The bluetooth controller has been using the flag '-Ofast' to keep
within a real-time limit. There are two problems with this; firstly,
when a project should be optimized for size it is standard to use -O2,
not -Ofast.
Secondly, optimization flags have been deemed to be non-portable, so
instead of directly using "-Ofast" we should use the intent-macro
OPTIMIZE_FOR_SPEED_FLAG to ensure toolchain portability.
Testing has shown that we are still within the real-time limit when
changing from -Ofast to -O2. -Ofast is about 1us and 1% faster, but
increases the code size by 13kB (5% of the available flash on a
nRF51).
Since the slowdown is comparatively small compared to the code size
increase we have decided to use -O2 in place of -Ofast.
Other optimization combinations were also measured and their results
can be seen below:
-Ofast in BLE Controller and in #pragma in entropy driver.
Memory region Used Size Region Size %age Used
FLASH: 138920 B 256 KB 52.99%
[bt] [INF] encode_control: l: 6, 6, 7; t: 75, 45, 132.
-O2 in BLE Controller and in #pragma in entropy driver.
Memory region Used Size Region Size %age Used
FLASH: 125840 B 256 KB 48.00%
[bt] [INF] encode_control: l: 6, 6, 7; t: 75, 51, 133
-O3 in BLE Controller and in #pragma in entropy driver.
Memory region Used Size Region Size %age Used
FLASH: 138920 B 256 KB 52.99%
[bt] [INF] encode_control: l: 6, 6, 7; t: 75, 50, 132.
No extra CFLAGS for BLE and entropy driver (pragmas removed), using
CONFIG_SIZE_OPTIMIZATIONS=y
Memory region Used Size Region Size %age Used
FLASH: 120124 B 256 KB 45.82%
[bt] [ERR] isr_rx_conn: assert: '!radio_is_ready()' failed
No extra CFLAGS for BLE and entropy driver (pragmas removed), using
CONFIG_SPEED_OPTIMIZATIONS=y
Memory region Used Size Region Size %age Used
FLASH: 138004 B 256 KB 52.64%
[bt] [INF] encode_control: l: 6, 6, 7; t: 61, 51, 130.
NB: RAM usage differences were insignificant.
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
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