Initial RAM optimization for ESP8266

by putting big constants into ROM, instead of residing in RAM.
Related to https://github.com/Samsung/jerryscript/issues/1224.

JerryScript-DCO-1.0-Signed-off-by: Slavey Karadzhov slaff@attachix.com

targets/esp8266/Makefile.esp8266

@@ -30,9 +30,21 @@ ESPTOOL    ?= /opt/Espressif/esptool-py/esptool.py
 
 # compile flags
 ESP_CFLAGS := -D__TARGET_ESP8266 -D__attr_always_inline___=
+
+MFORCE32 = `xtensa-lx106-elf-gcc --help=target | grep mforce-l32`
+
+ifneq ($(MFORCE32),)
+    # Your compiler supports the -mforce-l32 flag which means that 
+    # constants can be placed in ROM to free additional RAM
+    ESP_CFLAGS += -DJERRY_CONST_DATA="__attribute__((aligned(4))) __attribute__((section(\".irom.text\")))"
+endif
+
 ESP_CFLAGS += -Wl,-EL -fno-inline-functions
 ESP_CFLAGS += -ffunction-sections -fdata-sections
 ESP_CFLAGS += -mlongcalls -mtext-section-literals -mno-serialize-volatile
+ifneq ($(MFORCE32),)
+	ESP_CFLAGS += -mforce-l32
+endif
 
 # include path
 ESP_LIBS_INC :=$(CURDIR)/targets/esp8266/include

targets/esp8266/readme.md

@@ -68,3 +68,46 @@ Sample program here works with LED and a SW with below connection.
 * Connect GPIO0 between VCC > 4K resistor and GND
 
 If GPIO0 is High then LED is turned on longer. If L vice versa.
+
+### 6. Optimizing initial RAM usage (ESP8266 specific)
+The existing open source gcc compiler with Xtensa support stores const(ants) in
+the same limited RAM where our code needs to run. 
+
+It is possible to force the compiler to 1)store a constant into ROM and also 2) read it from there thus saving 1.1) RAM.
+It will require two things though:
+1. To add the attribute JERRY_CONST_DATA to your constant. For example
+
+```C
+static const lit_magic_size_t lit_magic_string_sizes[] =
+```
+
+can be modified to
+
+```C
+static const lit_magic_size_t lit_magic_string_sizes[] JERRY_CONST_DATA =
+```
+
+That is already done to some constants in jerry-core. 
+
+1.1) Below is a short list:
+
+
+Bytes  | Name
+-------- | ---------
+928 | magic_strings$2428
+610 | vm_decode_table
+424 | unicode_letter_interv_sps
+235 | cbc_flags
+232 | lit_magic_string_sizes
+212 | unicode_letter_interv_len
+196 | unicode_non_letter_ident_
+112 | unicode_letter_chars
+
+Which frees 2949 bytes in RAM. 
+
+2. To compile your code with compiler that supports the `-mforce-l32` parameter. You can check if your compiler is 
+supporting that parameter by calling:
+```bash
+xtensa-lx106-elf-gcc --help=target | grep mforce-l32
+```
+If the command above does not provide a result then you will need to upgrade your compiler.