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

1 files changed, 8 insertions, 2 deletions

diff --git a/include/llvm/Target/TargetOptions.h b/include/llvm/Target/TargetOptions.h
index 7730ab90e8..1c10059218 100644
--- a/include/llvm/Target/TargetOptions.h
+++ b/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(""),
-          FloatABIType(FloatABI::Default)
+          PositionIndependentExecutable(false), EnableSegmentedStacks(false),
+          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