[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
diff --git a/llvm/lib/Target/X86/X86TargetObjectFile.cpp b/llvm/lib/Target/X86/X86TargetObjectFile.cpp
index f8bcd61..1d1c32e 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 @@
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);