[AMDGPU] Override PSet for M0

This change returns empty PSet list for M0 register. Otherwise its
PSet as defined by tablegen is SReg_32. This results in incorrect
register pressure calculation every time an instruction uses M0.
Such uses count as SReg_32 PSet and inadequately increase pressure
on SGPRs.

Differential Revision: https://reviews.llvm.org/D29798

llvm-svn: 294691

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -1329,3 +1329,11 @@ unsigned SIRegisterInfo::getRegPressureSetLimit(const MachineFunction &MF,
 
   return AMDGPURegisterInfo::getRegPressureSetLimit(MF, Idx);
 }
+
+const int *SIRegisterInfo::getRegUnitPressureSets(unsigned RegUnit) const {
+  static const int Empty[] = { -1 };
+
+  if (hasRegUnit(AMDGPU::M0, RegUnit))
+    return Empty;
+  return AMDGPURegisterInfo::getRegUnitPressureSets(RegUnit);
+}

llvm/lib/Target/AMDGPU/SIRegisterInfo.h

@@ -211,6 +211,8 @@ public:
   unsigned getRegPressureSetLimit(const MachineFunction &MF,
                                   unsigned Idx) const override;
 
+  const int *getRegUnitPressureSets(unsigned RegUnit) const override;
+
 private:
   void buildSpillLoadStore(MachineBasicBlock::iterator MI,
                            unsigned LoadStoreOp,

llvm/test/CodeGen/AMDGPU/schedule-regpressure.mir

@@ -0,0 +1,57 @@
+# RUN: llc -march=amdgcn -misched=converge -run-pass machine-scheduler %s -o - -debug-only=misched 2>&1 | FileCheck %s
+# REQUIRES: asserts
+
+# Check there is no SReg_32 pressure created by DS_* instructions because of M0 use
+
+# CHECK: ScheduleDAGMILive::schedule starting
+# CHECK: SU({{.*}} = DS_READ_B32 {{.*}} %M0<imp-use>, %EXEC<imp-use>
+# CHECK: Pressure Diff : {{$}}
+# CHECK: SU({{.*}} DS_WRITE_B32
+
+---
+name:            mo_pset
+alignment:       0
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: sreg_128 }
+  - { id: 1, class: sgpr_64 }
+  - { id: 2, class: sreg_32_xm0 }
+  - { id: 3, class: sgpr_32 }
+  - { id: 4, class: vgpr_32 }
+  - { id: 5, class: sreg_32_xm0_xexec }
+  - { id: 6, class: vgpr_32 }
+  - { id: 7, class: vgpr_32 }
+  - { id: 8, class: vgpr_32 }
+liveins:
+  - { reg: '%sgpr4_sgpr5', virtual-reg: '%1' }
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  maxCallFrameSize: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+body:             |
+  bb.0:
+    liveins: %sgpr4_sgpr5
+
+    %1 = COPY %sgpr4_sgpr5
+    %5 = S_LOAD_DWORD_IMM %1, 0, 0 :: (non-temporal dereferenceable invariant load 4 from `i32 addrspace(2)* undef`)
+    %m0 = S_MOV_B32 -1
+    %7 = COPY %5
+    %6 = DS_READ_B32 %7, 0, 0, implicit %m0, implicit %exec
+    DS_WRITE_B32 %7, %6, 4, 0, implicit killed %m0, implicit %exec
+    S_ENDPGM
+
+...