[AMDGPU] Update buffer fat pointer docs for gfx1250, fix formatting (#167818)

Author: krzysz00

llvm/docs/AMDGPUUsage.rst

@@ -1011,9 +1011,9 @@ supported for the ``amdgcn`` target.
   bounds checking may be disabled, buffer fat pointers may choose to enable
   it or not). The cache swizzle support introduced in gfx942 may be used.
 
-  These pointers can be created by `addrspacecast` from a buffer resource
-  (`ptr addrspace(8)`) or by using `llvm.amdgcn.make.buffer.rsrc` to produce a
-  `ptr addrspace(7)` directly, which produces a buffer fat pointer with an initial
+  These pointers can be created by ``addrspacecast`` from a buffer resource
+  (``ptr addrspace(8)```) or by using `llvm.amdgcn.make.buffer.rsrc` to produce a
+  ``ptr addrspace(7)`` directly, which produces a buffer fat pointer with an initial
   offset of 0 and prevents the address space cast from being rewritten away.
 
   The ``align`` attribute on operations from buffer fat pointers is deemed to apply
@@ -1028,26 +1028,33 @@ supported for the ``amdgcn`` target.
 **Buffer Resource**
   The buffer resource pointer, in address space 8, is the newer form
   for representing buffer descriptors in AMDGPU IR, replacing their
-  previous representation as `<4 x i32>`. It is a non-integral pointer
-  that represents a 128-bit buffer descriptor resource (`V#`).
+  previous representation as ``<4 x i32>``. It is a non-integral pointer
+  that represents a 128-bit buffer descriptor resource (``V#``).
 
   Since, in general, a buffer resource supports complex addressing modes that cannot
   be easily represented in LLVM (such as implicit swizzled access to structured
-  buffers), it is **illegal** to perform non-trivial address computations, such as
-  ``getelementptr`` operations, on buffer resources. They may be passed to
-  AMDGPU buffer intrinsics, and they may be converted to and from ``i128``.
+  buffers), performing address computations such as ``getelementptr`` is not
+  recommended on ``ptr addrspace(8)``s (if such computations are performed, the
+  offset must be wavefront-uniform.) Note that such a usage of GEP is currently
+  **unimplemented** in the backend, as it would require a wrapping 48-bit
+  addition. Buffer resources may be passed to AMDGPU buffer intrinsics, and they
+  may be converted to and from ``i128``.
 
   Casting a buffer resource to a buffer fat pointer is permitted and adds an offset
   of 0.
 
   Buffer resources can be created from 64-bit pointers (which should be either
-  generic or global) using the `llvm.amdgcn.make.buffer.rsrc` intrinsic, which
+  generic or global) using the ``llvm.amdgcn.make.buffer.rsrc`` intrinsic, which
   takes the pointer, which becomes the base of the resource,
   the 16-bit stride (and swzizzle control) field stored in bits `63:48` of a `V#`,
   the 32-bit NumRecords/extent field (bits `95:64`), and the 32-bit flags field
   (bits `127:96`). The specific interpretation of these fields varies by the
   target architecture and is detailed in the ISA descriptions.
 
+  On gfx1250, the base pointer is instead truncated to 57 bits and the NumRecords
+  field is 45 bits, which necessitated a change to ``make.buffer.rsrcs``'s arguments
+  in order to make that field an ``i64``.
+
   When buffer resources are passed to buffer intrinsics such as
   ``llvm.amdgcn.raw.ptr.buffer.load`` or
   ``llvm.amdgcn.struct.ptr.buffer.store``, the ``align`` attribute on the
@@ -1079,9 +1086,9 @@ supported for the ``amdgcn`` target.
   the stride is the size of a structured element, the "add tid" flag must be 0,
   and the swizzle enable bits must be off.
 
-  These pointers can be created by `addrspacecast` from a buffer resource
-  (`ptr addrspace(8)`) or by using `llvm.amdgcn.make.buffer.rsrc` to produce a
-  `ptr addrspace(9)` directly, which produces a buffer strided pointer whose initial
+  These pointers can be created by ``addrspacecast`` from a buffer resource
+  (``ptr addrspace(8)``) or by using ``llvm.amdgcn.make.buffer.rsrc`` to produce a
+  ``ptr addrspace(9)``` directly, which produces a buffer strided pointer whose initial
   index and offset values are both 0. This prevents the address space cast from
   being rewritten away.