[ThinLTO]Record import type in GlobalValueSummary::GVFlags (#87597)

The motivating use case is to support import the function declaration
across modules to construct call graph edges for indirect calls [1]
when importing the function definition costs too much compile time
(e.g., the function is too large has no `noinline` attribute).
1. Currently, when the compiled IR module doesn't have a function
definition but its postlink combined summary contains the function
summary or a global alias summary with this function as aliasee, the
function definition will be imported from source module by IRMover. The
implementation is in FunctionImporter::importFunctions [2]
2. In order for FunctionImporter to import a declaration of a function,
both function summary and alias summary need to carry the def / decl
state. Specifically, all existing summary fields doesn't differ across
import modules, but the def / decl state of is decided by
`<ImportModule, Function>`.

This change encodes the def/decl state in `GlobalValueSummary::GVFlags`.

In the subsequent changes
1. The indexing step `computeImportForModule` [3]
will compute the set of definitions and the set of declarations for each
module, and passing on the information to bitcode writer.
2. Bitcode writer will look up the def/decl state and sets the state
when it writes out the flag value. This is demonstrated in
https://github.com/llvm/llvm-project/pull/87600
3. Function importer will read the def/decl state when reading the
combined summary to figure out two sets of global values, and IRMover
will be updated to import the declaration (aka linkGlobalValuePrototype [4])
into the destination module.

- The next change is https://github.com/llvm/llvm-project/pull/87600

[1] mentioned in rfc https://discourse.llvm.org/t/rfc-for-better-call-graph-sort-build-a-more-complete-call-graph-by-adding-more-indirect-call-edges/74029#support-cross-module-function-declaration-import-5
[2] 3b337242ee/llvm/lib/Transforms/IPO/FunctionImport.cpp (L1608-L1764)
[3] 3b337242ee/llvm/lib/Transforms/IPO/FunctionImport.cpp (L856)
[4] 3b337242ee/llvm/lib/Linker/IRMover.cpp (L605)

llvm/include/llvm/AsmParser/LLParser.h

@@ -301,6 +301,8 @@ namespace llvm {
                               bool &DSOLocal);
     void parseOptionalDSOLocal(bool &DSOLocal);
     void parseOptionalVisibility(unsigned &Res);
+    bool parseOptionalImportType(lltok::Kind Kind,
+                                 GlobalValueSummary::ImportKind &Res);
     void parseOptionalDLLStorageClass(unsigned &Res);
     bool parseOptionalCallingConv(unsigned &CC);
     bool parseOptionalAlignment(MaybeAlign &Alignment,

llvm/include/llvm/AsmParser/LLToken.h

@@ -370,6 +370,9 @@ enum Kind {
   kw_live,
   kw_dsoLocal,
   kw_canAutoHide,
+  kw_importType,
+  kw_definition,
+  kw_declaration,
   kw_function,
   kw_insts,
   kw_funcFlags,

llvm/include/llvm/IR/ModuleSummaryIndex.h

@@ -432,6 +432,18 @@ public:
   /// Sububclass discriminator (for dyn_cast<> et al.)
   enum SummaryKind : unsigned { AliasKind, FunctionKind, GlobalVarKind };
 
+  enum ImportKind : unsigned {
+    // The global value definition corresponding to the summary should be
+    // imported from source module
+    Definition = 0,
+
+    // When its definition doesn't exist in the destination module and not
+    // imported (e.g., function is too large to be inlined), the global value
+    // declaration corresponding to the summary should be imported, or the
+    // attributes from summary should be annotated on the function declaration.
+    Declaration = 1,
+  };
+
   /// Group flags (Linkage, NotEligibleToImport, etc.) as a bitfield.
   struct GVFlags {
     /// The linkage type of the associated global value.
@@ -472,14 +484,19 @@ public:
     /// means the symbol was externally visible.
     unsigned CanAutoHide : 1;
 
