Store constant tables in PROGMEM on AVR

matthijskooijman · matthijskooijman · commit ef6a7861a214 · 2016-02-20T12:24:04.000+01:00
When compiling for an Arduino Uno, this saves 4546 bytes of RAM (5860 -&gt; 1214)
at the expense of 302 additional bytes of code. Most of this additional
code is used by the AES implementation, probably because the use of
special PROGMEM loading instructions reduces the optimization's freedom
with regard to instruction ordering and register assignment.
diff --git a/src/lmic/oslmic.h b/src/lmic/oslmic.h
@@ -222,16 +222,43 @@ u2_t os_crc16 (xref2u1_t d, uint len);
 #define TABLE_GET_OSTIME(table, index) table_get_ostime(RESOLVE_TABLE(table), index)
 #define TABLE_GET_U1_TWODIM(table, index1, index2) table_get_u1(RESOLVE_TABLE(table)[index1], index2)
 
-inline u1_t table_get_u1(const u1_t *table, size_t index) { return table[index]; }
-inline s1_t table_get_s1(const s1_t *table, size_t index) { return table[index]; }
-inline u2_t table_get_u2(const u2_t *table, size_t index) { return table[index]; }
-inline s2_t table_get_s2(const s2_t *table, size_t index) { return table[index]; }
-inline u4_t table_get_u4(const u4_t *table, size_t index) { return table[index]; }
-inline s4_t table_get_s4(const s4_t *table, size_t index) { return table[index]; }
-inline ostime_t table_get_ostime(const ostime_t *table, size_t index) { return table[index]; }
-
-// Declare a table
-#define CONST_TABLE(type, name) const type RESOLVE_TABLE(name)
+#if defined(__AVR__)
+    #include <avr/pgmspace.h>
+    // Macro to define the getter functions. This loads data from
+    // progmem using pgm_read_xx, or accesses memory directly when the
+    // index is a constant so gcc can optimize it away;
+    #define TABLE_GETTER(postfix, type, pgm_type) \
+        inline type table_get ## postfix(const type *table, size_t index) { \
+            if (__builtin_constant_p(table[index])) \
+                return table[index]; \
+            return pgm_read_ ## pgm_type(&table[index]); \
+        }
+
+    TABLE_GETTER(_u1, u1_t, byte);
+    TABLE_GETTER(_s1, s1_t, byte);
+    TABLE_GETTER(_u2, u2_t, word);
+    TABLE_GETTER(_s2, s2_t, word);
+    TABLE_GETTER(_u4, u4_t, dword);
+    TABLE_GETTER(_s4, s4_t, dword);
+
+    // This assumes ostime_t is 4 bytes, so error out if it is not
+    typedef int check_sizeof_ostime_t[(sizeof(ostime_t) == 4) ? 0 : -1];
+    TABLE_GETTER(_ostime, ostime_t, dword);
+
+    // For AVR, store constants in PROGMEM, saving on RAM usage
+    #define CONST_TABLE(type, name) const type PROGMEM RESOLVE_TABLE(name)
+#else
+    inline u1_t table_get_u1(const u1_t *table, size_t index) { return table[index]; }
+    inline s1_t table_get_s1(const s1_t *table, size_t index) { return table[index]; }
+    inline u2_t table_get_u2(const u2_t *table, size_t index) { return table[index]; }
+    inline s2_t table_get_s2(const s2_t *table, size_t index) { return table[index]; }
+    inline u4_t table_get_u4(const u4_t *table, size_t index) { return table[index]; }
+    inline s4_t table_get_s4(const s4_t *table, size_t index) { return table[index]; }
+    inline ostime_t table_get_ostime(const ostime_t *table, size_t index) { return table[index]; }
+
+    // Declare a table
+    #define CONST_TABLE(type, name) const type RESOLVE_TABLE(name)
+#endif
 
 // ======================================================================
 // AES support
