[X86] Support for the mno-tls-direct-seg-refs flag

Allows to disable direct TLS segment access (%fs or %gs). GCC supports
a similar flag, it can be useful in some circumstances, e.g. when a thread
context block needs to be updated directly from user space. More info
and specific use cases: https://bugs.llvm.org/show_bug.cgi?id=16145

There is another revision for clang as well.
Related: D53102

All X86 CodeGen tests appear to pass:
```
[46/47] Running lit suite /SourceCache/llvm-trunk-8.0/test/CodeGen
Testing Time: 23.17s
  Expected Passes    : 3801
  Expected Failures  : 15
  Unsupported Tests  : 8021
```

Reviewed by: Craig Topper.

Patch by nruslan (Ruslan Nikolaev).

Differential Revision: https://reviews.llvm.org/D53103

llvm-svn: 344723

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index c06ad11589d..d6bcdcdf149 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -165,6 +165,9 @@ namespace {
     /// If true, selector should try to optimize for minimum code size.
     bool OptForMinSize;
 
+    /// Disable direct TLS access through segment registers.
+    bool IndirectTlsSegRefs;
+
   public:
     explicit X86DAGToDAGISel(X86TargetMachine &tm, CodeGenOpt::Level OptLevel)
         : SelectionDAGISel(tm, OptLevel), OptForSize(false),
@@ -177,6 +180,8 @@ namespace {
     bool runOnMachineFunction(MachineFunction &MF) override {
       // Reset the subtarget each time through.
       Subtarget = &MF.getSubtarget<X86Subtarget>();
+      IndirectTlsSegRefs = MF.getFunction().hasFnAttribute(
+                             "indirect-tls-seg-refs");
       SelectionDAGISel::runOnMachineFunction(MF);
       return true;
     }
@@ -981,6 +986,7 @@ bool X86DAGToDAGISel::matchLoadInAddress(LoadSDNode *N, X86ISelAddressMode &AM){
   // For more information see http://people.redhat.com/drepper/tls.pdf
   if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(Address))
     if (C->getSExtValue() == 0 && AM.Segment.getNode() == nullptr &&
+        !IndirectTlsSegRefs &&
         (Subtarget->isTargetGlibc() || Subtarget->isTargetAndroid() ||
          Subtarget->isTargetFuchsia()))
       switch (N->getPointerInfo().getAddrSpace()) {