[X86-64] Only treat PC-relative globals as memory content if they are…

… dynamically relocated

This makes sure that both of the following work as expected:

1. mov eax, [rip + myglob@GOTPCREL]
   used for shared libraries compiled to position independent code (-fPIC)
   Here myglob is dynamically relocated through the GOT
2. mov eax, [rip + myglob]
   used for non-PIC code to access a global directly without any
   indirection through the GOT

[Test] Added one test to verify RIP-relative globals work as expected

X86/X86MachineInstructionRaiser.cpp

@@ -46,6 +46,8 @@ using namespace llvm;
 using namespace mctoll;
 using namespace X86RegisterUtils;
 
+std::set<std::string> X86MachineInstructionRaiser::DynRelocatedGlobalVariables;
+
 // Constructor
 
 X86MachineInstructionRaiser::X86MachineInstructionRaiser(MachineFunction &MF,
@@ -1881,18 +1883,18 @@ bool X86MachineInstructionRaiser::raiseMoveFromMemInstr(const MachineInstr &MI,
   // Following are the exceptions when MemRefValue needs to be considered as
   // memory content and not as memory reference.
   if (IsPCRelMemRef) {
-    // If it is a PC-relative global variable with an initializer, it is memory
-    // content and should not be loaded from.
-    if (auto GV = dyn_cast<GlobalVariable>(MemRefValue))
-      LoadFromMemrefValue = !(GV->hasInitializer());
-    // If it is not a PC-relative constant expression accessed using
-    // GetElementPtrInst, it is memory content and should not be loaded from.
-    else {
-      const ConstantExpr *CExpr = dyn_cast<ConstantExpr>(MemRefValue);
-      if (CExpr != nullptr) {
-        LoadFromMemrefValue =
-            (CExpr->getOpcode() == Instruction::GetElementPtr);
+    // If it is a PC-relative dynamically relocated global variable, it is
+    // memory content and should not be loaded from.
+    if (auto GV = dyn_cast<GlobalVariable>(MemRefValue)) {
+      if (isDynRelocatedGlobalVariable(GV->getName().str())) {
+        LoadFromMemrefValue = false;
+      } else {
+        LoadFromMemrefValue = true;
       }
+      // If it is not a PC-relative constant expression accessed using
+      // GetElementPtrInst, it is memory content and should not be loaded from.
+    } else if (auto *CExpr = dyn_cast<ConstantExpr>(MemRefValue)) {
+      LoadFromMemrefValue = (CExpr->getOpcode() == Instruction::GetElementPtr);
     }
   }
 

X86/X86MachineInstructionRaiser.h

@@ -82,6 +82,10 @@ class X86MachineInstructionRaiser : public MachineInstructionRaiser {
   // at the exit of the MBB.
   std::map<int, MCPhysRegSizeMap> PerMBBDefinedPhysRegMap;
 
+  // Stores all global variables that are dynamically relocated, e.g. through PLT
+  // Usually these are global variables in shared objects
+  static std::set<std::string> DynRelocatedGlobalVariables;
+
   static const uint8_t FPUSTACK_SZ = 8;
   struct {
     int8_t TOP;
@@ -218,6 +222,11 @@ class X86MachineInstructionRaiser : public MachineInstructionRaiser {
   bool isEffectiveAddrValue(Value *Val);
 
   std::vector<JumpTableInfo> jtList;
+
+  // Check if there exists a dynamically relocated variable with the given name
+  inline bool isDynRelocatedGlobalVariable(std::string Name) {
+    return DynRelocatedGlobalVariables.find(Name) != DynRelocatedGlobalVariables.end();
+  }
 };
 
