this
  code inhibits cloning of (some) "global" metadata that's effectively
  owned by the module.
- The code maintaining the "all compile units must be explicitly
  referenced by !llvm.dbg.cu" invariant is now only active in the
  DifferentModule case, where a function is being cloned into a new
  module in isolation.
- CoroSplit.cpp's call to CloneFunctionInto in CoroCloner::create
  uses LocalChangeOnly, since fa635d730f74f3285b77cc1537f1692184b8bf5b
  only set `ModuleLevelChanges` to trigger cloning of local metadata.
- CloneModule drops its unsound use of RF_ReuseAndMutateDistinctMDs
  and special handling of !llvm.dbg.cu.
- Fixed some outdated header docs and left a couple of FIXMEs.

Differential Revision: https://reviews.llvm.org/D96531

1 files changed, 94 insertions, 46 deletions

diff --git a/llvm/lib/Transforms/Utils/CloneFunction.cpp b/llvm/lib/Transforms/Utils/CloneFunction.cpp
index a953ba5..dd3d535 100644
--- a/llvm/lib/Transforms/Utils/CloneFunction.cpp
+++ b/llvm/lib/Transforms/Utils/CloneFunction.cpp
@@ -83,8 +83,8 @@ BasicBlock *llvm::CloneBasicBlock(const BasicBlock *BB, ValueToValueMapTy &VMap,
 //
 void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
                              ValueToValueMapTy &VMap,
-                             bool ModuleLevelChanges,
-                             SmallVectorImpl<ReturnInst*> &Returns,
+                             CloneFunctionChangeType Changes,
+                             SmallVectorImpl<ReturnInst *> &Returns,
                              const char *NameSuffix, ClonedCodeInfo *CodeInfo,
                              ValueMapTypeRemapper *TypeMapper,
                              ValueMaterializer *Materializer) {
@@ -95,6 +95,8 @@ void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
     assert(VMap.count(&I) && "No mapping from source argument specified!");
 #endif
 
+  bool ModuleLevelChanges = Changes > CloneFunctionChangeType::LocalChangesOnly;
+
   // Copy all attributes other than those stored in the AttributeList.  We need
   // to remap the parameter indices of the AttributeList.
   AttributeList NewAttrs = NewFunc->getAttributes();
@@ -123,21 +125,37 @@ void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
       AttributeList::get(NewFunc->getContext(), OldAttrs.getFnAttributes(),
                          OldAttrs.getRetAttributes(), NewArgAttrs));
 
-  bool MustCloneSP =
-      OldFunc->getParent() && OldFunc->getParent() == NewFunc->getParent();
-  DISubprogram *SP = OldFunc->getSubprogram();
-  if (SP) {
-    assert(!MustCloneSP || ModuleLevelChanges);
-    // Add mappings for some DebugInfo nodes that we don't want duplicated
-    // even if they're distinct.
-    auto &MD = VMap.MD();
-    MD[SP->getUnit()].reset(SP->getUnit());
-    MD[SP->getType()].reset(SP->getType());
-    MD[SP->getFile()].reset(SP->getFile());
-    // If we're not cloning into the same module, no need to clone the
-    // subprogram
-    if (!MustCloneSP)
-      MD[SP].reset(SP);
+  // When we remap instructions within the same module, we want to avoid
+  // duplicating inlined DISubprograms, so record all subprograms we find as we
+  // duplicate instructions and then freeze them in the MD map. We also record
+  // information about dbg.value and dbg.declare to avoid duplicating the
+  // types.
+  Optional<DebugInfoFinder> DIFinder;
+
+  // Track the subprogram attachment that needs to be cloned to fine-tune the
+  // mapping within the same module.
+  DISubprogram *SPClonedWithinModule = nullptr;
+  if (Changes < CloneFunctionChangeType::DifferentModule) {
+    assert((NewFunc->getParent() == nullptr ||
+            NewFunc->getParent() == OldFunc->getParent()) &&
+           "Expected NewFunc to have the same parent, or no parent");
+
+    // Need to find subprograms, types, and compile units.
+    DIFinder.emplace();
+
+    SPClonedWithinModule = OldFunc->getSubprogram();
+  } else {
+    assert((NewFunc->getParent() == nullptr ||
+            NewFunc->getParent() != OldFunc->getParent()) &&
+           "Set SameModule to true if the new function is in the same module");
+
+    if (Changes == CloneFunctionChangeType::DifferentModule) {
+      assert(NewFunc->getParent() &&
+             "Need parent of new function to maintain debug info invariants");
+
+      // Need to find all the compile units.
+      DIFinder.emplace();
+    }
   }
 
   // Everything else beyond this point deals with function instructions,
@@ -145,13 +163,6 @@ void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
   if (OldFunc->isDeclaration())
     return;
 
-  // When we remap instructions, we want to avoid duplicating inlined
-  // DISubprograms, so record all subprograms we find as we duplicate
-  // instructions and then freeze them in the MD map.
-  // We also record information about dbg.value and dbg.declare to avoid
-  // duplicating the types.
-  DebugInfoFinder DIFinder;
-
   // Loop over all of the basic blocks in the function, cloning them as
   // appropriate.  Note that we save BE this way in order to handle cloning of
   // recursive functions into themselves.
@@ -161,7 +172,7 @@ void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
 
