MISched: Don't schedule regions with 0 instructions

[arsenm]

Summary

I think this is correct, but may not necessarily be the correct fix
for the assertion I'm really trying to solve. If a scheduling region
was found that only has dbg_value instructions, the RegPressure
tracker would end up in an inconsistent state because it would skip
over any debug instructions and point to an instruction outside of the
scheduling region. It may still be possible for this to happen if
there are some real schedulable instructions between dbg_values, but I
haven't managed to break this.

The testcase is extremely sensitive and I'm not sure how to make it
more resistent to future scheduler changes that would avoid stressing
this situation.

See also D59144

[arsenm]

ping

[niravd]

LGTM.

[arsenm]

r356926

lib/CodeGen/MachineScheduler.cpp

for(MachineBasicBlock::iterator RegionEnd = MBB->end();
     // The next region starts above the previous region. Look backward in the
     // instruction stream until we find the nearest boundary.
     unsigned NumRegionInstrs = 0;
     I = RegionEnd;
     for (;I != MBB->begin(); --I) {
       MachineInstr &MI = *std::prev(I);
       if (isSchedBoundary(&MI, &*MBB, MF, TII))
         break;
-      if (!MI.isDebugInstr())
+      if (!MI.isDebugInstr()) {
         // MBB::size() uses instr_iterator to count. Here we need a bundle to
         // count as a single instruction.
         ++NumRegionInstrs;
       }
+    }
 
+    // It's possible we found a scheduling region that only has debug
+    // instructions. Don't bother scheduling these.
+    if (NumRegionInstrs != 0)
       Regions.push_back(SchedRegion(I, RegionEnd, NumRegionInstrs));
   }
 
   if (RegionsTopDown)
     std::reverse(Regions.begin(), Regions.end());
 }
 
 /// Main driver for both MachineScheduler and PostMachineScheduler.
 void MachineSchedulerBase::scheduleRegions(ScheduleDAGInstrs &Scheduler,

