[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/Target/TargetLoweringObjectFile.h

@@ -41,10 +41,14 @@ class TargetLoweringObjectFile : public MCObjectFileInfo {
     const TargetLoweringObjectFile&) = delete;
   void operator=(const TargetLoweringObjectFile&) = delete;
 
+protected:
+  bool SupportIndirectSymViaGOTPCRel;
+
 public:
   MCContext &getContext() const { return *Ctx; }
 
-  TargetLoweringObjectFile() : MCObjectFileInfo(), Ctx(nullptr), DL(nullptr) {}
+  TargetLoweringObjectFile() : MCObjectFileInfo(), Ctx(nullptr), DL(nullptr),
+                               SupportIndirectSymViaGOTPCRel(false) {}
 
   virtual ~TargetLoweringObjectFile();
 
@@ -158,6 +162,18 @@ public:
     return nullptr;
   }
 
+  /// \brief Target supports replacing a data "PC"-relative access to a symbol
+  /// through another symbol, by accessing the later via a GOT entry instead?
+  bool supportIndirectSymViaGOTPCRel() const {
+    return SupportIndirectSymViaGOTPCRel;
+  }
+
+  /// \brief Get the target specific PC relative GOT entry relocation
+  virtual const MCExpr *getIndirectSymViaGOTPCRel(const MCSymbol *Sym,
+                                                  int64_t Offset) const {
+    return nullptr;
+  }
+
 protected:
   virtual const MCSection *
   SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,