[MC][NFCI] Factor out ELF section unique ID calculation

[tmatheson]

Summary

Split from D100944. The logic for determining the unique ID had become
quite difficult to reason about, so I have factored this out into a
separate function.

[MaskRay]

Thanks!

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

 public:
   LoweringDiagnosticInfo(const Twine &DiagMsg,
                          DiagnosticSeverity Severity = DS_Error)
       : DiagnosticInfo(DK_Lowering, Severity), Msg(DiagMsg) {}
   void print(DiagnosticPrinter &DP) const override { DP << Msg; }
 };
 }
 
+/// Calculate an appropriate unique ID for a section, and update Flags,
+/// EntrySize and NextUniqueID where appropriate.
+static unsigned
+calcUniqueIDUpdateFlagsAndSize(const GlobalObject *GO, StringRef SectionName,
+                               SectionKind Kind, const TargetMachine &TM,
+                               MCContext &Ctx, Mangler &Mang, unsigned &Flags,
+                               unsigned &EntrySize, unsigned &NextUniqueID,
+                               const bool Retain, const bool ForceUnique) {
+  // Increment uniqueID if we are forced to emit a unique section.
+  // This works perfectly fine with section attribute or pragma section as the
+  // sections with the same name are grouped together by the assembler.
+  if (ForceUnique)
+    return NextUniqueID++;
+
+  // A section can have at most one associated section. Put each global with
+  // MD_associated in a unique section.
+  const bool Associated = GO->getMetadata(LLVMContext::MD_associated);
+  if (Associated) {
+    Flags |= ELF::SHF_LINK_ORDER;
+    return NextUniqueID++;
+  }
+
+  if (Retain) {
+    if (Ctx.getAsmInfo()->useIntegratedAssembler() ||
+        Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36))
+      Flags |= ELF::SHF_GNU_RETAIN;
+    return NextUniqueID++;
+  }
+
+  // If two symbols with differing sizes end up in the same mergeable section
+  // that section can be assigned an incorrect entry size. To avoid this we
+  // usually put symbols of the same size into distinct mergeable sections with
+  // the same name. Doing so relies on the ",unique ," assembly feature. This
+  // feature is not avalible until bintuils version 2.35
+  // (https://sourceware.org/bugzilla/show_bug.cgi?id=25380).
+  const bool SupportsUnique = Ctx.getAsmInfo()->useIntegratedAssembler() ||
+                              Ctx.getAsmInfo()->binutilsIsAtLeast(2, 35);
+  if (!SupportsUnique) {
+    Flags &= ~ELF::SHF_MERGE;
+    EntrySize = 0;
+    return MCContext::GenericSectionID;
+  }
+
+  const bool SymbolMergeable = Flags & ELF::SHF_MERGE;
+  const bool SeenSectionNameBefore =
+      Ctx.isELFGenericMergeableSection(SectionName);
+  // If this is the first ocurrence of this section name, treat it as the
+  // generic section
+  if (!SymbolMergeable && !SeenSectionNameBefore)
+    return MCContext::GenericSectionID;
+
+  // Symbols must be placed into sections with compatible entry sizes. Generate
+  // unique sections for symbols that have not been assigned to compatible
+  // sections.
+  const auto PreviousID =
+      Ctx.getELFUniqueIDForEntsize(SectionName, Flags, EntrySize);
+  if (PreviousID)
+    return *PreviousID;
+
+  // If the user has specified the same section name as would be created
+  // implicitly for this symbol e.g. .rodata.str1.1, then we don't need
+  // to unique the section as the entry size for this symbol will be
+  // compatible with implicitly created sections.
+  SmallString<128> ImplicitSectionNameStem =
+      getELFSectionNameForGlobal(GO, Kind, Mang, TM, EntrySize, false);
+  if (SymbolMergeable &&
+      Ctx.isELFImplicitMergeableSectionNamePrefix(SectionName) &&
+      SectionName.startswith(ImplicitSectionNameStem))
+    return MCContext::GenericSectionID;
+
+  // We have seen this section name before, but with different flags or entity
+  // size. Create a new unique ID.
+  return NextUniqueID++;
+}
+
 static MCSection *selectExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM,
     MCContext &Ctx, Mangler &Mang, unsigned &NextUniqueID,
     bool Retain, bool ForceUnique) {
   StringRef SectionName = GO->getSection();
 
