Bug 1865383 - Check slice budget when we start marking or sweeping r=sfink

The patch forces a budget check when we start marking or sweeping unless we
have just started a GC slice in the current state.

This turned out more complicated than expected because the change caused a test
that called gcslice(1) in a loop to not make any progress. This was because the
original stepAndForceCheck() method ate the single unit of work budget. The fix
was to remove the step part of this method. I don't think any of the callers
need it to step the budget as well.

Differential Revision: https://phabricator.services.mozilla.com/D193925

js/public/SliceBudget.h

@@ -112,14 +112,12 @@ class JS_PUBLIC_API SliceBudget {
     counter -= steps;
   }
 
-  // Do enough steps to force an "expensive" (time) check on the next call to
-  // isOverBudget. Useful when switching between major phases of an operation
-  // like a cycle collection.
-  void stepAndForceCheck() {
+  // Force an "expensive" (time) check on the next call to isOverBudget. Useful
+  // when switching between major phases of an operation like a cycle
+  // collection.
+  void forceCheck() {
     if (isTimeBudget()) {
       counter = 0;
-    } else {
-      counter--;
     }
   }
 

js/src/gc/GC.cpp

@@ -3000,6 +3000,13 @@ IncrementalProgress GCRuntime::markUntilBudgetExhausted(
 
   AutoMajorGCProfilerEntry s(this);
 
+  if (initialState != State::Mark) {
+    sliceBudget.forceCheck();
+    if (sliceBudget.isOverBudget()) {
+      return NotFinished;
+    }
+  }
+
   if (processTestMarkQueue() == QueueYielded) {
     return NotFinished;
   }

js/src/gc/ParallelMarking.cpp

@@ -221,7 +221,7 @@ bool ParallelMarkTask::requestWork(AutoLockGC& lock) {
     return false;  // All other tasks are empty. We're finished.
   }
 
-  budget.stepAndForceCheck();
+  budget.forceCheck();
   if (budget.isOverBudget()) {
     return false;  // Over budget or interrupted.
   }

js/src/gc/Sweeping.cpp

@@ -2326,9 +2326,15 @@ IncrementalProgress GCRuntime::performSweepActions(SliceBudget& budget) {
   }
 #endif
 
-  if (initialState == State::Sweep &&
-      markDuringSweeping(gcx, budget) == NotFinished) {
-    return NotFinished;
+  if (initialState == State::Sweep) {
+    if (markDuringSweeping(gcx, budget) == NotFinished) {
+      return NotFinished;
+    }
+  } else {
+    budget.forceCheck();
+    if (budget.isOverBudget()) {
+      return NotFinished;
+    }
   }
 
   // Then continue running sweep actions.

xpcom/base/CycleCollectedJSRuntime.cpp

@@ -1397,7 +1397,7 @@ bool CycleCollectedJSRuntime::TraceNativeGrayRoots(
     TraceAdditionalNativeGrayRoots(aTracer);
 
     mHolderIter.emplace(mJSHolders, aWhich);
-    aBudget.stepAndForceCheck();
+    aBudget.forceCheck();
   } else {
     // Holders may have been removed between slices, so we may need to update
     // the iterator.

xpcom/base/nsCycleCollector.cpp

@@ -3506,7 +3506,7 @@ bool nsCycleCollector::Collect(CCReason aReason, ccIsManual aIsManual,
         break;
     }
     if (continueSlice) {
-      aBudget.stepAndForceCheck();
+      aBudget.forceCheck();
       continueSlice = !aBudget.isOverBudget();
     }
   } while (continueSlice);