[AsmPrinter] Access pointers to globals via pcrel GOT entries

Front-ends could use global unnamed_addr to hold pointers to other
symbols, like @gotequivalent below:

@foo = global i32 42
@gotequivalent = private unnamed_addr constant i32* @foo

@delta = global i32 trunc (i64 sub (i64 ptrtoint (i32** @gotequivalent to i64),
                                    i64 ptrtoint (i32* @delta to i64))
                           to i32)

The global @delta holds a data "PC"-relative offset to @gotequivalent,
an unnamed pointer to @foo. The darwin/x86-64 assembly output for this follows:

 .globl  _foo
_foo:
 .long   42

 .globl  _gotequivalent
_gotequivalent:
 .quad   _foo

 .globl  _delta
_delta:
 .long   _gotequivalent-_delta

Since unnamed_addr indicates that the address is not significant, only
the content, we can optimize the case above by replacing pc-relative
accesses to "GOT equivalent" globals, by a PC relative access to the GOT
entry of the final symbol instead. Therefore, "delta" can contain a pc
relative relocation to foo's GOT entry and we avoid the emission of
"gotequivalent", yielding the assembly code below:

 .globl  _foo
_foo:
 .long   42

 .globl  _delta
_delta:
 .long   _foo@GOTPCREL+4

There are a couple of advantages of doing this: (1) Front-ends that need
to emit a great deal of data to store pointers to external symbols could
save space by not emitting such "got equivalent" globals and (2) IR
constructs combined with this opt opens a way to represent GOT pcrel
relocations by using the LLVM IR, which is something we previously had
no way to express.

Differential Revision: http://reviews.llvm.org/D6922

rdar://problem/18534217

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230264 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/AsmPrinter.h

@@ -97,6 +97,11 @@ public:
   /// default, this is equal to CurrentFnSym.
   MCSymbol *CurrentFnSymForSize;
 
+  /// Map global GOT equivalent MCSymbols to GlobalVariables and keep track of
+  /// its number of uses by other globals.
+  typedef std::pair<const GlobalVariable *, unsigned> GOTEquivUsePair;
+  DenseMap<const MCSymbol *, GOTEquivUsePair> GlobalGOTEquivs;
+
 private:
   // The garbage collection metadata printer table.
   void *GCMetadataPrinters; // Really a DenseMap.
@@ -242,6 +247,21 @@ public:
   /// \brief Print a general LLVM constant to the .s file.
   void EmitGlobalConstant(const Constant *CV);
 
+  /// \brief Unnamed constant global variables solely contaning a pointer to
+  /// another globals variable act like a global variable "proxy", or GOT
+  /// equivalents, i.e., it's only used to hold the address of the latter. One
+  /// optimization is to replace accesses to these proxies by using the GOT
+  /// entry for the final global instead. Hence, we select GOT equivalent
+  /// candidates among all the module global variables, avoid emitting them
+  /// unnecessarily and finally replace references to them by pc relative
+  /// accesses to GOT entries.
+  void computeGlobalGOTEquivs(Module &M);
+
+  /// \brief Constant expressions using GOT equivalent globals may not be
+  /// eligible for PC relative GOT entry conversion, in such cases we need to
+  /// emit the proxies we previously omitted in EmitGlobalVariable.
+  void emitGlobalGOTEquivs();
+
   //===------------------------------------------------------------------===//
   // Overridable Hooks
   //===------------------------------------------------------------------===//