AMDGPU/MC: Add .amdgpu_lds directive

Summary:
The directive defines a symbol as an group/local memory (LDS) symbol.
LDS symbols behave similar to common symbols for the purposes of ELF,
using the processor-specific SHN_AMDGPU_LDS as section index.

It is the linker and/or runtime loader's job to "instantiate" LDS symbols
and resolve relocations that reference them.

It is not possible to initialize LDS memory (not even zero-initialize
as for .bss).

We want to be able to link together objects -- starting with relocatable
objects, but possible expanding to shared objects in the future -- that
access LDS memory in a flexible way.

LDS memory is in an address space that is entirely separate from the
address space that contains the program image (code and normal data),
so having program segments for it doesn't really make sense.

Furthermore, we want to be able to compile multiple kernels in a
compilation unit which have disjoint use of LDS memory. In that case,
we may want to place LDS symbols differently for different kernels
to save memory (LDS memory is very limited and physically private to
each kernel invocation), so we can't simply place LDS symbols in a
.lds section.

Hence this solution where LDS symbols always stay undefined.

Change-Id: I08cbc37a7c0c32f53f7b6123aa0afc91dbc1748f

Reviewers: arsenm, rampitec, t-tye, b-sumner, jsjodin

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61493

llvm-svn: 364296

1 files changed, 14 insertions, 13 deletions

diff --git a/llvm/docs/AMDGPUUsage.rst b/llvm/docs/AMDGPUUsage.rst
index 5aecdfc3c22..e035c62c1c4 100644
--- a/llvm/docs/AMDGPUUsage.rst
+++ b/llvm/docs/AMDGPUUsage.rst
@@ -851,15 +851,16 @@ Symbols include the following:
   .. table:: AMDGPU ELF Symbols
      :name: amdgpu-elf-symbols-table
 
-     ===================== ============== ============= ==================
-     Name                  Type           Section       Description
-     ===================== ============== ============= ==================
-     *link-name*           ``STT_OBJECT`` - ``.data``   Global variable
-                                          - ``.rodata``
-                                          - ``.bss``
-     *link-name*\ ``.kd``  ``STT_OBJECT`` - ``.rodata`` Kernel descriptor
-     *link-name*           ``STT_FUNC``   - ``.text``   Kernel entry point
-     ===================== ============== ============= ==================
+     ===================== ================== ================ ==================
+     Name                  Type               Section          Description
+     ===================== ================== ================ ==================
+     *link-name*           ``STT_OBJECT``     - ``.data``      Global variable
+                                              - ``.rodata``
+					      - ``.bss``
+     *link-name*\ ``.kd``  ``STT_OBJECT``     - ``.rodata``    Kernel descriptor
+     *link-name*           ``STT_FUNC``       - ``.text``      Kernel entry point
+     *link-name*           ``STT_OBJECT``     - SHN_AMDGPU_LDS Global variable in LDS
+     ===================== ================== ================ ==================
 
 Global variable
   Global variables both used and defined by the compilation unit.
@@ -871,10 +872,10 @@ Global variable
   will resolve relocations using the definition provided by another code object
   or explicitly defined by the runtime.
 
-  All global symbols, whether defined in the compilation unit or external, are
-  accessed by the machine code indirectly through a GOT table entry. This
-  allows them to be preemptable. The GOT table is only supported when the target
-  triple OS is ``amdhsa`` (see :ref:`amdgpu-target-triples`).
+  If the symbol resides in local/group memory (LDS) then its section is the
+  special processor-specific section name ``SHN_AMDGPU_LDS``, and the
+  ``st_value`` field describes alignment requirements as it does for common
+  symbols.
 
   .. TODO
      Add description of linked shared object symbols. Seems undefined symbols