[AMDGPU][MC][DOC] Updated AMD GPU assembler description

Stage 2: added detailed description of operands

See bug 36572: https://bugs.llvm.org/show_bug.cgi?id=36572

llvm-svn: 349368
diff --git a/llvm/docs/AMDGPU/gfx7_src_exp.rst b/llvm/docs/AMDGPU/gfx7_src_exp.rst
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+..
+    **************************************************
+    *                                                *
+    *   Automatically generated file, do not edit!   *
+    *                                                *
+    **************************************************
+
+.. _amdgpu_synid7_src_exp:
+
+vsrc
+===========================
+
+Data to copy to export buffers. This is an optional operand. Must be specified as :ref:`off<amdgpu_synid_off>` if not used.
+
+:ref:`compr<amdgpu_synid_compr>` modifier indicates use of compressed (16-bit) data. This limits number of source operands from 4 to 2:
+
+* src0 and src1 must specify the first register (or :ref:`off<amdgpu_synid_off>`).
+* src2 and src3 must specify the second register (or :ref:`off<amdgpu_synid_off>`).
+
+An example:
+
+.. code-block:: nasm
+
+  exp mrtz v3, v3, off, off compr
+
+*Size:* 1 dword.
+
+*Operands:* :ref:`v<amdgpu_synid_v>`, :ref:`off<amdgpu_synid_off>`