 #endif // LLVM_TOOLS_LLVM_MCTOLL_X86_X86MACHINEINSTRUCTIONRAISER_H

X86/X86MachineInstructionRaiserUtils.cpp

@@ -189,18 +189,18 @@ Value *X86MachineInstructionRaiser::loadMemoryRefValue(
   // Following are the exceptions when MemRefValue needs to be considered as
   // memory content and not as memory reference.
   if (IsPCRelMemRef) {
-    // If it is a PC-relative global variable with an initializer, it is memory
-    // content and should not be loaded from.
-    if (auto GV = dyn_cast<GlobalVariable>(MemRefValue))
-      LoadFromMemrefValue = !(GV->hasInitializer());
+    // If it is a PC-relative dynamically relocated global variable, it is
+    // memory content and should not be loaded from.
+    if (auto GV = dyn_cast<GlobalVariable>(MemRefValue)) {
+      if (isDynRelocatedGlobalVariable(GV->getName().str())) {
+        LoadFromMemrefValue = false;
+      } else {
+        LoadFromMemrefValue = true;
+      }
       // If it is not a PC-relative constant expression accessed using
       // GetElementPtrInst, it is memory content and should not be loaded from.
-    else {
-      const ConstantExpr *CExpr = dyn_cast<ConstantExpr>(MemRefValue);
-      if (CExpr != nullptr) {
-        LoadFromMemrefValue =
-            (CExpr->getOpcode() == Instruction::GetElementPtr);
-      }
+    } else if (auto *CExpr = dyn_cast<ConstantExpr>(MemRefValue)) {
+      LoadFromMemrefValue = (CExpr->getOpcode() == Instruction::GetElementPtr);
     }
   }
 
@@ -745,17 +745,16 @@ Value *X86MachineInstructionRaiser::createPCRelativeAccesssValue(
               break;
             }
           }
-
           Constant *GlobalInit;
           if (IncludedFileInfo::IsExternalVariable(Symname->str())) {
             GlobalInit = nullptr;
             Lnkg = GlobalValue::ExternalLinkage;
+          } else if (DynRelocType == ELF::R_X86_64_GLOB_DAT) {
+            GlobalInit = ConstantInt::get(GlobalValTy, SymbVal);
+            DynRelocatedGlobalVariables.insert(Symname->str());
           } else {
-            GlobalInit = (DynRelocType == ELF::R_X86_64_GLOB_DAT)
-                             ? ConstantInt::get(GlobalValTy, SymbVal)
-                             : nullptr;
+            GlobalInit = nullptr;
           }
-
           auto GlobalVal = new GlobalVariable(*(MR->getModule()), GlobalValTy,
                                               false /* isConstant */, Lnkg,
                                               GlobalInit, Symname->data());

test/asm_test/X86/raise-pcrel-global-variable.s

@@ -0,0 +1,46 @@
+// REQUIRES: x86_64-linux
+// RUN: clang -O0 -o %t %s
+// RUN: llvm-mctoll -d -I /usr/include/stdio.h %t
+// RUN: clang -o %t-dis %t-dis.ll
+// RUN: %t-dis 2>&1 | FileCheck %s
+// CHECK: i = 0
+// CHECK: i = 10
+// CHECK-EMPTY
+
+.text
+.intel_syntax noprefix
+.file "raise-call64r-float.s"
+
+.globl    main                    # -- Begin function main
+.p2align    4, 0x90
+.type    main,@function
+main:                                   # @main
+    movabs rdi, offset .L.str
+    mov esi, [rip + i]
+    mov al, 0
+    call printf
+
+    mov dword ptr [rip + i], 10
+
+    movabs rdi, offset .L.str
+    mov esi, [rip + i]
+    mov al, 0
+    call printf
+
+    xor rax, rax
+    ret
+
+
+.type   .L.str,@object                  # @.str
+.section        .rodata.str1.1,"aMS",@progbits,1
+.L.str:
+    .asciz  "i = %d\n"
+    .size   .L.str, 8
+
+.bss
+.type   i,@object
+.globl i
+.p2align 2
+i:
+    .long 0
+    .size i, 4