[AMDGPU] Update AMDGPUUsage.rst

[scott.linder]

Summary

• Fix typo "competing" -> "completing".

• Document reason waintcnt is added to store and not load for sequential consistent.

Cherry-pick of patch by Tony Tye

[nhaehnle]

Duplicate of D89596?

[scott.linder]

In D89595#2339181, @nhaehnle wrote:

Duplicate of D89596?

Yes, I think my arc threw a lot of PHP deprecation warnings at me and I ended up pushing again without noticing.

llvm/docs/AMDGPUUsage.rst

load atomic  seq_cst      - workgroup    - global   1. s_waitcnt lgkmcnt(0)         1. s_waitcnt lgkmcnt(0) &
                                                              release is                      release is
                                                              prevented by the                prevented by the
                                                              waitcnt of                      waitcnt of
                                                              the release, but                the release, but
                                                              there is nothing                there is nothing
                                                              preventing a store              preventing a store
                                                              release followed by             release followed by
                                                              load acquire from               load acquire from
-                                                             competing out of                competing out of
-                                                             order.)                         order.)
+                                                             completing out of               completing out of
+                                                             order. The waitcnt              order. The waitcnt
+                                                             could be placed after           could be placed after
+                                                             seq_store or before             seq_store or before
+                                                             the seq_load. We                the seq_load. We
+                                                             choose the load to              choose the load to
+                                                             make the waitcnt be             make the waitcnt be
+                                                             as late as possible             as late as possible
+                                                             so that the store               so that the store
+                                                             may have already                may have already
+                                                             completed.)                     completed.)
 
                                                          2. *Following                   2. *Following
                                                             instructions same as            instructions same as
                                                             corresponding load              corresponding load
                                                             atomic acquire,                 atomic acquire,
                                                             except must generated           except must generated
                                                             all instructions even           all instructions even
                                                             for OpenCL.*                    for OpenCL.*
load atomic  seq_cst      - workgroup    - local    *Same as corresponding
                                                                                              release is
                                                                                              prevented by the
                                                                                              waitcnt of
                                                                                              the release, but
                                                                                              there is nothing
                                                                                              preventing a store
                                                                                              release followed by
                                                                                              load acquire from
-                                                                                             competing out of
-                                                                                             order.)
+                                                                                             completing out of
+                                                                                             order. The waitcnt
+                                                                                             could be placed after
+                                                                                             seq_store or before
+                                                                                             the seq_load. We
+                                                                                             choose the load to
+                                                                                             make the waitcnt be
+                                                                                             as late as possible
+                                                                                             so that the store
+                                                                                             may have already
+                                                                                             completed.)
 
                                                                                          2. *Following
                                                                                             instructions same as
                                                                                             corresponding load
                                                                                             atomic acquire,
                                                                                             except must generated
                                                                                             all instructions even
                                                                                             for OpenCL.*
load atomic  seq_cst      - agent        - global   1. s_waitcnt lgkmcnt(0) &       1. s_waitcnt lgkmcnt(0) &
                                                              release is                      release is
                                                              prevented by the                prevented by the
                                                              waitcnt of                      waitcnt of
                                                              the release, but                the release, but
                                                              there is nothing                there is nothing
                                                              preventing a store              preventing a store
                                                              release followed by             release followed by
                                                              load acquire from               load acquire from
-                                                             competing out of                competing out of
-                                                             order.)                         order.)
+                                                             completing out of               completing out of
+                                                             order. The waitcnt              order. The waitcnt
+                                                             could be placed after           could be placed after
+                                                             seq_store or before             seq_store or before
+                                                             the seq_load. We                the seq_load. We
+                                                             choose the load to              choose the load to
+                                                             make the waitcnt be             make the waitcnt be
+                                                             as late as possible             as late as possible
+                                                             so that the store               so that the store
+                                                             may have already                may have already
+                                                             completed.)                     completed.)
 
                                                          2. *Following                   2. *Following
                                                             instructions same as            instructions same as
                                                             corresponding load              corresponding load
                                                             atomic acquire,                 atomic acquire,
                                                             except must generated           except must generated
                                                             all instructions even           all instructions even
                                                             for OpenCL.*                    for OpenCL.*