+    /// This field is written by the ThinLTO indexing step to postlink combined
+    /// summary. The value is interpreted as 'ImportKind' enum defined above.
+    unsigned ImportType : 1;
+
     /// Convenience Constructors
     explicit GVFlags(GlobalValue::LinkageTypes Linkage,
                      GlobalValue::VisibilityTypes Visibility,
                      bool NotEligibleToImport, bool Live, bool IsLocal,
-                     bool CanAutoHide)
+                     bool CanAutoHide, ImportKind ImportType)
         : Linkage(Linkage), Visibility(Visibility),
           NotEligibleToImport(NotEligibleToImport), Live(Live),
-          DSOLocal(IsLocal), CanAutoHide(CanAutoHide) {}
+          DSOLocal(IsLocal), CanAutoHide(CanAutoHide),
+          ImportType(static_cast<unsigned>(ImportType)) {}
   };
 
 private:
@@ -564,6 +581,12 @@ public:
 
   bool canAutoHide() const { return Flags.CanAutoHide; }
 
+  bool shouldImportAsDecl() const {
+    return Flags.ImportType == GlobalValueSummary::ImportKind::Declaration;
+  }
+
+  void setImportKind(ImportKind IK) { Flags.ImportType = IK; }
+
   GlobalValue::VisibilityTypes getVisibility() const {
     return (GlobalValue::VisibilityTypes)Flags.Visibility;
   }
@@ -813,7 +836,7 @@ public:
             GlobalValue::LinkageTypes::AvailableExternallyLinkage,
             GlobalValue::DefaultVisibility,
             /*NotEligibleToImport=*/true, /*Live=*/true, /*IsLocal=*/false,
-            /*CanAutoHide=*/false),
+            /*CanAutoHide=*/false, GlobalValueSummary::ImportKind::Definition),
         /*NumInsts=*/0, FunctionSummary::FFlags{}, /*EntryCount=*/0,
         std::vector<ValueInfo>(), std::move(Edges),
         std::vector<GlobalValue::GUID>(),

llvm/include/llvm/IR/ModuleSummaryIndexYAML.h

@@ -138,6 +138,7 @@ template <> struct MappingTraits<TypeIdSummary> {
 struct FunctionSummaryYaml {
   unsigned Linkage, Visibility;
   bool NotEligibleToImport, Live, IsLocal, CanAutoHide;
+  unsigned ImportType;
   std::vector<uint64_t> Refs;
   std::vector<uint64_t> TypeTests;
   std::vector<FunctionSummary::VFuncId> TypeTestAssumeVCalls,
@@ -183,6 +184,7 @@ template <> struct MappingTraits<FunctionSummaryYaml> {
     io.mapOptional("Live", summary.Live);
     io.mapOptional("Local", summary.IsLocal);
     io.mapOptional("CanAutoHide", summary.CanAutoHide);
+    io.mapOptional("ImportType", summary.ImportType);
     io.mapOptional("Refs", summary.Refs);
     io.mapOptional("TypeTests", summary.TypeTests);
     io.mapOptional("TypeTestAssumeVCalls", summary.TypeTestAssumeVCalls);
@@ -227,7 +229,8 @@ template <> struct CustomMappingTraits<GlobalValueSummaryMapTy> {
               static_cast<GlobalValue::LinkageTypes>(FSum.Linkage),
               static_cast<GlobalValue::VisibilityTypes>(FSum.Visibility),
               FSum.NotEligibleToImport, FSum.Live, FSum.IsLocal,
-              FSum.CanAutoHide),
+              FSum.CanAutoHide,
+              static_cast<GlobalValueSummary::ImportKind>(FSum.ImportType)),
           /*NumInsts=*/0, FunctionSummary::FFlags{}, /*EntryCount=*/0, Refs,
           ArrayRef<FunctionSummary::EdgeTy>{}, std::move(FSum.TypeTests),
           std::move(FSum.TypeTestAssumeVCalls),
@@ -251,9 +254,9 @@ template <> struct CustomMappingTraits<GlobalValueSummaryMapTy> {
               static_cast<bool>(FSum->flags().NotEligibleToImport),
               static_cast<bool>(FSum->flags().Live),
               static_cast<bool>(FSum->flags().DSOLocal),
-              static_cast<bool>(FSum->flags().CanAutoHide), Refs,
-              FSum->type_tests(), FSum->type_test_assume_vcalls(),
-              FSum->type_checked_load_vcalls(),
+              static_cast<bool>(FSum->flags().CanAutoHide),
+              FSum->flags().ImportType, Refs, FSum->type_tests(),
+              FSum->type_test_assume_vcalls(), FSum->type_checked_load_vcalls(),
               FSum->type_test_assume_const_vcalls(),
               FSum->type_checked_load_const_vcalls()});
           }