[ARM] Add support for embedded position-independent code

This patch adds support for some new relocation models to the ARM
backend:

* Read-only position independence (ROPI): Code and read-only data is accessed
  PC-relative. The offsets between all code and RO data sections are known at
  static link time. This does not affect read-write data.
* Read-write position independence (RWPI): Read-write data is accessed relative
  to the static base register (r9). The offsets between all writeable data
  sections are known at static link time. This does not affect read-only data.

These two modes are independent (they specify how different objects
should be addressed), so they can be used individually or together. They
are otherwise the same as the "static" relocation model, and are not
compatible with SysV-style PIC using a global offset table.

These modes are normally used by bare-metal systems or systems with
small real-time operating systems. They are designed to avoid the need
for a dynamic linker, the only initialisation required is setting r9 to
an appropriate value for RWPI code.

I have only added support to SelectionDAG, not FastISel, because
FastISel is currently disabled for bare-metal targets where these modes
would be used.

Differential Revision: https://reviews.llvm.org/D23195

llvm-svn: 278015

1 files changed, 7 insertions, 6 deletions

diff --git a/llvm/lib/Target/TargetLoweringObjectFile.cpp b/llvm/lib/Target/TargetLoweringObjectFile.cpp
index f863f429f43..3be4c1d5155 100644
--- a/llvm/lib/Target/TargetLoweringObjectFile.cpp
+++ b/llvm/lib/Target/TargetLoweringObjectFile.cpp
@@ -208,12 +208,13 @@ SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalValue *GV,
       }
 
     } else {
-      // In static relocation model, the linker will resolve all addresses, so
-      // the relocation entries will actually be constants by the time the app
-      // starts up.  However, we can't put this into a mergable section, because
-      // the linker doesn't take relocations into consideration when it tries to
-      // merge entries in the section.
-      if (ReloModel == Reloc::Static)
+      // In static, ROPI and RWPI relocation models, the linker will resolve
+      // all addresses, so the relocation entries will actually be constants by
+      // the time the app starts up.  However, we can't put this into a
+      // mergable section, because the linker doesn't take relocations into
+      // consideration when it tries to merge entries in the section.
+      if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI ||
+          ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI)
         return SectionKind::getReadOnly();
 
       // Otherwise, the dynamic linker needs to fix it up, put it in the