[AMDGPU] prefer non-mfma in post-RA schedule

[rampitec]

Summary

MFMA instructions shall not be scheduled back to back
to avoid MAI SIMD stall. Tell post-RA schedule we would
prefer some other instruction instead.

[kerbowa]

Minor nit, but test should disable-power-sched otherwise I think MFMA will be scheduled apart anyway.

[rampitec]

In D84883#2182887, @kerbowa wrote:

Minor nit, but test should disable-power-sched otherwise I think MFMA will be scheduled apart anyway.

It will not schedule s_cmp in between, but in general you are right.

[rampitec]

Added -amdgpu-disable-power-sched in test.

[kerbowa]

Actually MFMA would need more height to ensure post-RA tries to schedule them first and together. You could have their results be implicit uses on later instruction like S_ENDPGM or something.

[rampitec]

In D84883#2182909, @kerbowa wrote:

Actually MFMA would need more height to ensure post-RA tries to schedule them first and together. You could have their results be implicit uses on later instruction like S_ENDPGM or something.

W/o this patch they are scheduled back to back:

S_ADD_U32
S_ADDC_U32
V_MFMA_F32_32X32X1F32
V_MFMA_F32_32X32X1F32
S_CMP_LG_U32

[kerbowa]

LGTM either way

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h

public:
   void EmitInstruction(SUnit *SU) override;
   void EmitInstruction(MachineInstr *MI) override;
   HazardType getHazardType(SUnit *SU, int Stalls) override;
   void EmitNoop() override;
   unsigned PreEmitNoops(MachineInstr *) override;
   unsigned PreEmitNoopsCommon(MachineInstr *);
   void AdvanceCycle() override;
   void RecedeCycle() override;
+  bool ShouldPreferAnother(SUnit *SU) override;
 };
 
 } // end namespace llvm
 
 #endif //LLVM_LIB_TARGET_AMDGPUHAZARDRECOGNIZERS_H

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.cpp

for (const MachineOperand &Op : MI->explicit_uses()) {
 
     WaitStatesNeededForUse = VALUWriteAccVgprReadLdStDepVALUWaitStates -
       getWaitStatesSince(IsVALUAccVgprReadCheckFn, MaxWaitStates);
     WaitStatesNeeded = std::max(WaitStatesNeeded, WaitStatesNeededForUse);
   }
 
   return WaitStatesNeeded;
 }
+
+bool GCNHazardRecognizer::ShouldPreferAnother(SUnit *SU) {
+  if (!SU->isInstr())
+    return false;
+
+  MachineInstr *MAI = nullptr;
+  auto IsMFMAFn = [&MAI] (MachineInstr *MI) {
+    MAI = nullptr;
+    if (SIInstrInfo::isMAI(*MI) &&
+        MI->getOpcode() != AMDGPU::V_ACCVGPR_WRITE_B32 &&
+        MI->getOpcode() != AMDGPU::V_ACCVGPR_READ_B32)
+      MAI = MI;
+    return MAI != nullptr;
+  };
+
+  MachineInstr *MI = SU->getInstr();
+  if (IsMFMAFn(MI)) {
+    int W = getWaitStatesSince(IsMFMAFn, 16);
+    if (MAI)
+      return W < (int)TSchedModel.computeInstrLatency(MAI);
+  }
+
+  return false;
+}

llvm/test/CodeGen/AMDGPU/sched-prefer-non-mfma.mir

+# RUN: llc -march=amdgcn -mcpu=gfx908 -run-pass post-RA-sched -amdgpu-disable-power-sched -verify-machineinstrs -o - %s | FileCheck -check-prefix=GCN %s
+
+# GCN-LABEL: name: unrelated_mfma
+# GCN: V_MFMA_F32_32X32X1F32
+# GCN: S_CMP_LG_U32
+# GCN: V_MFMA_F32_32X32X1F32
+
+---
+name:            unrelated_mfma
+body:             |
+  bb.0.entry:
+    renamable $agpr0_agpr1_agpr2_agpr3_agpr4_agpr5_agpr6_agpr7_agpr8_agpr9_agpr10_agpr11_agpr12_agpr13_agpr14_agpr15_agpr16_agpr17_agpr18_agpr19_agpr20_agpr21_agpr22_agpr23_agpr24_agpr25_agpr26_agpr27_agpr28_agpr29_agpr30_agpr31 = V_MFMA_F32_32X32X1F32 $vgpr67, $vgpr66, $agpr0_agpr1_agpr2_agpr3_agpr4_agpr5_agpr6_agpr7_agpr8_agpr9_agpr10_agpr11_agpr12_agpr13_agpr14_agpr15_agpr16_agpr17_agpr18_agpr19_agpr20_agpr21_agpr22_agpr23_agpr24_agpr25_agpr26_agpr27_agpr28_agpr29_agpr30_agpr31, 0, 0, 0, implicit $mode, implicit $exec
+    renamable $agpr32_agpr33_agpr34_agpr35_agpr36_agpr37_agpr38_agpr39_agpr40_agpr41_agpr42_agpr43_agpr44_agpr45_agpr46_agpr47_agpr48_agpr49_agpr50_agpr51_agpr52_agpr53_agpr54_agpr55_agpr56_agpr57_agpr58_agpr59_agpr60_agpr61_agpr62_agpr63 = V_MFMA_F32_32X32X1F32 $vgpr69, $vgpr68, killed $agpr32_agpr33_agpr34_agpr35_agpr36_agpr37_agpr38_agpr39_agpr40_agpr41_agpr42_agpr43_agpr44_agpr45_agpr46_agpr47_agpr48_agpr49_agpr50_agpr51_agpr52_agpr53_agpr54_agpr55_agpr56_agpr57_agpr58_agpr59_agpr60_agpr61_agpr62_agpr63, 0, 0, 0, implicit $mode, implicit $exec
+    renamable $sgpr2 = S_ADD_U32 renamable $sgpr2, 4, implicit-def $scc
+    renamable $sgpr3 = S_ADDC_U32 renamable $sgpr3, 0, implicit-def dead $scc, implicit killed $scc
+    S_CMP_LG_U32 renamable $sgpr2, 64, implicit-def $scc
+
+...