[liftoff] Mark tagged safepoint slots in reversed order

Setting the highest bit first saves cost for repeatedly growing the
underlying bitvector.

R=jkummerow@chromium.org

Bug: v8:13063
Change-Id: Ic324caa20c91dd6f55760944c3dafe7f1dc018b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3776340
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81849}

src/base/small-vector.h

@@ -100,6 +100,12 @@ class SmallVector {
   T* end() { return end_; }
   const T* end() const { return end_; }
 
+  auto rbegin() { return std::reverse_iterator{end_}; }
+  auto rbegin() const { return std::reverse_iterator{end_}; }
+
+  auto rend() { return std::reverse_iterator{begin_}; }
+  auto rend() const { return std::reverse_iterator{begin_}; }
+
   size_t size() const { return end_ - begin_; }
   bool empty() const { return end_ == begin_; }
   size_t capacity() const { return end_of_storage_ - begin_; }

src/wasm/baseline/liftoff-assembler.cc

@@ -569,7 +569,9 @@ void LiftoffAssembler::CacheState::GetTaggedSlotsForOOLCode(
 
 void LiftoffAssembler::CacheState::DefineSafepoint(
     SafepointTableBuilder::Safepoint& safepoint) {
-  for (const auto& slot : stack_state) {
+  // Go in reversed order to set the higher bits first; this avoids cost for
+  // growing the underlying bitvector.
+  for (const auto& slot : base::Reversed(stack_state)) {
     if (is_reference(slot.kind())) {
       DCHECK(slot.is_stack());
       safepoint.DefineTaggedStackSlot(GetSafepointIndexForStackSlot(slot));