[sil-combine] Add peephole: alloc_ref/set_deallocating/dealloc_ref -> remove these instructions

Discovered this missing peephole while looking at the performance of swiftlang#7420

Author: swiftix

lib/SILOptimizer/SILCombiner/SILCombiner.h

@@ -190,6 +190,7 @@ class SILCombiner :
   SILInstruction *visitUpcastInst(UpcastInst *UCI);
   SILInstruction *visitLoadInst(LoadInst *LI);
   SILInstruction *visitAllocStackInst(AllocStackInst *AS);
+  SILInstruction *visitAllocRefInst(AllocRefInst *AR);
   SILInstruction *visitSwitchEnumAddrInst(SwitchEnumAddrInst *SEAI);
   SILInstruction *visitInjectEnumAddrInst(InjectEnumAddrInst *IEAI);
   SILInstruction *visitPointerToAddressInst(PointerToAddressInst *PTAI);

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -391,6 +391,33 @@ SILInstruction *SILCombiner::visitAllocStackInst(AllocStackInst *AS) {
   return eraseInstFromFunction(*AS);
 }
 
+SILInstruction *SILCombiner::visitAllocRefInst(AllocRefInst *AR) {
+  if (!AR)
+    return nullptr;
+  // Check if the only uses are deallocating stack or deallocating.
+  SmallPtrSet<SILInstruction *, 16> ToDelete;
+  bool HasNonRemovableUses = false;
+  for (auto UI = AR->use_begin(), UE = AR->use_end(); UI != UE;) {
+    auto *Op = *UI;
+    ++UI;
+    auto *User = Op->getUser();
+    if (!isa<DeallocRefInst>(User) && !isa<SetDeallocatingInst>(User)) {
+      HasNonRemovableUses = true;
+      break;
+    }
+    ToDelete.insert(User);
+  }
+
+  if (HasNonRemovableUses)
+    return nullptr;
+
+  // Remove the instruction and all its uses.
+  for (auto *I : ToDelete)
+    eraseInstFromFunction(*I);
+  eraseInstFromFunction(*AR);
+  return nullptr;
+}
+
 SILInstruction *SILCombiner::visitLoadInst(LoadInst *LI) {
   // (load (upcast-ptr %x)) -> (upcast-ref (load %x))
   Builder.setCurrentDebugScope(LI->getDebugScope());

test/SILOptimizer/sil_combine.sil

@@ -3137,3 +3137,27 @@ bb0(%0 : $VV):
   return %26 : $()
 }
 
+// CHECK-LABEL: sil @remove_unused_alloc_ref
+// CHECK-NEXT: bb0
+// CHECK-NEXT: %0 = tuple ()
+// CHECK-NEXT: return %0 : $()
+sil @remove_unused_alloc_ref : $@convention(thin) () -> () {
+bb0:
+  %1 = alloc_ref $B
+  dealloc_ref %1 : $B
+  %3 = tuple ()
+  return %3 : $()
+}
+
+// CHECK-LABEL: sil @remove_unused_alloc_ref_stack
+// CHECK-NEXT: bb0
+// CHECK-NEXT: %0 = tuple ()
+// CHECK-NEXT: return %0 : $()
+sil @remove_unused_alloc_ref_stack : $@convention(thin) () -> () {
+bb0:
+  %1 = alloc_ref [stack] $B
+  set_deallocating %1 : $B
+  dealloc_ref [stack] %1 : $B
+  %3 = tuple ()
+  return %3 : $()
+}