8372513: Shenandoah: ShenandoahMaxRegionSize can produce an unaligned heap alignment

Reviewed-by: jsikstro, eosterlund, shade

src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp

@@ -37,6 +37,7 @@
 #include "runtime/globals_extension.hpp"
 #include "runtime/java.hpp"
 #include "utilities/defaultStream.hpp"
+#include "utilities/powerOfTwo.hpp"
 
 void ShenandoahArguments::initialize() {
 #if !(defined AARCH64 || defined AMD64 || defined PPC64 || defined RISCV64)
@@ -205,7 +206,7 @@ void ShenandoahArguments::initialize() {
 }
 
 size_t ShenandoahArguments::conservative_max_heap_alignment() {
-  size_t align = ShenandoahMaxRegionSize;
+  size_t align = next_power_of_2(ShenandoahMaxRegionSize);
   if (UseLargePages) {
     align = MAX2(align, os::large_page_size());
   }

src/hotspot/share/runtime/arguments.cpp

@@ -1482,6 +1482,7 @@ void Arguments::set_conservative_max_heap_alignment() {
                                           os::vm_allocation_granularity(),
                                           os::max_page_size(),
                                           GCArguments::compute_heap_alignment());
+  assert(is_power_of_2(_conservative_max_heap_alignment), "Expected to be a power-of-2");
 }
 
 jint Arguments::set_ergonomics_flags() {

test/hotspot/jtreg/gc/shenandoah/options/TestRegionSizeArgs.java

@@ -192,6 +192,17 @@ public class TestRegionSizeArgs {
             output.shouldHaveExitValue(0);
         }
 
+        // This used to assert that _conservative_max_heap_alignment is not a power-of-2.
+        {
+            OutputAnalyzer output = ProcessTools.executeLimitedTestJava("-XX:+UnlockExperimentalVMOptions",
+                    "-XX:+UseShenandoahGC",
+                    "-Xms100m",
+                    "-Xmx1g",
+                    "-XX:ShenandoahMaxRegionSize=33m",
+                    "-version");
+            output.shouldHaveExitValue(0);
+        }
+
     }
 
     private static void testMaxRegionSize() throws Exception {