[PATCH] D53103: Support for the mno-tls-direct-seg-refs flag

[Ruslan Nikolaev via Phabricator via llvm-commits]

nruslan created this revision.
nruslan added reviewers: hans, craig.topper, clang.
Herald added a subscriber: llvm-commits.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D53103

Files:
  docs/LangRef.rst
  lib/Target/X86/X86ISelDAGToDAG.cpp
  test/CodeGen/X86/tls.ll


Index: test/CodeGen/X86/tls.ll
===================================================================
--- test/CodeGen/X86/tls.ll
+++ test/CodeGen/X86/tls.ll
@@ -453,3 +453,20 @@
 
   ret i32* @i6
 }
+
+define i32 @f17() #0 {
+; X86_LINUX-LABEL: f17:
+; X86_LINUX:      movl %gs:0, %eax
+; X86_LINUX-NEXT: movl i1 at NTPOFF(%eax), %eax
+; X86_LINUX-NEXT: ret
+; X64_LINUX-LABEL: f17:
+; X64_LINUX:      movq %fs:0, %rax
+; X64_LINUX-NEXT: movl i1 at TPOFF(%rax), %eax
+; X64_LINUX-NEXT: ret
+
+entry:
+	%tmp1 = load i32, i32* @i1
+	ret i32 %tmp1
+}
+
+attributes #0 = { "indirect-tls-seg-refs" }
Index: lib/Target/X86/X86ISelDAGToDAG.cpp
===================================================================
--- lib/Target/X86/X86ISelDAGToDAG.cpp
+++ lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -165,6 +165,9 @@
     /// 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 @@
     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;
     }
@@ -979,6 +984,7 @@
   // 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()) {
Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -1450,6 +1450,10 @@
 ``noredzone``
     This attribute indicates that the code generator should not use a
     red zone, even if the target-specific ABI normally permits it.
+``indirect-tls-seg-refs``
+    This attribute indicates that the code generator should not use
+    direct TLS access through segment registers, even if the
+    target-specific ABI normally permits it.
 ``noreturn``
     This function attribute indicates that the function never returns
     normally. This produces undefined behavior at runtime if the


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