Performance optimizations

[WonderMr]

What's new

10 safe performance optimizations for STM32WB55 (Cortex-M4 @ 64MHz).

Compiler & build:

1. -Og → -Os for release builds — enables most -O2 passes: function inlining, dead code elimination, loop optimization, aggressive register allocation, tail call optimization, and common subexpression elimination (site_scons/firmwareopts.scons)

Memory management (correctness + perf):
2. Disable heap memset on free() in release — configHEAP_CLEAR_MEMORY_ON_FREE is now conditional on FURI_DEBUG. Saves ~500+ memset calls/sec during active GUI/protocol work (FreeRTOSConfig.h)
3. Fix calloc() to explicitly zero memory — with heap-clear disabled in release, calloc() now does its own memset(0) to guarantee zero-initialized returns (memmgr.c)
4. Fix realloc() to copy min(old_size, new_size) bytes — original copied size (new) bytes, reading past old allocation when growing. Added memmgr_heap_get_block_size() that reads usable size from Heap_4 BlockLink_t header. Also added NULL-guard on pvPortMalloc result to preserve original allocation on OOM (memmgr.c, memmgr_heap.c/.h, api_symbols.csv)

Branch prediction:
5. __builtin_expect hints on furi_check/assert/break — crash code moves to end of function, hot path becomes fall-through (0 pipeline penalty on Cortex-M4 3-stage pipeline). Affects ~2300+ call sites across the firmware (check.h)

DMA:
6. SPI TX via DMA with RX drain — furi_hal_spi_bus_tx() now delegates to DMA when scheduler is running, freeing the CPU during display updates (~1KB/frame @ 20fps) and radio TX. TX-only path sets up RX DMA channel draining into a dummy byte to prevent OVR accumulation. Polling fallback preserved for pre-scheduler context (furi_hal_spi.c)

Hot function inlining:
7. attribute((flatten)) on furi_get_tick() — forces inlining of FreeRTOS wrappers at call sites (kernel.c)
8. attribute((flatten)) on hot thread functions — applied to furi_thread_get_current_id(), furi_thread_get_current(), furi_thread_flags_get() (thread.c)

String formatting:
9. In-place vprintf for furi_string_cat_vprintf() — formats directly into destination buffer at current offset, growing only if needed. Eliminates temporary FuriString allocation (malloc + format + memcpy + free) per call (string.c)

Power:
10. Reduce configEXPECTED_IDLE_TIME_BEFORE_SLEEP from 4 to 2 ticks — allows FreeRTOS tickless idle to enter STOP mode more aggressively (2ms threshold instead of 4ms). Reduces average power consumption (FreeRTOSConfig.h)

Verification

• Build: ./fbt COMPACT=1 DEBUG=0 — compiles without warnings/errors
• Build: ./fbt DEBUG=1 — debug build still compiles and links
• Boot device, navigate Settings → About — FW version shown correctly
• Open SubGHz, NFC, IR apps — UI responsive, no hangs
• Verify formatted strings render correctly in UI (menus, popups, dialogs)
• SPI peripherals working: display renders, Sub-GHz TX/RX functional
• Leave device idle 30s — confirm no increased battery drain or wake-up issues
• Stress test: rapid app switching, menu scrolling — no crashes or visual artifacts
• Verify calloc-dependent code (e.g. allocating zero-initialized buffers) works correctly in release build

Checklist (For Reviewer)

• PR has description of feature/bug or link to Confluence/Jira task
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix