Resource Optimization (Low-End Systems)

Purpose

Guide agents through flash and RAM optimization on constrained devices: compiler size flags, linker map analysis, stack usage measurement, dead code elimination, and size-vs-speed tradeoffs — for bare-metal and small RTOS images.

When to Use

Firmware exceeds flash budget
Stack overflow in production only
Choosing -Os vs -O2 on MCU
Finding unexpected .rodata or .bss growth

Workflow

1. Size measurement toolchain

arm-none-eabi-size -A firmware.elf
arm-none-eabi-objdump -h firmware.elf
nm --size-sort -S firmware.elf | tail -20

2. Linker map file

/* linker.ld */
OUTPUT_FORMAT("elf32-littlearm")
ENTRY(Reset_Handler)

SECTIONS
{
  /* ... */
}

/* Generate map */
/* gcc ... -Wl,-Map=firmware.map */

grep -E '\.text|\.rodata|\.data|\.bss' firmware.map | head

Identify largest symbols and unexpected library pull-in.

3. Compiler flags

Flag	Effect
`-Os`	Size-first optimization
`-ffunction-sections -fdata-sections`	Per-symbol sections
`-Wl,--gc-sections`	Drop unused sections
`-flto`	Cross-TU dead code elimination
`-specs=nano.specs`	Smaller newlib (GCC ARM)

CFLAGS += -Os -ffunction-sections -fdata-sections
LDFLAGS += -Wl,--gc-sections -Wl,--print-memory-usage

4. Stack analysis

# Static (if built with -fstack-usage)
find . -name '*.su' -exec cat {} \;

# Linker stack symbol
grep _estack firmware.map

Runtime: fill stack with pattern (0xDEADBEEF), run tests, scan high-water mark. FreeRTOS: uxTaskGetStackHighWaterMark.

5. RAM categories

Section	Tactic
`.bss`	Shrink buffers; pool allocators
`.data`	Move const to flash (`const` → `.rodata`)
Heap	Avoid malloc; fixed pools
Stack	Reduce nesting; smaller ISR stacks

6. Size vs speed decision

Hot path in ISR or 1 kHz loop?
├── Yes → -O2 for that file (#pragma GCC optimize)
└── No  → -Os globally

7. Agent usage

/resource-optimization-lowend Find top 10 flash consumers in firmware.map and suggest -gc-sections fixes

Common Problems

Symptom	Cause	Fix
printf pulls 20+ KB	Full newlib printf	`_write` retarget; tiny printf
`--gc-sections` broke IRQ	Section collected	`KEEP()` in linker script
Stack overflow late	Deep call + IRQ nest	Measure HW stack; increase `_estack`
RAM zero but big ELF	`.data` not loaded	Check VMA/LMA
LTO link fail	Mixed compiler versions	Same GCC for all objects

Related Skills

skills/embedded/linker-scripts — MEMORY/Sections layout
skills/baremetal/baremetal-startup — stack symbol
skills/baremetal/low-power-embedded — RAM retention
skills/compilers/gcc — optimization flags
skills/rust/rust-no-std — embedded Rust size patterns