[heap] MinorMC implies separate GC phases

This is a necessary assumption for concurrent marking in MinorMC and
will simplify the code as it allows MinorMC to reuse the same marking
bitmap as full GCs.

Bug: v8:12612
Change-Id: I5e9be45c7d84320721ce7f7578dee1eb972d6f6a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732933Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81465}

src/flags/flag-definitions.h

@@ -1934,6 +1934,7 @@ DEFINE_NEG_NEG_IMPLICATION(text_is_readable, partial_constant_pool)
 DEFINE_BOOL(trace_minor_mc_parallel_marking, false,
             "trace parallel marking for the young generation")
 DEFINE_BOOL(minor_mc, false, "perform young generation mark compact GCs")
+DEFINE_IMPLICATION(minor_mc, separate_gc_phases)
 
 //
 // Dev shell flags

src/heap/heap.cc

@@ -2606,6 +2606,7 @@ void Heap::MarkCompact() {
 
 void Heap::MinorMarkCompact() {
   DCHECK(FLAG_minor_mc);
+  CHECK_EQ(NOT_IN_GC, gc_state());
   DCHECK(new_space());
 
   if (FLAG_trace_incremental_marking && !incremental_marking()->IsStopped()) {

src/heap/mark-compact.cc

@@ -5623,6 +5623,7 @@ class MinorMarkCompactCollector::RootMarkingVisitor : public RootVisitor {
 };
 
 void MinorMarkCompactCollector::CollectGarbage() {
+  DCHECK(!heap()->mark_compact_collector()->in_use());
   // Minor MC does not support processing the ephemeron remembered set.
   DCHECK(heap()->ephemeron_remembered_set_.empty());