Modify gc_stats API to be able to select needed ones

Allows to introduce more fields to `Stats` struct in future without
compromising performance by calculating extra unnecessary fields. This
will prevent `core.memory.GC` API from getting bloated as new stats
become desired.

Author: mihails-strasuns-sociomantic

src/core/memory.d

@@ -138,7 +138,7 @@ private
     }
 
     extern (C) BlkInfo_ gc_query( void* p ) pure nothrow;
-    extern (C) GC.Stats gc_stats ( ) nothrow @nogc;
+    extern (C) GC.Stats gc_stats ( ulong fields ) nothrow @nogc;
 
     extern (C) void gc_addRoot( in void* p ) nothrow @nogc;
     extern (C) void gc_addRange( in void* p, size_t sz, const TypeInfo ti = null ) nothrow @nogc;
@@ -170,6 +170,12 @@ struct GC
         size_t freeSize;
     }
 
+    /**
+     * Auto-generated bitflag enum with values of identical names as `Stats`
+     * struct. Optionally used to specify exact stats to calculate.
+     */
+    mixin(generateFieldEnum!Stats);
+
     /**
      * Enables automatic garbage collection behavior if collections have
      * previously been suspended by a call to disable.  This function is
@@ -677,7 +683,7 @@ struct GC
      */
     static Stats stats() nothrow
     {
-        return gc_stats();
+        return gc_stats(ulong.max);
     }
 
     /**
@@ -1167,3 +1173,50 @@ unittest
     int* pint; __delete(pint);
     S* ps; __delete(ps);
 }
+
+/**
+    For a given struct `S` generated bitflag enum with a value for each of
+    struct fields.
+ */
+private string generateFieldEnum(alias S)()
+{
+    import core.internal.string;
+
+    string code = "enum " ~ __traits(identifier, S) ~ "Fields\n{\n";
+    ulong shift = 0;
+    char[3] buf;
+
+    foreach (idx, _; S.init.tupleof)
+    {
+        auto init = "1UL << " ~ unsignedToTempString(shift, buf);
+        code ~= __traits(identifier, S.tupleof[idx]) ~ " = " ~ init ~ ",\n";
+        ++shift;
+    }
+
+    code ~= "}";
+
+    return code;
+}
+
+unittest
+{
+    static struct Dummy
+    {
+        int a, b, c;
+    }
+
+    enum code = generateFieldEnum!Dummy();
+
+    static assert (code == "enum DummyFields
+{
+a = 1UL << 0,
+b = 1UL << 1,
+c = 1UL << 2,
+}");
+
+    mixin(code);
+
+    assert(DummyFields.a == 1);
+    assert(DummyFields.b == 2);
+    assert(DummyFields.c == 4);
+}

src/gc/gcinterface.d

@@ -146,7 +146,7 @@ interface GC
      * Retrieve statistics about garbage collection.
      * Useful for debugging and tuning.
      */
-    core.memory.GC.Stats stats() nothrow;
+    core.memory.GC.Stats stats(ulong fields) nothrow;
 
     /**
      * add p to list of roots

src/gc/impl/conservative/gc.d

@@ -1191,11 +1191,11 @@ class ConservativeGC : GC
     }
 
 
-    core.memory.GC.Stats stats() nothrow
+    core.memory.GC.Stats stats(ulong fields) nothrow
     {
         typeof(return) ret;
 
-        runLocked!(getStatsNoSync, otherTime, numOthers)(ret);
+        runLocked!(getStatsNoSync, otherTime, numOthers)(ret, fields);
 
         return ret;
     }
@@ -1204,29 +1204,37 @@ class ConservativeGC : GC
     //
     //
     //
-    private void getStatsNoSync(out core.memory.GC.Stats stats) nothrow
+    private void getStatsNoSync(out core.memory.GC.Stats stats, ulong fields) nothrow
     {
-        foreach (pool; gcx.pooltable[0 .. gcx.npools])
+        alias Flags = core.memory.GC.StatsFields;
+
+        if ((fields & Flags.usedSize) || (fields & Flags.freeSize))
         {
-            foreach (bin; pool.pagetable[0 .. pool.npages])
+            // calculates both if any is requested as it is part of the same
+            // iteration process
+
+            foreach (pool; gcx.pooltable[0 .. gcx.npools])
             {
-                if (bin == B_FREE)
-                    stats.freeSize += PAGESIZE;
-                else
-                    stats.usedSize += PAGESIZE;
+                foreach (bin; pool.pagetable[0 .. pool.npages])
+                {
+                    if (bin == B_FREE)
+                        stats.freeSize += PAGESIZE;
+                    else
+                        stats.usedSize += PAGESIZE;
+                }
             }
-        }
 
-        size_t freeListSize;
-        foreach (n; 0 .. B_PAGE)
-        {
-            immutable sz = binsize[n];
-            for (List *list = gcx.bucket[n]; list; list = list.next)
-                freeListSize += sz;
-        }
+            size_t freeListSize;
+            foreach (n; 0 .. B_PAGE)
+            {
+                immutable sz = binsize[n];
+                for (List *list = gcx.bucket[n]; list; list = list.next)
+                    freeListSize += sz;
+            }
 
-        stats.usedSize -= freeListSize;
-        stats.freeSize += freeListSize;
+            stats.usedSize -= freeListSize;
+            stats.freeSize += freeListSize;
+        }
     }
 }
 

src/gc/impl/manual/gc.d

@@ -190,7 +190,7 @@ class ManualGC : GC
         return BlkInfo.init;
     }
 
-    core.memory.GC.Stats stats() nothrow
+    core.memory.GC.Stats stats(ulong fields) nothrow
     {
         return typeof(return).init;
     }

src/gc/impl/proto/gc.d

@@ -155,7 +155,7 @@ class ProtoGC : GC
         return BlkInfo.init;
     }
 
-    core.memory.GC.Stats stats() nothrow
+    core.memory.GC.Stats stats(ulong fields) nothrow
     {
         return typeof(return).init;
     }

src/gc/proxy.d

@@ -184,9 +184,9 @@ extern (C)
         return instance.query( p );
     }
 
-    core.memory.GC.Stats gc_stats() nothrow
+    core.memory.GC.Stats gc_stats(ulong fields) nothrow
     {
-        return instance.stats();
+        return instance.stats(fields);
     }
 
     void gc_addRoot( void* p ) nothrow @nogc