Teach LLVM about a PIE option which, when enabled on top of PIC, makes

optimizations which are valid for position independent code being linked
into a single executable, but not for such code being linked into
a shared library.

I discussed the design of this with Eric Christopher, and the decision
was to support an optional bit rather than a completely separate
relocation model. Fundamentally, this is still PIC relocation, its just
that certain optimizations are only valid under a PIC relocation model
when the resulting code won't be in a shared library. The simplest path
to here is to expose a single bit option in the TargetOptions. If folks
have different/better designs, I'm all ears. =]

I've included the first optimization based upon this: changing TLS
models to the *Exec models when PIE is enabled. This is the LLVM
component of PR12380 and is all of the hard work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154294 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetOptions.h

@@ -42,8 +42,8 @@ namespace llvm {
           GuaranteedTailCallOpt(false), DisableTailCalls(false),
           StackAlignmentOverride(0), RealignStack(true),
           DisableJumpTables(false), EnableFastISel(false),
-          EnableSegmentedStacks(false), TrapFuncName(""),
+          PositionIndependentExecutable(false), EnableSegmentedStacks(false),
-          FloatABIType(FloatABI::Default)
+          TrapFuncName(""), FloatABIType(FloatABI::Default)
     {}
 
     /// PrintMachineCode - This flag is enabled when the -print-machineinstrs
@@ -164,6 +164,12 @@ namespace llvm {
     /// compile time.
     unsigned EnableFastISel : 1;
 
+    /// PositionIndependentExecutable - This flag indicates whether the code
+    /// will eventually be linked into a single executable, despite the PIC
+    /// relocation model being in use. It's value is undefined (and irrelevant)
+    /// if the relocation model is anything other than PIC.
+    unsigned PositionIndependentExecutable : 1;
+
     unsigned EnableSegmentedStacks : 1;
 
     /// getTrapFunctionName - If this returns a non-empty string, this means

lib/Target/TargetMachine.cpp

@@ -82,7 +82,8 @@ TLSModel::Model TargetMachine::getTLSModel(const GlobalValue *GV) const {
   // For variables, is internal different from hidden?
   bool isHidden = GV->hasHiddenVisibility();
 
-  if (getRelocationModel() == Reloc::PIC_) {
+  if (getRelocationModel() == Reloc::PIC_ &&
+      !Options.PositionIndependentExecutable) {
     if (isLocal || isHidden)
       return TLSModel::LocalDynamic;
     else

test/CodeGen/X86/tls-pie.ll

@@ -0,0 +1,64 @@
+; RUN: llc < %s -march=x86 -mtriple=i386-linux-gnu -relocation-model=pic -enable-pie \
+; RUN:   | FileCheck -check-prefix=X32 %s
+; RUN: llc < %s -march=x86-64 -mtriple=x86_64-linux-gnu -relocation-model=pic -enable-pie \
+; RUN:   | FileCheck -check-prefix=X64 %s
+
+@i = thread_local global i32 15
+@i2 = external thread_local global i32
+
+define i32 @f1() {
+; X32: f1:
+; X32:      movl %gs:i@NTPOFF, %eax
+; X32-NEXT: ret
+; X64: f1:
+; X64:      movabsq $i@TPOFF, %rax
+; X64-NEXT: movl %fs:(%rax), %eax
+; X64-NEXT: ret
+
+entry:
+	%tmp1 = load i32* @i
+	ret i32 %tmp1
+}
+
+define i32* @f2() {
+; X32: f2:
+; X32:      movl %gs:0, %eax
+; X32-NEXT: leal i@NTPOFF(%eax), %eax
+; X32-NEXT: ret
+; X64: f2:
+; X64:      movq %fs:0, %rax
+; X64-NEXT: addq $i@TPOFF, %rax
+; X64-NEXT: ret
+
+entry:
+	ret i32* @i
+}
+
+define i32 @f3() {
+; X32: f3:
+; X32:      movl i2@INDNTPOFF, %eax
+; X32-NEXT: movl %gs:(%eax), %eax
+; X32-NEXT: ret
+; X64: f3:
+; X64:      movq i2@GOTTPOFF(%rip), %rax
+; X64-NEXT: movl %fs:(%rax), %eax
+; X64-NEXT: ret
+
+entry:
+	%tmp1 = load i32* @i2
+	ret i32 %tmp1
+}
+
+define i32* @f4() {
+; X32: f4:
+; X32:      movl %gs:0, %eax
+; X32-NEXT: addl i2@INDNTPOFF, %eax
+; X32-NEXT: ret
+; X64: f4:
+; X64:      movq %fs:0, %rax
+; X64-NEXT: addq i2@GOTTPOFF(%rip), %rax
+; X64-NEXT: ret
+
+entry:
+	ret i32* @i2
+}

tools/llc/llc.cpp

@@ -262,6 +262,11 @@ TrapFuncName("trap-func", cl::Hidden,
   cl::desc("Emit a call to trap function rather than a trap instruction"),
   cl::init(""));
 
+static cl::opt<bool>
+EnablePIE("enable-pie",
+  cl::desc("Assume the creation of a position independent executable."),
+  cl::init(false));
+
 static cl::opt<bool>
 SegmentedStacks("segmented-stacks",
   cl::desc("Use segmented stacks if possible."),
@@ -467,6 +472,7 @@ int main(int argc, char **argv) {
   Options.RealignStack = EnableRealignStack;
   Options.DisableJumpTables = DisableSwitchTables;
   Options.TrapFuncName = TrapFuncName;
+  Options.PositionIndependentExecutable = EnablePIE;
   Options.EnableSegmentedStacks = SegmentedStacks;
 
   std::auto_ptr<TargetMachine>