[llvm] 1ae9575 - [AMDGPU][NFC] Update a comment about FLAT v/s LDSDMA

[Sameer Sahasrabuddhe via llvm-commits]

Author: Sameer Sahasrabuddhe
Date: 2025-12-08T20:49:19+05:30
New Revision: 1ae957515cccf8825733fbe100ff32864543d248

URL: https://github.com/llvm/llvm-project/commit/1ae957515cccf8825733fbe100ff32864543d248
DIFF: https://github.com/llvm/llvm-project/commit/1ae957515cccf8825733fbe100ff32864543d248.diff

LOG: [AMDGPU][NFC] Update a comment about FLAT v/s LDSDMA

The change in #170263 does not do justice to common knowledge in the backend.
Fix the comment to reflect the relation between FLAT encoding, flat pointer
access, and LDSDMA operations.

Added: 
    

Modified: 
    llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

Removed: 
    


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diff  --git a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
index 3b59045337b10..664ce126e64a1 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
@@ -2297,12 +2297,11 @@ void SIInsertWaitcnts::updateEventWaitcntAfter(MachineInstr &Inst,
       ScoreBrackets->updateByEvent(LDS_ACCESS, Inst);
     }
 
-    // If this is a truly flat memory operation, then it accesss both VMEM and
-    // LDS, so note it - it will require that both the VM and LGKM be flushed to
-    // zero if it is pending when a VM or LGKM dependency occurs.
-    //
-    // For example, LDS DMA operations have FLAT set in their TSFlags for
-    // unspecified reasons, but they are not flat operations)
+    // Async/LDSDMA operations have FLAT encoding but do not actually use flat
+    // pointers. They do have two operands that each access global and LDS, thus
+    // making it appear at this point that they are using a flat pointer. Filter
+    // them out, and for the rest, generate a dependency on flat pointers so
+    // that both VM and LGKM counters are flushed.
     if (!SIInstrInfo::isLDSDMA(Inst) && FlatASCount > 1)
       ScoreBrackets->setPendingFlat();
   } else if (SIInstrInfo::isVMEM(Inst) &&