Move the section name from GlobalObject to the LLVMContext

Summary:
Convention wisdom says that bytes in Function are precious, and the
vast, vast majority of globals do not live in special sections. Even
when they do, they tend to live in the same section. Store the section
name on the LLVMContext in a StringSet, and maintain a map from
GlobalObject* to section name like we do for metadata, prefix data, etc.

The fact that we've survived this long wasting at least three pointers
of space in Function suggests that Function bytes are perhaps not as
precious as we once thought. Given that most functions have metadata
attachments when debug info is enabled, we might consider adding a
pointer here to make that access more efficient.

Reviewers: jlebar, dexonsmith, mehdi_amini

Subscribers: mehdi_amini, aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D28150

llvm-svn: 291613

Author: rnk

llvm/include/llvm/IR/GlobalObject.h

@@ -37,11 +37,11 @@ class GlobalObject : public GlobalValue {
     setGlobalValueSubClassData(0);
   }
 
-  std::string Section;     // Section to emit this into, empty means default
   Comdat *ObjComdat;
   enum {
     LastAlignmentBit = 4,
     HasMetadataHashEntryBit,
+    HasSectionHashEntryBit,
 
     GlobalObjectBits,
   };
@@ -66,8 +66,26 @@ class GlobalObject : public GlobalValue {
   unsigned getGlobalObjectSubClassData() const;
   void setGlobalObjectSubClassData(unsigned Val);
 
-  bool hasSection() const { return !getSection().empty(); }
-  StringRef getSection() const { return Section; }
+  /// Check if this global has a custom object file section.
+  ///
+  /// This is more efficient than calling getSection() and checking for an empty
+  /// string.
+  bool hasSection() const {
+    return getGlobalValueSubClassData() & (1 << HasSectionHashEntryBit);
+  }
+
+  /// Get the custom section of this global if it has one.
+  ///
+  /// If this global does not have a custom section, this will be empty and the
+  /// default object file section (.text, .data, etc) will be used.
+  StringRef getSection() const {
+    return hasSection() ? getSectionImpl() : StringRef();
+  }
+
+  /// Change the section for this global.
+  ///
+  /// Setting the section to the empty string tells LLVM to choose an
+  /// appropriate default object file section.
   void setSection(StringRef S);
 
   bool hasComdat() const { return getComdat() != nullptr; }
@@ -134,14 +152,20 @@ class GlobalObject : public GlobalValue {
   void clearMetadata();
 
 private:
+  void setGlobalObjectFlag(unsigned Bit, bool Val) {
+    unsigned Mask = 1 << Bit;
+    setGlobalValueSubClassData((~Mask & getGlobalValueSubClassData()) |
+                               (Val ? Mask : 0u));
+  }
+
   bool hasMetadataHashEntry() const {
     return getGlobalValueSubClassData() & (1 << HasMetadataHashEntryBit);
   }
   void setHasMetadataHashEntry(bool HasEntry) {
-    unsigned Mask = 1 << HasMetadataHashEntryBit;
-    setGlobalValueSubClassData((~Mask & getGlobalValueSubClassData()) |
-                               (HasEntry ? Mask : 0u));
+    setGlobalObjectFlag(HasMetadataHashEntryBit, HasEntry);
   }
+
+  StringRef getSectionImpl() const;
 };
 
 } // end namespace llvm

llvm/lib/IR/Globals.cpp

@@ -24,6 +24,7 @@
 #include "llvm/IR/Operator.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "LLVMContextImpl.h"
 using namespace llvm;
 
 //===----------------------------------------------------------------------===//
@@ -37,6 +38,10 @@ static_assert(sizeof(GlobalValue) ==
                   sizeof(Constant) + 2 * sizeof(void *) + 2 * sizeof(unsigned),
               "unexpected GlobalValue size growth");
 
+// GlobalObject adds a comdat.
+static_assert(sizeof(GlobalObject) == sizeof(GlobalValue) + sizeof(void *),
+              "unexpected GlobalObject size growth");
+
 bool GlobalValue::isMaterializable() const {
   if (const Function *F = dyn_cast<Function>(this))
     return F->isMaterializable();
@@ -160,11 +165,24 @@ Comdat *GlobalValue::getComdat() {
   return cast<GlobalObject>(this)->getComdat();
 }
 
-void GlobalObject::setSection(StringRef S) {
-  Section = S;
+StringRef GlobalObject::getSectionImpl() const {
+  assert(hasSection());
+  return getContext().pImpl->GlobalObjectSections[this];
+}
 
-  // The C api requires this to be null terminated.
-  Section.c_str();
+void GlobalObject::setSection(StringRef S) {
+  // Do nothing if we're clearing the section and it is already empty.
+  if (!hasSection() && S.empty())
+    return;
+
+  // Get or create a stable section name string and put it in the table in the
+  // context.
+  S = getContext().pImpl->SectionStrings.insert(S).first->first();
+  getContext().pImpl->GlobalObjectSections[this] = S;
+
+  // Update the HasSectionHashEntryBit. Setting the section to the empty string
+  // means this global no longer has a section.
+  setGlobalObjectFlag(HasSectionHashEntryBit, !S.empty());
 }
 
 bool GlobalValue::isDeclaration() const {

llvm/lib/IR/LLVMContextImpl.h

@@ -26,6 +26,7 @@
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -1194,6 +1195,12 @@ class LLVMContextImpl {
   /// Collection of per-GlobalObject metadata used in this context.
   DenseMap<const GlobalObject *, MDGlobalAttachmentMap> GlobalObjectMetadata;
 
+  /// Collection of per-GlobalObject sections used in this context.
+  DenseMap<const GlobalObject *, StringRef> GlobalObjectSections;
+
+  /// Stable collection of section strings.
+  StringSet<> SectionStrings;
+
   /// DiscriminatorTable - This table maps file:line locations to an
   /// integer representing the next DWARF path discriminator to assign to
   /// instructions in different blocks at the same location.