[Liftoff] Add loop-header stack checks

This is needed to ensure that code can be interrupted. It will be
covered by a test once we support if-constructs in Liftoff.

Drive-by: Separate handling of blocks and loops, as there is only one
line in common.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: Ic22ca5e65c8d03a5d504289ec2a9e30cb97dc220
Reviewed-on: https://chromium-review.googlesource.com/853858
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50403}

src/wasm/baseline/liftoff-compiler.cc

@@ -322,32 +322,32 @@ class LiftoffCompiler {
     BindUnboundLabels(decoder);
   }
 
-  void Block(Decoder* decoder, Control* new_block) {
-    // Note: This is called for blocks and loops.
-    DCHECK_EQ(new_block, decoder->control_at(0));
+  void Block(Decoder* decoder, Control* block) {
+    TraceCacheState(decoder);
+    block->label_state.stack_base = __ cache_state()->stack_height();
+  }
 
+  void Loop(Decoder* decoder, Control* loop) {
     TraceCacheState(decoder);
+    loop->label_state.stack_base = __ cache_state()->stack_height();
 
-    new_block->label_state.stack_base = __ cache_state()->stack_height();
+    // Before entering a loop, spill all locals to the stack, in order to free
+    // the cache registers, and to avoid unnecessarily reloading stack values
+    // into registers at branches.
+    // TODO(clemensh): Come up with a better strategy here, involving
+    // pre-analysis of the function.
+    __ SpillLocals();
 
-    if (new_block->is_loop()) {
-      // Before entering a loop, spill all locals to the stack, in order to free
-      // the cache registers, and to avoid unnecessarily reloading stack values
-      // into registers at branches.
-      // TODO(clemensh): Come up with a better strategy here, involving
-      // pre-analysis of the function.
-      __ SpillLocals();
+    // Loop labels bind at the beginning of the block.
+    __ bind(loop->label.get());
 
-      // Loop labels bind at the beginning of the block, block labels at the
-      // end.
-      __ bind(new_block->label.get());
+    // Save the current cache state for the merge when jumping to this loop.
+    loop->label_state.Split(*__ cache_state());
 
-      new_block->label_state.Split(*__ cache_state());
-    }
+    // Execute a stack check in the loop header.
+    StackCheck(decoder->position());
   }
 
-  void Loop(Decoder* decoder, Control* block) { Block(decoder, block); }
-
   void Try(Decoder* decoder, Control* block) { unsupported(decoder, "try"); }
   void If(Decoder* decoder, const Value& cond, Control* if_block) {
     unsupported(decoder, "if");