[llvm] AMDGPU: Document more backend recognized attributes (PR #80239)

[Jay Foad via llvm-commits]

================
@@ -1449,6 +1449,42 @@ The AMDGPU backend supports the following LLVM IR attributes.
                                              the frame. This is an internal detail of how LDS variables are lowered,
                                              language front ends should not set this attribute.
 
+     "amdgpu-gds-size"                       Bytes expected to be allocated at the start of GDS memory at entry.
+
+     "amdgpu-git-ptr-high"                   The hard-wired high half of the address of the global information table
+                                             for AMDPAL OS type. 0xffffffff represents no hard-wired high half, since
+                                             current hardware only allows a 16 bit value.
+
+     "amdgpu-32bit-address-high-bits"        Assumed high 32-bits for 32-bit address spaces which are really truncated
+                                             64-bit addresses (i.e., addrspace(6))
+
+     "amdgpu-color-export"                   Indicates shader exports color information if set to 1.
+                                             Defaults to 1 for :ref:`amdgpu_ps <amdgpu-cc>`, and 0 for other calling
+                                             conventions. Determines the necessity and type of null exports when a shader
+                                             terminates early by killing lanes.
+
+     "amdgpu-depth-export"                   Indicates shader exports depth information if set to 1. Determines the
+                                             necessity and type of null exports when a shader terminates early by killing
+                                             lanes. A depth-only shader will export to depth channel when no null export
+                                             target is available (GFX11+).
+
+     "InitialPSInputAddr"                    Set the initial value of the `spi_ps_input_addr` register for
+                                             :ref:`amdgpu_ps <amdgpu-cc>` shaders. Any bits enabled by this value will
+                                             be enabled in the final register value.
+
+     "amdgpu-wave-priority-threshold"        VALU instruction count threshold for adjusting wave priority. If exceeded,
+                                             temporarily raise the wave priority at the start of the shader function
+                                             until its last VMEM instructions to allow younger waves to issue their VMEM
+                                             instructions as well.
+
+     "amdgpu-memory-bound"                   Set internally by backend
+
+     "amdgpu-wave-limiter"                   Set internally by backend
----------------
jayfoad wrote:

[LLPC](https://github.com/GPUOpen-Drivers/llpc) sets these in some cases. Any explicit setting takes precedence over the backend's internal heuristics. But I'm not sure if we want to document that.

https://github.com/llvm/llvm-project/pull/80239