[ScheduleDAG] Fix removing edges with weak deps

[kerbowa]

Summary

In SUnit::removePred edges are removed from the Preds and Succs lists before
updating the bookkeeping. This could result in incorrect values for
NumPreds/SuccsLeft and cause WeakPreds/SuccsLeft to underflow, since the
incorrect SDep will be used to update these values.

[rampitec]

LGTM

llvm/lib/CodeGen/ScheduleDAG.cpp

void SUnit::removePred(const SDep &D) {
   if (I == Preds.end())
     return;
   // Find the corresponding successor in N.
   SDep P = D;
   P.setSUnit(this);
   SUnit *N = D.getSUnit();
   SmallVectorImpl<SDep>::iterator Succ = llvm::find(N->Succs, P);
   assert(Succ != N->Succs.end() && "Mismatching preds / succs lists!");
-  N->Succs.erase(Succ);
-  Preds.erase(I);
   // Update the bookkeeping.
   if (P.getKind() == SDep::Data) {
     assert(NumPreds > 0 && "NumPreds will underflow!");
     assert(N->NumSuccs > 0 && "NumSuccs will underflow!");
     --NumPreds;
     --N->NumSuccs;
   }
   if (!N->isScheduled) {
-    if (D.isWeak())
+    if (D.isWeak()) {
+      assert(WeakPredsLeft > 0 && "WeakPredsLeft will underflow!");
       --WeakPredsLeft;
-    else {
+    } else {
       assert(NumPredsLeft > 0 && "NumPredsLeft will underflow!");
       --NumPredsLeft;
     }
   }
   if (!isScheduled) {
-    if (D.isWeak())
+    if (D.isWeak()) {
+      assert(WeakSuccsLeft > 0 && "WeakSuccsLeft will underflow!");
       --N->WeakSuccsLeft;
-    else {
+    } else {
       assert(N->NumSuccsLeft > 0 && "NumSuccsLeft will underflow!");
       --N->NumSuccsLeft;
     }
   }
+  N->Succs.erase(Succ);
+  Preds.erase(I);
   if (P.getLatency() != 0) {
     this->setDepthDirty();
     N->setHeightDirty();
   }
 }
 
 void SUnit::setDepthDirty() {
   if (!isDepthCurrent) return;

llvm/test/CodeGen/AMDGPU/sched-barrier-hang-weak-dep.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=gfx908 -run-pass=machine-scheduler -verify-misched -o - %s | FileCheck %s
+
+# This would hang after removing edges from the SCHED_BARRIER since the number
+# of Preds/Succs would be left in an inconsistent state.
+
+---
+name: sched_barrier_cluster_dep_hang
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: sched_barrier_cluster_dep_hang
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:sreg_64 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:vgpr_32 = IMPLICIT_DEF
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   SCHED_BARRIER 128
+  ; CHECK-NEXT:   [[GLOBAL_LOAD_DWORD_SADDR:%[0-9]+]]:vgpr_32 = GLOBAL_LOAD_DWORD_SADDR [[DEF]], [[DEF1]], 512, 0, implicit $exec :: (load (s32))
+  ; CHECK-NEXT:   [[GLOBAL_LOAD_DWORD_SADDR1:%[0-9]+]]:vgpr_32 = GLOBAL_LOAD_DWORD_SADDR [[DEF]], [[DEF1]], 0, 0, implicit $exec :: (load (s32))
+  ; CHECK-NEXT:   SCHED_BARRIER 128
+  ; CHECK-NEXT:   [[V_MUL_LO_U32_e64_:%[0-9]+]]:vgpr_32 = nsw V_MUL_LO_U32_e64 [[GLOBAL_LOAD_DWORD_SADDR]], [[GLOBAL_LOAD_DWORD_SADDR]], implicit $exec
+  ; CHECK-NEXT:   [[V_MUL_LO_U32_e64_1:%[0-9]+]]:vgpr_32 = nsw V_MUL_LO_U32_e64 [[GLOBAL_LOAD_DWORD_SADDR1]], [[GLOBAL_LOAD_DWORD_SADDR1]], implicit $exec
+  ; CHECK-NEXT:   GLOBAL_STORE_DWORD_SADDR [[DEF1]], [[V_MUL_LO_U32_e64_]], [[DEF]], 512, 0, implicit $exec :: (store (s32))
+  ; CHECK-NEXT:   GLOBAL_STORE_DWORD_SADDR [[DEF1]], [[V_MUL_LO_U32_e64_1]], [[DEF]], 0, 0, implicit $exec :: (store (s32))
+  ; CHECK-NEXT:   S_ENDPGM 0
+  bb.0:
+    %0:sreg_64 = IMPLICIT_DEF
+    %1:vgpr_32 = IMPLICIT_DEF
+
+  bb.1:
+    SCHED_BARRIER 128
+    %3:vgpr_32 = GLOBAL_LOAD_DWORD_SADDR %0, %1, 0, 0, implicit $exec :: (load (s32))
+    %5:vgpr_32 = GLOBAL_LOAD_DWORD_SADDR %0, %1, 512, 0, implicit $exec :: (load (s32))
+    SCHED_BARRIER 128
+    %6:vgpr_32 = nsw V_MUL_LO_U32_e64 %5, %5, implicit $exec
+    %4:vgpr_32 = nsw V_MUL_LO_U32_e64 %3, %3, implicit $exec
+    GLOBAL_STORE_DWORD_SADDR %1, %6, %0, 512, 0, implicit $exec :: (store (s32))
+    GLOBAL_STORE_DWORD_SADDR %1, %4, %0, 0, 0, implicit $exec :: (store (s32))
+    S_ENDPGM 0
+...