[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

llvm-svn: 230264

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86TargetObjectFile.cpp b/llvm/lib/Target/X86/X86TargetObjectFile.cpp
index f8bcd616e11..1d1c32eb2cb 100644
--- a/llvm/lib/Target/X86/X86TargetObjectFile.cpp
+++ b/llvm/lib/Target/X86/X86TargetObjectFile.cpp
@@ -21,6 +21,11 @@
 using namespace llvm;
 using namespace dwarf;
 
+X86_64MachoTargetObjectFile::X86_64MachoTargetObjectFile()
+  : TargetLoweringObjectFileMachO() {
+  SupportIndirectSymViaGOTPCRel = true;
+}
+
 const MCExpr *X86_64MachoTargetObjectFile::getTTypeGlobalReference(
     const GlobalValue *GV, unsigned Encoding, Mangler &Mang,
     const TargetMachine &TM, MachineModuleInfo *MMI,
@@ -46,6 +51,17 @@ MCSymbol *X86_64MachoTargetObjectFile::getCFIPersonalitySymbol(
   return TM.getSymbol(GV, Mang);
 }
 
+const MCExpr *X86_64MachoTargetObjectFile::getIndirectSymViaGOTPCRel(
+    const MCSymbol *Sym, int64_t Offset) const {
+  // On Darwin/X86-64, we need to use foo@GOTPCREL+4 to access the got entry
+  // from a data section. In case there's an additional offset, then use
+  // foo@GOTPCREL+4+<offset>.
+  const MCExpr *Res =
+    MCSymbolRefExpr::Create(Sym, MCSymbolRefExpr::VK_GOTPCREL, getContext());
+  const MCExpr *Off = MCConstantExpr::Create(Offset+4, getContext());
+  return MCBinaryExpr::CreateAdd(Res, Off, getContext());
+}
+
 void
 X86LinuxTargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM) {
   TargetLoweringObjectFileELF::Initialize(Ctx, TM);