   // Check if '#pragma clang section' name is applicable.
   // Note that pragma directive overrides -ffunction-section, -fdata-section
static MCSection *selectExplicitSectionGlobal(
   unsigned Flags = getELFSectionFlags(Kind);
   if (const Comdat *C = getELFComdat(GO)) {
     Group = C->getName();
     IsComdat = C->getSelectionKind() == Comdat::Any;
     Flags |= ELF::SHF_GROUP;
   }
 
   unsigned EntrySize = getEntrySizeForKind(Kind);
+  const unsigned UniqueID = calcUniqueIDUpdateFlagsAndSize(
+      GO, SectionName, Kind, TM, Ctx, Mang, Flags, EntrySize, NextUniqueID,
+      Retain, ForceUnique);
 
-  // A section can have at most one associated section. Put each global with
-  // MD_associated in a unique section.
-  unsigned UniqueID = MCContext::GenericSectionID;
   const MCSymbolELF *LinkedToSym = getLinkedToSymbol(GO, TM);
-  const bool Associated = GO->getMetadata(LLVMContext::MD_associated);
-  if (Associated || Retain) {
-    UniqueID = NextUniqueID++;
-    if (Associated)
-      Flags |= ELF::SHF_LINK_ORDER;
-    if (Retain && (Ctx.getAsmInfo()->useIntegratedAssembler() ||
-                   Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36)))
-      Flags |= ELF::SHF_GNU_RETAIN;
-  } else {
-    if (Ctx.getAsmInfo()->useIntegratedAssembler() ||
-        Ctx.getAsmInfo()->binutilsIsAtLeast(2, 35)) {
-      // Symbols must be placed into sections with compatible entry
-      // sizes. Generate unique sections for symbols that have not
-      // been assigned to compatible sections.
-      if (Flags & ELF::SHF_MERGE) {
-        auto maybeID = Ctx.getELFUniqueIDForEntsize(SectionName, Flags,
-                                                    EntrySize);
-        if (maybeID)
-          UniqueID = *maybeID;
-        else {
-          // If the user has specified the same section name as would be created
-          // implicitly for this symbol e.g. .rodata.str1.1, then we don't need
-          // to unique the section as the entry size for this symbol will be
-          // compatible with implicitly created sections.
-          SmallString<128> ImplicitSectionNameStem = getELFSectionNameForGlobal(
-              GO, Kind, Mang, TM, EntrySize, false);
-          if (!(Ctx.isELFImplicitMergeableSectionNamePrefix(SectionName) &&
-                SectionName.startswith(ImplicitSectionNameStem)))
-            UniqueID = NextUniqueID++;
-        }
-      } else {
-        // We need to unique the section if the user has explicity
-        // assigned a non-mergeable symbol to a section name for
-        // a generic mergeable section.
-        if (Ctx.isELFGenericMergeableSection(SectionName)) {
-          auto maybeID = Ctx.getELFUniqueIDForEntsize(
-              SectionName, Flags, EntrySize);
-          UniqueID = maybeID ? *maybeID : NextUniqueID++;
-        }
-      }
-    } else {
-      // If two symbols with differing sizes end up in the same mergeable
-      // section that section can be assigned an incorrect entry size. To avoid
-      // this we usually put symbols of the same size into distinct mergeable
-      // sections with the same name. Doing so relies on the ",unique ,"
-      // assembly feature. This feature is not avalible until bintuils
-      // version 2.35 (https://sourceware.org/bugzilla/show_bug.cgi?id=25380).
-      Flags &= ~ELF::SHF_MERGE;
-      EntrySize = 0;
-    }
-  }
-
-  // Increment uniqueID if we are forced to emit a unique section.
-  // This works perfectly fine with section attribute or pragma section as the
-  // sections with the same name are grouped together by the assembler.
-  if (ForceUnique && UniqueID == MCContext::GenericSectionID)
-    UniqueID = NextUniqueID++;
-
   MCSectionELF *Section = Ctx.getELFSection(
       SectionName, getELFSectionType(SectionName, Kind), Flags, EntrySize,
       Group, IsComdat, UniqueID, LinkedToSym);
   // Make sure that we did not get some other section with incompatible sh_link.
   // This should not be possible due to UniqueID code above.
   assert(Section->getLinkedToSymbol() == LinkedToSym &&
          "Associated symbol mismatch between sections");