test/CodeGen/AMDGPU/sched-assert-onlydbg-value-empty-region.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=machine-scheduler -verify-machineinstrs %s -o - | FileCheck %s
+
+# The sequence of DBG_VALUEs forms a scheduling region with 0 real
+# instructions. The RegPressure tracker would end up skipping over any
+# debug instructions, so it would point to the instruction
+# before/outside of the region, hitting this assert:
+#  assert((BotRPTracker.getPos() == RegionEnd ||
+#          (RegionEnd->isDebugInstr() &&
+#           BotRPTracker.getPos() == priorNonDebug(RegionEnd, RegionBegin))) &&
+#         "Can't find the region bottom");
+
+--- |
+  define amdgpu_kernel void @only_dbg_value_sched_region() #0 {
+    ret void
+  }
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!2}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 8.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, nameTableKind: None)
+  !1 = !DIFile(filename: "foo", directory: "/dev/null")
+  !2 = !{i32 2, !"Debug Info Version", i32 3}
+  !3 = !{i32 0, i32 256}
+  !4 = !DILocation(line: 93, column: 2, scope: !5)
+  !5 = distinct !DISubprogram(name: "func", scope: !6, file: !6, line: 92, scopeLine: 92, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+  !6 = !DIFile(filename: "arst.cl", directory: "/dev/null")
+  !7 = !DILocalVariable(name: "p", arg: 2, scope: !5, file: !6, line: 92)
+  !8 = !DILocation(line: 92, column: 47, scope: !5)
+  !9 = !{}
+
+...
+---
+name:            only_dbg_value_sched_region
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+  waveLimiter:     true
+body:             |
+  ; CHECK-LABEL: name: only_dbg_value_sched_region
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: $vgpr0
+  ; CHECK:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; CHECK:   [[DEF:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   [[GLOBAL_LOAD_DWORDX2_:%[0-9]+]]:vreg_64 = GLOBAL_LOAD_DWORDX2 [[DEF]], 0, 0, 0, implicit $exec
+  ; CHECK:   [[GLOBAL_LOAD_DWORD:%[0-9]+]]:vgpr_32 = GLOBAL_LOAD_DWORD [[DEF]], 8, 0, 0, implicit $exec
+  ; CHECK:   undef %4.sub1:vreg_64 = V_ADD_U32_e32 [[COPY]], [[COPY]], implicit $exec
+  ; CHECK:   %4.sub0:vreg_64 = V_MOV_B32_e32 111, implicit $exec
+  ; CHECK:   [[DEF1:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   [[DEF2:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   [[DEF3:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   undef %11.sub1:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   [[DEF4:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   [[DEF5:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+  ; CHECK:   [[V_MOV_B32_e32_:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+  ; CHECK:   [[V_MOV_B32_e32_1:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+  ; CHECK:   [[DEF6:%[0-9]+]]:vgpr_32 = IMPLICIT_DEF
+  ; CHECK:   [[DEF7:%[0-9]+]]:vgpr_32 = IMPLICIT_DEF
+  ; CHECK:   [[COPY1:%[0-9]+]]:vreg_64 = COPY [[GLOBAL_LOAD_DWORDX2_]]
+  ; CHECK:   undef %6.sub0:vreg_64 = V_ADD_F32_e32 [[DEF]].sub0, [[COPY1]].sub0, implicit $exec
+  ; CHECK:   dead undef %6.sub1:vreg_64 = V_ADD_F32_e32 [[DEF]].sub1, [[COPY1]].sub0, implicit $exec
+  ; CHECK:   [[GLOBAL_LOAD_DWORD1:%[0-9]+]]:vgpr_32 = GLOBAL_LOAD_DWORD [[COPY1]], 0, 0, 0, implicit $exec
+  ; CHECK:   [[DEF8:%[0-9]+]]:vgpr_32 = IMPLICIT_DEF
+  ; CHECK:   undef %19.sub0:vreg_64 = V_ADD_F32_e32 [[GLOBAL_LOAD_DWORD1]], [[GLOBAL_LOAD_DWORDX2_]].sub0, implicit $exec
