2.9 KiB
Building an Embedded Program
The following command cross compiles the program to the ARM Cortex-M4 architecture.
$ cargo build --bin hello
The default in a Cargo project is to compile for the host (native compilation) but the beginner/apps project has been configured for cross compilation. This configuration can be see in the Cargo configuration file (.cargo/config):
# .cargo/config
[build]
target = "thumbv7em-none-eabi" # = ARM Cortex-M4
The output of the compilation process will be an ELF (Executable and Linkable Format) file. The file will be placed in the target/thumbv7em-none-eabi directory.
$ file target/thumbv7em-none-eabi/debug/hello
hello: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, with debug_info, not stripped
Binary size
ELF files contain metadata like debug information and their size on disk is not a good indication of the amount of Flash the program will use once it's loaded on the target device's memory.
To display the amount of Flash the program will occupy on the target device use the cargo-size tool (part of the cargo-binutils package):
$ cargo size --bin hello -- -A
hello :
section size addr
.vector_table 256 0x0
.text 9740 0x100
.rodata 4568 0x270c
.data 8 0x20000000
.bss 2124 0x20000008
.uninit 0 0x20000854
This gives you a breakdown of the program's static memory usage per linker section.
The .vector_table section contains the vector table, a data structure required by the Cortex-M ISA. The .text section contains the instructions the program will execute. The .rodata section contains constants like strings literals. These three sections are contiguously located in Flash memory -- Flash memory spans from address 0x0000_0000 to 0x0010_0000 (1 MB).
The next three sections, .data, .bss and .uninit, are located in RAM -- RAM memory spans the address range 0x2000_0000 - 0x2004_0000 (256 KB). These sections contain statically allocated variables (static variables).
Another other useful tool to analyze the binary size of a program is cargo-bloat:
$ cargo bloat --bin hello
File .text Size Crate Name
0.7% 13.5% 1.3KiB std <char as core::fmt::Debug>::fmt
0.5% 9.6% 928B hello hello::__cortex_m_rt_main
0.4% 8.4% 804B std core::str::slice_error_fail
0.4% 8.0% 768B std core::fmt::Formatter::pad
0.3% 6.4% 614B std core::fmt::num::<impl core::fmt::Debug for usize>::fmt
(..)
5.1% 100.0% 9.4KiB .text section size, the file size is 184.5KiB
This breakdowns the size of the .text section by function. This breakdown can be used to identify the largest functions in the program; those could then be modified to make them smaller.