update makefile

Makefile

@@ -6,7 +6,12 @@ INCLUDE = include
 
 PROJECT=shuffle3
 
-COMMON_FLAGS = -Wall -pedantic $(addprefix -I,$(INCLUDE)) -fno-strict-aliasing
+# Currently supported:
+#	_FS_SPILL_BUFFER:	Use file backed buffer instead of memory backed one for unshuffling. See `shuffle3.h`.
+#	DEBUG:		Pretend we're building a debug release even though we're not. Will enable additional output messages and may interfere with some optimisations
+FEATURE_FLAGS?=
+
+COMMON_FLAGS = $(addprefix -D,$(FEATURE_FLAGS)) -Wall -pedantic $(addprefix -I,$(INCLUDE)) -fno-strict-aliasing
 
 OPT_FLAGS?= -march=native -fgraphite -fopenmp -floop-parallelize-all -ftree-parallelize-loops=4 \
 	    -floop-interchange -ftree-loop-distribution -floop-strip-mine -floop-block \

include/shuffle.hpp

@@ -29,6 +29,7 @@ namespace rng {
 		if(!span.size()) return;
 
 #ifdef _FS_SPILL_BUFFER
+#warning "_FS_SPILL_BUFFER is still experimental"
 		file_vector<std::size_t> rng_values(span.size());
 #else
 		std::vector<std::size_t> rng_values(span.size());

include/shuffle3.h

@@ -18,6 +18,8 @@ extern "C" {
 
 //#define _FS_SPILL_BUFFER /* Use a file-backed buffer when unshuffling in cases of too high memory usage. Will cause massive slowdowns but can stop OOMs when unshuffling large files */
 
+//
+
 extern const char* _prog_name;
 
 #ifdef __cplusplus