[llvm] [AMDGPU] Fix no waitcnt produced between LDS DMA and ds_read on gfx10 (PR #75245)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Stanislav Mekhanoshin (rampitec)

<details>
<summary>Changes</summary>

BUFFER_LOAD_DWORD_LDS was incorrectly touching vscnt instead of the vmcnt. This is VMEM load and DS store, so it shall use vmcnt.

---
Full diff: https://github.com/llvm/llvm-project/pull/75245.diff


3 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp (+8-11) 
- (modified) llvm/lib/Target/AMDGPU/SIInstrInfo.h (+12) 
- (modified) llvm/test/CodeGen/AMDGPU/lds-dma-waits.ll (+3-8) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
index ede4841b8a5fd..8967369cf21b6 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
@@ -452,7 +452,9 @@ class SIInsertWaitcnts : public MachineFunctionPass {
   // FLAT instruction.
   WaitEventType getVmemWaitEventType(const MachineInstr &Inst) const {
     assert(SIInstrInfo::isVMEM(Inst) || SIInstrInfo::isFLAT(Inst));
-    if (!ST->hasVscnt())
+    // LDS DMA loads are also stores, but on the LDS side. On the VMEM side
+    // these should use VM_CNT.
+    if (!ST->hasVscnt() || SIInstrInfo::mayWriteLDSThroughDMA(Inst))
       return VMEM_ACCESS;
     if (Inst.mayStore() && !SIInstrInfo::isAtomicRet(Inst)) {
       // FLAT and SCRATCH instructions may access scratch. Other VMEM
@@ -544,14 +546,6 @@ void WaitcntBrackets::setExpScore(const MachineInstr *MI,
   }
 }
 
-// MUBUF and FLAT LDS DMA operations need a wait on vmcnt before LDS written
-// can be accessed. A load from LDS to VMEM does not need a wait.
-static bool mayWriteLDSThroughDMA(const MachineInstr &MI) {
-  return SIInstrInfo::isVALU(MI) &&
-         (SIInstrInfo::isMUBUF(MI) || SIInstrInfo::isFLAT(MI)) &&
-         MI.getOpcode() != AMDGPU::BUFFER_STORE_LDS_DWORD;
-}
-
 void WaitcntBrackets::updateByEvent(const SIInstrInfo *TII,
                                     const SIRegisterInfo *TRI,
                                     const MachineRegisterInfo *MRI,
@@ -703,7 +697,10 @@ void WaitcntBrackets::updateByEvent(const SIInstrInfo *TII,
         setRegScore(RegNo, T, CurrScore);
       }
     }
-    if (Inst.mayStore() && (TII->isDS(Inst) || mayWriteLDSThroughDMA(Inst))) {
+    if (Inst.mayStore() &&
+        (TII->isDS(Inst) || TII->mayWriteLDSThroughDMA(Inst))) {
+      // MUBUF and FLAT LDS DMA operations need a wait on vmcnt before LDS
+      // written can be accessed. A load from LDS to VMEM does not need a wait.
       setRegScore(SQ_MAX_PGM_VGPRS + EXTRA_VGPR_LDS, T, CurrScore);
     }
   }
@@ -1178,7 +1175,7 @@ bool SIInsertWaitcnts::generateWaitcntInstBefore(MachineInstr &MI,
         if (AS != AMDGPUAS::LOCAL_ADDRESS && AS != AMDGPUAS::FLAT_ADDRESS)
           continue;
         // No need to wait before load from VMEM to LDS.
-        if (mayWriteLDSThroughDMA(MI))
+        if (TII->mayWriteLDSThroughDMA(MI))
           continue;
         unsigned RegNo = SQ_MAX_PGM_VGPRS + EXTRA_VGPR_LDS;
         // VM_CNT is only relevant to vgpr or LDS.
diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.h b/llvm/lib/Target/AMDGPU/SIInstrInfo.h
index e794d8cf7cc22..32a81d67cc246 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.h
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.h
@@ -546,6 +546,14 @@ class SIInstrInfo final : public AMDGPUGenInstrInfo {
     return get(Opcode).TSFlags & SIInstrFlags::DS;
   }
 
+  static bool isLDSDMA(const MachineInstr &MI) {
+    return isVALU(MI) && (isMUBUF(MI) || isFLAT(MI));
+  }
+
+  bool isLDSDMA(uint16_t Opcode) {
+    return isVALU(Opcode) && (isMUBUF(Opcode) || isFLAT(Opcode));
+  }
+
   static bool isGWS(const MachineInstr &MI) {
     return MI.getDesc().TSFlags & SIInstrFlags::GWS;
   }
@@ -667,6 +675,10 @@ class SIInstrInfo final : public AMDGPUGenInstrInfo {
                                   SIInstrFlags::IsAtomicNoRet);
   }
 
+  static bool mayWriteLDSThroughDMA(const MachineInstr &MI) {
+    return isLDSDMA(MI) && MI.getOpcode() != AMDGPU::BUFFER_STORE_LDS_DWORD;
+  }
+
   static bool isWQM(const MachineInstr &MI) {
     return MI.getDesc().TSFlags & SIInstrFlags::WQM;
   }
diff --git a/llvm/test/CodeGen/AMDGPU/lds-dma-waits.ll b/llvm/test/CodeGen/AMDGPU/lds-dma-waits.ll
index 688f0314f7d8f..58c6406fac7aa 100644
--- a/llvm/test/CodeGen/AMDGPU/lds-dma-waits.ll
+++ b/llvm/test/CodeGen/AMDGPU/lds-dma-waits.ll
@@ -9,15 +9,10 @@ declare void @llvm.amdgcn.global.load.lds(ptr addrspace(1) nocapture %gptr, ptr
 
 ; FIXME: vmcnt(0) is too strong, it shall use vmcnt(2) before the first
 ;        ds_read_b32 and vmcnt(0) before the second.
-; FIXME: GFX10 does not get a waitcount at all.
 
 ; GCN-LABEL: {{^}}buffer_load_lds_dword_2_arrays:
 ; GCN-COUNT-4: buffer_load_dword
-; GFX9: s_waitcnt vmcnt(0)
-
-; FIXME:
-; GFX10-NOT: s_waitcnt
-
+; GCN: s_waitcnt vmcnt(0)
 ; GCN: ds_read_b32
 
 ; FIXME:
@@ -49,9 +44,9 @@ main_body:
 ; GFX9: s_waitcnt vmcnt(0)
 ; GFX9-COUNT-2: ds_read_b32
 
-; FIXME:
-; GFX10-NOT: s_waitcnt
+; FIXME: can be vmcnt(2)
 
+; GFX10: s_waitcnt vmcnt(0)
 ; GFX10: ds_read_b32
 
 ; FIXME:

``````````

</details>


https://github.com/llvm/llvm-project/pull/75245