Make enlarge a local function inside reserve

Author: andralex

src/dmd/backend/outbuf.d

@@ -82,9 +82,45 @@ struct Outbuffer
      */
     void reserve(size_t nbytes)
     {
+        pragma(inline, true);  // inline fast path
+
+        // non-inline function for the heavy/infrequent reallocation case
+        @trusted static void enlarge(ref Outbuffer b, size_t nbytes)
+        {
+            pragma(inline, false);  // do not inline slow path
+
+            if (b.buf is null)
+            {
+                // Special-case the overwhelmingly most frequent situation
+                if (nbytes < 64) nbytes = 64;
+                b.p = b.buf = cast(ubyte*) malloc(nbytes);
+                b.pend = b.buf + nbytes;
+            }
+            else
+            {
+                const size_t used = b.p - b.buf;
+                const size_t oldlen = b.pend - b.buf;
+                // Ensure exponential growth, oldlen * 2 for small sizes, oldlen * 1.5 for big sizes
+                const size_t minlen = oldlen + (oldlen >> (oldlen > 1024 * 64));
+
+                size_t len = used + nbytes;
+                if (len < minlen)
+                    len = minlen;
+                // Round up to cache line size
+                len = (len + 63) & ~63;
+
+                b.buf = cast(ubyte*) realloc(b.buf, len);
+
+                b.pend = b.buf + len;
+                b.p = b.buf + used;
+            }
+            if (!b.buf)
+                err_nomem();
+        }
+
         // Keep small so it is inlined
         if (pend - p < nbytes)
-            enlarge(nbytes);
+            enlarge(this, nbytes);
     }
 
     // Reserve nbytes in buffer