AMDGPU: Document why we use (non-volatile) BUFFER_WBINVL1 in graphics

[nhaehnle]

Summary

Graphics never used volatile vs. non-volatile in anything even
approaching a principled way, and trying to fix all the drivers years
later is fraught with unnecessary risk.

Let's just document why things are the way they are and be done with it.

Change-Id: If937c95834b712da8561e262bc41c5669e45c78c

[nhaehnle]

It seems the MTYPE numbering was changed on gfx9, reflect that in the comment.

[t-tye]

llvm/lib/Target/AMDGPU/SIMemoryLegalizer.cpp

904

BUFFER_WBINVL1

[nhaehnle]

After our internal discussion of this fizzled out, I am no longer actually sure whether this analysis is correct.

[arsenm]

Should this be abandoned?

[arsenm]

Can this be abandoned?

llvm/lib/Target/AMDGPU/SIMemoryLegalizer.cpp

 //===- SIMemoryLegalizer.cpp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
bool SIGfx7CacheControl::insertAcquire(MachineBasicBlock::iterator &MI,
 
   bool Changed = false;
 
   MachineBasicBlock &MBB = *MI->getParent();
   DebugLoc DL = MI->getDebugLoc();
 
   const GCNSubtarget &STM = MBB.getParent()->getSubtarget<GCNSubtarget>();
 
+  // Graphics environments never added a principled use of volatile vs.
+  // non-volatile, and the default MTYPE in buffer descriptors (MTYPE value 0)
+  // is NC_NV (non-coherent, non-volatile) on gfx8 (and maybe on gfx7, but not
+  // on gfx9), so a non-volatile wbinv is required.
   const unsigned InvalidateL1 = STM.isAmdPalOS() || STM.isMesa3DOS()
                                     ? AMDGPU::BUFFER_WBINVL1
                                     : AMDGPU::BUFFER_WBINVL1_VOL;
 
   if (Pos == Position::AFTER)
     ++MI;
 
   if ((AddrSpace & SIAtomicAddrSpace::GLOBAL) != SIAtomicAddrSpace::NONE) {