     // Create a new basic block and copy instructions into it!
     BasicBlock *CBB = CloneBasicBlock(&BB, VMap, NameSuffix, NewFunc, CodeInfo,
-                                      ModuleLevelChanges ? &DIFinder : nullptr);
+                                      DIFinder ? &*DIFinder : nullptr);
 
     // Add basic block mapping.
     VMap[&BB] = CBB;
@@ -183,15 +194,38 @@ void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
       Returns.push_back(RI);
   }
 
-  for (DISubprogram *ISP : DIFinder.subprograms())
-    if (ISP != SP)
-      VMap.MD()[ISP].reset(ISP);
+  if (Changes < CloneFunctionChangeType::DifferentModule &&
+      (SPClonedWithinModule || DIFinder->subprogram_count() > 0)) {
+    // Turn on module-level changes, since we need to clone (some of) the
+    // debug info metadata.
+    //
+    // FIXME: Metadata effectively owned by a function should be made
+    // local, and only that local metadata should be cloned.
+    ModuleLevelChanges = true;
+
+    auto mapToSelfIfNew = [&VMap](MDNode *N) {
+      // Avoid clobbering an existing mapping.
+      (void)VMap.MD().try_emplace(N, N);
+    };
+
+    // Avoid cloning what the subprogram references.
+    if (SPClonedWithinModule) {
+      mapToSelfIfNew(SPClonedWithinModule->getUnit());
+      mapToSelfIfNew(SPClonedWithinModule->getType());
+      mapToSelfIfNew(SPClonedWithinModule->getFile());
+    }
+
+    // Avoid cloning other subprograms, compile units, and types.
+    for (DISubprogram *ISP : DIFinder->subprograms())
+      if (ISP != SPClonedWithinModule)
+        mapToSelfIfNew(ISP);
 
-  for (DICompileUnit *CU : DIFinder.compile_units())
-    VMap.MD()[CU].reset(CU);
+    for (DICompileUnit *CU : DIFinder->compile_units())
+      mapToSelfIfNew(CU);
 
-  for (DIType *Type : DIFinder.types())
-    VMap.MD()[Type].reset(Type);
+    for (DIType *Type : DIFinder->types())
+      mapToSelfIfNew(Type);
+  }
 
   // Duplicate the metadata that is attached to the cloned function.
   // Subprograms/CUs/types that were already mapped to themselves won't be
@@ -218,19 +252,33 @@ void llvm::CloneFunctionInto(Function *NewFunc, const Function *OldFunc,
                        ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges,
                        TypeMapper, Materializer);
 
-  // Register all DICompileUnits of the old parent module in the new parent module
-  auto* OldModule = OldFunc->getParent();
+  // Only update !llvm.dbg.cu for DifferentModule (not CloneModule). In the
+  // same module, the compile unit will already be listed (or not). When
+  // cloning a module, CloneModule() will handle creating the named metadata.
+  if (Changes != CloneFunctionChangeType::DifferentModule)
+    return;
+
+  // Update !llvm.dbg.cu with compile units added to the new module if this
+  // function is being cloned in isolation.
+  //
+  // FIXME: This is making global / module-level changes, which doesn't seem
+  // like the right encapsulation  Consider dropping the requirement to update
+  // !llvm.dbg.cu (either obsoleting the node, or restricting it to
+  // non-discardable compile units) instead of discovering compile units by
+  // visiting the metadata attached to global values, which would allow this
+  // code to be deleted. Alternatively, perhaps give responsibility for this
+  // update to CloneFunctionInto's callers.
   auto* NewModule = NewFunc->getParent();
-  if (OldModule && NewModule && OldModule != NewModule && DIFinder.compile_unit_count()) {
-    auto* NMD = NewModule->getOrInsertNamedMetadata("llvm.dbg.cu");
-    // Avoid multiple insertions of the same DICompileUnit to NMD.
-    SmallPtrSet<const void*, 8> Visited;
-    for (auto* Operand : NMD->operands())
-      Visited.insert(Operand);
-    for (auto* Unit : DIFinder.compile_units())
-      // VMap.MD()[Unit] == Unit
-      if (Visited.insert(Unit).second)
-        NMD->addOperand(Unit);
+  auto *NMD = NewModule->getOrInsertNamedMetadata("llvm.dbg.cu");
+  // Avoid multiple insertions of the same DICompileUnit to NMD.
+  SmallPtrSet<const void *, 8> Visited;
+  for (auto *Operand : NMD->operands())
+    Visited.insert(Operand);
+  for (auto *Unit : DIFinder->compile_units()) {
+    MDNode *MappedUnit =
+        MapMetadata(Unit, VMap, RF_None, TypeMapper, Materializer);
+    if (Visited.insert(MappedUnit).second)
+      NMD->addOperand(MappedUnit);
   }
 }
 
@@ -269,8 +317,8 @@ Function *llvm::CloneFunction(Function *F, ValueToValueMapTy &VMap,
     }
 
   SmallVector<ReturnInst*, 8> Returns;  // Ignore returns cloned.
-  CloneFunctionInto(NewF, F, VMap, F->getSubprogram() != nullptr, Returns, "",
-                    CodeInfo);
+  CloneFunctionInto(NewF, F, VMap, CloneFunctionChangeType::LocalChangesOnly,
+                    Returns, "", CodeInfo);
 
   return NewF;
 }