Bug 1139456 - Do not let compaction set the alloc threshold to unreasonably small sizes, r=terrence

--HG--
extra : rebase_source : 2842e2aaaa97b355c61637d421c8a3fb02d70d57

js/src/jit-test/tests/gc/bug-1138390.js

@@ -17,12 +17,12 @@ var g = newGlobal();
 
 // Start an off thread compilation that will not run until GC has finished
 if ("gcstate" in this)
-   assertEq("mark", gcstate());
+   assertEq(gcstate(), "mark");
 g.offThreadCompileScript('23;', {});
 
 // Wait for the compilation to finish, which must finish the GC first
 assertEq(23, g.runOffThreadScript());
 if ("gcstate" in this)
-   assertEq("none", gcstate());
+   assertEq(gcstate(), "none");
 
 print("done");

js/src/jsgc.cpp

@@ -1714,7 +1714,7 @@ ZoneHeapThreshold::computeZoneTriggerBytes(double growthFactor, size_t lastBytes
                                            const GCSchedulingTunables &tunables)
 {
     size_t base = gckind == GC_SHRINK
-                ? lastBytes
+                ? Max(lastBytes, tunables.minEmptyChunkCount() * ChunkSize)
                 : Max(lastBytes, tunables.gcZoneAllocThresholdBase());
     double trigger = double(base) * growthFactor;
     return size_t(Min(double(tunables.gcMaxBytes()), trigger));