I have been using ARM embedded gcc with STM32F103 for a long time. recentlly, I found the binary size is much smaller compiled by Keil than gcc. It's huge different, lets say 10k bytes with Keil and 40k bytes with gcc. After some research there's solution.
1. Optimization level to code size
CFLAGS += -Os
2.Remove debug
#CFLAGS += -g
3.Add garbage collection flag
CFLAGS += -Wl,--gc-sections
4. Using nano-newlib instead of newlib
CFLAGS += --specs=nano.specs
5.Other flags
CFLAGS += -fno-common -mfloat-abi=soft -fsingle-precision-constant -fomit-frame-pointer -ffunction-sections -fdata-sections
6.Below are the results, the red text is code size in bytes
CFLAGS += -Os
arm-none-eabi-size main.elf
text data bss dec hex filename
99652 2332 4560 106544 1a030 main.elf
CFLAGS += -Os
CFLAGS += -Wl,--gc-sections
arm-none-eabi-size main.elf
text data bss dec hex filename
88712 2260 3748 94720 17200 main.elf
CFLAGS += -Os
CFLAGS += -Wl,--gc-sections
CFLAGS += --specs=nano.specs
arm-none-eabi-size main.elf
text data bss dec hex filename
31716 1156 1620 34492 86bc main.elf
CFLAGS += -Os
CFLAGS += -Wl,--gc-sections
CFLAGS += --specs=nano.specs
CFLAGS += -fno-common -mfloat-abi=soft -fsingle-precision-constant -fomit-frame-pointer -ffunction-sections -fdata-sections
arm-none-eabi-size main.elf
text data bss dec hex filename
22572 1136 1612 25320 62e8 main.elf
