[Liftoff] Avoid quadratic behaviour in stack transfers

We currently iterate the list of unexecuted register moves repeatedly,
always executing the moves whose destination register is not being used
as source register any more. This can lead to quadratic execution times
if only a small number of moves is processed in every iteration.

This CL refactors this such that we iterate the moves at most three
times: Once for executing moves which can be executed right away (fast
path) and for computing the source register use counts. A second time
to execute all remaining non-cyclic moves, and a third time to execute
cyclic moves.
During the second and third iteration, whenever we decrement the source
register use count, we check whether it drops to zero and execute the
respective move right away.

R=ahaas@chromium.org

Bug: v8:6600, v8:8423
Change-Id: I503328f5ae5f0208e35d53c71b4c289d75799892
Reviewed-on: https://chromium-review.googlesource.com/c/1397703
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58588}

src/wasm/baseline/liftoff-assembler.cc

@@ -248,63 +248,67 @@ class StackTransferRecipe {
            register_move(dst)->type == type;
   }
 
-  void ExecuteMoves() {
-    if (move_dst_regs_.is_empty()) return;
+  void ExecuteMove(LiftoffRegister dst, uint32_t* src_reg_use_count) {
+    RegisterMove* move = register_move(dst);
+    DCHECK_EQ(0, src_reg_use_count[dst.liftoff_code()]);
+    asm_->Move(dst, move->src, move->type);
+    ClearExecutedMove(dst, src_reg_use_count);
+  }
 
-    if ((move_dst_regs_ & move_src_regs_).is_empty()) {
-      // No overlap in src and dst registers. Just execute the moves in any
-      // order.
-      for (LiftoffRegister dst : move_dst_regs_) {
-        RegisterMove* move = register_move(dst);
+  void ClearExecutedMove(LiftoffRegister dst, uint32_t* src_reg_use_count) {
+    DCHECK(move_dst_regs_.has(dst));
+    move_dst_regs_.clear(dst);
+    RegisterMove* move = register_move(dst);
+    DCHECK_LT(0, src_reg_use_count[move->src.liftoff_code()]);
+    if (--src_reg_use_count[move->src.liftoff_code()] != 0) return;
+    // src count dropped to zero. If this is a destination register, execute
+    // that move now.
+    if (!move_dst_regs_.has(move->src)) return;
+    ExecuteMove(move->src, src_reg_use_count);
+  }
+
+  void ExecuteMoves() {
+    // Execute all moves whose {dst} is not being used as src any more.
+    // Simultaneously, compute use counters of src registers for the remaining
+    // passes.
+    uint32_t src_reg_use_count[kAfterMaxLiftoffRegCode] = {0};
+    for (LiftoffRegister dst : move_dst_regs_) {
+      RegisterMove* move = register_move(dst);
+      if (!move_src_regs_.has(dst)) {
         asm_->Move(dst, move->src, move->type);
+        move_dst_regs_.clear(dst);
+      } else {
+        ++src_reg_use_count[move->src.liftoff_code()];
       }
-      move_dst_regs_ = {};
-      return;
     }
+    // {move_src_regs_} is outdated and not being used any more.
+    move_src_regs_ = {};
 
-    // Keep use counters of src registers.
-    uint32_t src_reg_use_count[kAfterMaxLiftoffRegCode] = {0};
+    // Now iterate all remaining moves again and execute all that can be
+    // executed according to {src_reg_use_count}. If any src count drops to
+    // zero, also (transitively) execute the corresponding move to that
+    // register.
     for (LiftoffRegister dst : move_dst_regs_) {
-      ++src_reg_use_count[register_move(dst)->src.liftoff_code()];
+      // Check if already handled via transitivity in {ClearExecutedMove}.
+      if (!move_dst_regs_.has(dst)) continue;
+      RegisterMove* move = register_move(dst);
+      if (src_reg_use_count[move->src.liftoff_code()]) continue;
+      ExecuteMove(dst, src_reg_use_count);
     }
-    // Now repeatedly iterate the list of register moves, and execute those
-    // whose dst register does not appear as src any more. The remaining moves
-    // are compacted during this iteration.
-    // If no more moves can be executed (because of a cycle), spill one
-    // register to the stack, add a RegisterLoad to reload it later, and
-    // continue.
+
+    // All remaining moves are parts of a cycle. Just spill the first one, then
+    // process all remaining moves in that cycle. Repeat for all cycles.
     uint32_t next_spill_slot = asm_->cache_state()->stack_height();
     while (!move_dst_regs_.is_empty()) {
-      bool freed_src_reg = false;
-      for (LiftoffRegister dst : move_dst_regs_) {
-        // If the dst register is still being used as src, we cannot execute
-        // this move yet.
-        if (src_reg_use_count[dst.liftoff_code()] != 0) continue;
-
-        RegisterMove* move = register_move(dst);
-        asm_->Move(dst, move->src, move->type);
-        DCHECK_LT(0, src_reg_use_count[move->src.liftoff_code()]);
-        if (--src_reg_use_count[move->src.liftoff_code()] == 0) {
-          freed_src_reg = true;
-        }
-        move_dst_regs_.clear(dst);
-      }
-      // If the loop above did free at least one src register, then there is a
-      // chance that another run executes more moves. Thus run again.
-      if (freed_src_reg) continue;
-
-      // There is a cycle. Spill one register, then continue.
       // TODO(clemensh): Use an unused register if available.
       LiftoffRegister dst = move_dst_regs_.GetFirstRegSet();
-      move_dst_regs_.clear(dst);
       RegisterMove* move = register_move(dst);
       LiftoffRegister spill_reg = move->src;
       asm_->Spill(next_spill_slot, spill_reg, move->type);
       // Remember to reload into the destination register later.
       LoadStackSlot(dst, next_spill_slot, move->type);
-      DCHECK_EQ(1, src_reg_use_count[spill_reg.liftoff_code()]);
-      src_reg_use_count[spill_reg.liftoff_code()] = 0;
       ++next_spill_slot;
+      ClearExecutedMove(dst, src_reg_use_count);
     }
   }