From 64e6719ee870f88db445feac0a1d5f597a7549e3 Mon Sep 17 00:00:00 2001
From: Rafael Espindola <rafael.espindola@gmail.com>
Date: Wed, 20 Oct 2010 16:46:08 +0000
Subject: [PATCH] Handle _GLOBAL_OFFSET_TABLE_ correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116932 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Target/X86/X86MCCodeEmitter.cpp | 27 +++++++++++++++++++++++++++
 test/MC/ELF/global-offset.s         | 18 ++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 test/MC/ELF/global-offset.s

diff --git a/lib/Target/X86/X86MCCodeEmitter.cpp b/lib/Target/X86/X86MCCodeEmitter.cpp
index 62f434d0fd1..47e91a8159c 100644
--- a/lib/Target/X86/X86MCCodeEmitter.cpp
+++ b/lib/Target/X86/X86MCCodeEmitter.cpp
@@ -18,6 +18,7 @@
 #include "llvm/MC/MCCodeEmitter.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCInst.h"
+#include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
@@ -193,6 +194,25 @@ static bool Is32BitMemOperand(const MCInst &MI, unsigned Op) {
   return false;
 }
 
+/// StartsWithGlobalOffsetTable - Return true for the simple cases where this
+/// expression starts with _GLOBAL_OFFSET_TABLE_. This is a needed to support
+/// PIC on ELF i386 as that symbol is magic. We check only simple case that
+/// are know to be used: _GLOBAL_OFFSET_TABLE_ by itself or at the start
+/// of a binary expression.
+static bool StartsWithGlobalOffsetTable(const MCExpr *Expr) {
+  if (Expr->getKind() == MCExpr::Binary) {
+    const MCBinaryExpr *BE = static_cast<const MCBinaryExpr *>(Expr);
+    Expr = BE->getLHS();
+  }
+
+  if (Expr->getKind() != MCExpr::SymbolRef)
+    return false;
+
+  const MCSymbolRefExpr *Ref = static_cast<const MCSymbolRefExpr*>(Expr);
+  const MCSymbol &S = Ref->getSymbol();
+  return S.getName() == "_GLOBAL_OFFSET_TABLE_";
+}
+
 void X86MCCodeEmitter::
 EmitImmediate(const MCOperand &DispOp, unsigned Size, MCFixupKind FixupKind,
               unsigned &CurByte, raw_ostream &OS,
@@ -209,6 +229,13 @@ EmitImmediate(const MCOperand &DispOp, unsigned Size, MCFixupKind FixupKind,
   // If we have an immoffset, add it to the expression.
   const MCExpr *Expr = DispOp.getExpr();
 
+  if (StartsWithGlobalOffsetTable(Expr)) {
+    // FIXME: We should probably change the FixupKind to a special one so that
+    // other parts of MC don't have to check the symbol name.
+    assert(ImmOffset == 0);
+    ImmOffset = CurByte;
+  }
+
   // If the fixup is pc-relative, we need to bias the value to be relative to
   // the start of the field, not the end of the field.
   if (FixupKind == MCFixupKind(X86::reloc_pcrel_4byte) ||
diff --git a/test/MC/ELF/global-offset.s b/test/MC/ELF/global-offset.s
new file mode 100644
index 00000000000..aa6328760d4
--- /dev/null
+++ b/test/MC/ELF/global-offset.s
@@ -0,0 +1,18 @@
+// RUN: llvm-mc -filetype=obj -triple i386-pc-linux-gnu %s -o - | elf-dump  --dump-section-data | FileCheck  %s
+
+// We test that _GLOBAL_OFFSET_TABLE_ will account for the two bytes at the
+// start of the addl.
+
+        addl    $_GLOBAL_OFFSET_TABLE_, %ebx
+
+// CHECK:      ('sh_name', 0x00000001) # '.text'
+// CHECK-NEXT: ('sh_type',
+// CHECK-NEXT: ('sh_flags',
+// CHECK-NEXT: ('sh_addr',
+// CHECK-NEXT: ('sh_offset',
+// CHECK-NEXT: ('sh_size',
+// CHECK-NEXT: ('sh_link',
+// CHECK-NEXT: ('sh_info',
+// CHECK-NEXT: ('sh_addralign',
+// CHECK-NEXT: ('sh_entsize',
+// CHECK-NEXT: ('_section_data', '81c30200 0000')