+  ; CHECK:   %19.sub1:vreg_64 = V_ADD_F32_e32 [[GLOBAL_LOAD_DWORD]], [[GLOBAL_LOAD_DWORD]], implicit $exec
+  ; CHECK:   GLOBAL_STORE_DWORDX2 %19, %4, 32, 0, 0, implicit $exec
+  ; CHECK:   %11.sub0:vreg_64 = GLOBAL_LOAD_DWORD [[DEF1]], 0, 0, 0, implicit $exec
+  ; CHECK:   [[DEF2]].sub0:vreg_64 = GLOBAL_LOAD_DWORD [[DEF3]], 0, 0, 0, implicit $exec
+  ; CHECK:   dead %20:vgpr_32 = GLOBAL_LOAD_DWORD %11, 0, 0, 0, implicit $exec
+  ; CHECK:   dead %21:vgpr_32 = GLOBAL_LOAD_DWORD [[DEF4]], 0, 0, 0, implicit $exec
+  ; CHECK:   [[V_LSHLREV_B64_:%[0-9]+]]:vreg_64 = V_LSHLREV_B64 2, [[DEF2]], implicit $exec
+  ; CHECK:   dead %22:vgpr_32 = GLOBAL_LOAD_DWORD [[DEF5]], 0, 0, 0, implicit $exec
+  ; CHECK:   S_NOP 0, implicit [[DEF7]], implicit [[V_LSHLREV_B64_]].sub0, implicit [[DEF6]], implicit [[V_MOV_B32_e32_]]
+  ; CHECK:   GLOBAL_STORE_DWORD [[DEF5]], [[V_MOV_B32_e32_1]], 0, 0, 0, implicit $exec
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.2(0x80000000)
+  ; CHECK:   S_SETREG_IMM32_B32 0, 1
+  ; CHECK:   DBG_VALUE %21, $noreg
+  ; CHECK:   DBG_VALUE %22, $noreg
+  ; CHECK:   DBG_VALUE [[V_LSHLREV_B64_]], $noreg
+  ; CHECK:   S_SETREG_IMM32_B32 0, 1
+  ; CHECK: bb.2:
+  ; CHECK:   S_NOP 0, implicit [[COPY]]
+  ; CHECK:   S_NOP 0, implicit [[DEF8]]
+  ; CHECK:   S_ENDPGM 0
+  bb.0:
+    liveins: $vgpr0
+
+    %0:vgpr_32 = COPY $vgpr0
+    %1:vreg_64 = IMPLICIT_DEF
+    %2:vreg_64 = GLOBAL_LOAD_DWORDX2 %1, 0, 0, 0, implicit $exec
+    %3:vgpr_32 = GLOBAL_LOAD_DWORD %1, 8, 0, 0, implicit $exec
+    undef %4.sub1:vreg_64 = V_ADD_U32_e32 %0, %0, implicit $exec
+    %4.sub0:vreg_64 = V_MOV_B32_e32 111, implicit $exec
+    %5:vreg_64 = COPY %2
+    undef %6.sub0:vreg_64 = V_ADD_F32_e32 %1.sub0, %5.sub0, implicit $exec
+    %6.sub1:vreg_64 = V_ADD_F32_e32 %1.sub1, %5.sub0, implicit $exec
+    %7:vgpr_32 = GLOBAL_LOAD_DWORD %5, 0, 0, 0, implicit $exec
+    %8:vreg_64 = IMPLICIT_DEF
+    %9:vreg_64 = IMPLICIT_DEF
+    %10:vreg_64 = IMPLICIT_DEF
+    undef %11.sub1:vreg_64 = IMPLICIT_DEF
+    %12:vgpr_32 = IMPLICIT_DEF
+    %13:vgpr_32 = IMPLICIT_DEF
+    %14:vreg_64 = IMPLICIT_DEF
+    %15:vreg_64 = IMPLICIT_DEF
+    %16:vgpr_32 = IMPLICIT_DEF
+    %17:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+    %18:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+    undef %19.sub0:vreg_64 = V_ADD_F32_e32 %7, %2.sub0, implicit $exec
+    %19.sub1:vreg_64 = V_ADD_F32_e32 %3, %3, implicit $exec
+    GLOBAL_STORE_DWORDX2 %19, %4, 32, 0, 0, implicit $exec
+    %11.sub0:vreg_64 = GLOBAL_LOAD_DWORD %9, 0, 0, 0, implicit $exec
+    %8.sub0:vreg_64 = GLOBAL_LOAD_DWORD %10, 0, 0, 0, implicit $exec
+    %20:vgpr_32 = GLOBAL_LOAD_DWORD %11, 0, 0, 0, implicit $exec
+    %21:vgpr_32 = GLOBAL_LOAD_DWORD %14, 0, 0, 0, implicit $exec
+    %22:vgpr_32 = GLOBAL_LOAD_DWORD %15, 0, 0, 0, implicit $exec
+    %23:vreg_64 = V_LSHLREV_B64 2, %8, implicit $exec
+    S_NOP 0, implicit %13, implicit %23.sub0, implicit %12, implicit %17
+    GLOBAL_STORE_DWORD %15, %18, 0, 0, 0, implicit $exec
+
+  bb.1:
+    S_SETREG_IMM32_B32 0, 1
+    DBG_VALUE %21, $noreg, !7, !DIExpression(DW_OP_constu, 1, DW_OP_swap, DW_OP_xderef), debug-location !8
+    DBG_VALUE %22, $noreg, !7, !DIExpression(DW_OP_constu, 1, DW_OP_swap, DW_OP_xderef), debug-location !8
+    DBG_VALUE %23, $noreg, !7, !DIExpression(DW_OP_constu, 1, DW_OP_swap, DW_OP_xderef), debug-location !8
+    S_SETREG_IMM32_B32 0, 1
+
+  bb.2:
+    S_NOP 0, implicit %0
+    S_NOP 0, implicit %16
+    S_ENDPGM 0
+
+...