Add a runtime flag to enable MMTk

[angussidney]

In my initial iteration of the Ruby/MMTk binding, the Ruby compiler modifications are enabled/disabled via a compiler flag. Although this should be functional, it won't be ideal because the entire language must be rebuilt whenever we want to turn the third party heap on/off (e.g. to perform comparative benchmarks etc).

My initial attempt to implement a runtime flag was to add a feature as a command line option to the ruby interpreter. However, this attempt proved unfruitful because the VM/heap is initialised before the command line arguments are parsed, which makes it impossible to configure MMTk usage without majorly refactoring the code (something which I am not comfortable doing).

Chris's suggestion was to use an environment variable instead. The get_param function in vm.c may be useful for this.

[angussidney]

My (now discarded) changes for adding the command line option:

diff --git a/ruby.c b/ruby.c
index 96d8f75a56..025cd09071 100644
--- a/ruby.c
+++ b/ruby.c
@@ -73,6 +73,21 @@ void rb_warning_category_update(unsigned int mask, unsigned int bits);
 
 #define COMMA ,
 #define FEATURE_BIT(bit) (1U << feature_##bit)
+#ifdef USE_THIRD_PARTY_HEAP
+#define EACH_FEATURES(X, SEP) \
+    X(gems) \
+    SEP \
+    X(did_you_mean) \
+    SEP \
+    X(rubyopt) \
+    SEP \
+    X(frozen_string_literal) \
+    SEP \
+    X(jit) \
+    SEP \
+    X(third_party_heap) \
+    /* END OF FEATURES */
+#else
 #define EACH_FEATURES(X, SEP) \
     X(gems) \
     SEP \
@@ -84,6 +99,7 @@ void rb_warning_category_update(unsigned int mask, unsigned int bits);
     SEP \
     X(jit) \
     /* END OF FEATURES */
+#endif
 #define EACH_DEBUG_FEATURES(X, SEP) \
     X(frozen_string_literal) \
     /* END OF DEBUG FEATURES */
@@ -193,7 +209,10 @@ enum {
 	& ~FEATURE_BIT(gems)
 #endif
 	& ~FEATURE_BIT(frozen_string_literal)
-        & ~FEATURE_BIT(jit)
+	& ~FEATURE_BIT(jit)
+#ifdef USE_THIRD_PARTY_HEAP
+	& ~FEATURE_BIT(third_party_heap)
+#endif
 	)
 };
 
@@ -298,7 +317,10 @@ usage(const char *name, int help)
 	M("did_you_mean", "",   "did_you_mean (default: "DEFAULT_RUBYGEMS_ENABLED")"),
 	M("rubyopt", "",        "RUBYOPT environment variable (default: enabled)"),
 	M("frozen-string-literal", "", "freeze all string literals (default: disabled)"),
-        M("jit", "",            "JIT compiler (default: disabled)"),
+	M("jit", "",            "JIT compiler (default: disabled)"),
+#ifdef USE_THIRD_PARTY_HEAP
+	M("third_party_heap", "", "third party heap and GC (default: disabled)"),
+#endif
     };
     static const struct message warn_categories[] = {
         M("deprecated", "",       "deprecated features"),

(yes... I could have made a branch and linked to it here, but I was too lazy to selectively pick out the correct lines of code to commit)