Bluetooth: Controller: Support Link Time Optimizations (LTO)

Support for using Link Time Optimization (LTO) when
building application with open source Bluetooth Low
Energy Controller. This reduces code space usage.

Current Memory Usage at the time of this PR (hci_ipc):

BT_CTLR_OPTIMIZE_FOR_SPEED:
Memory region         Used Size  Region Size  %age Used
           FLASH:      260112 B       256 KB     99.22%
             RAM:       60136 B        64 KB     91.76%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

BT_CTLR_OPTIMIZE_FOR_SIZE:
Memory region         Used Size  Region Size  %age Used
           FLASH:      232292 B       256 KB     88.61%
             RAM:       60128 B        64 KB     91.75%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

BT_CTLR_OPTIMIZE_FOR_APP_DEFAULT:
Memory region         Used Size  Region Size  %age Used
           FLASH:      232292 B       256 KB     88.61%
             RAM:       60128 B        64 KB     91.75%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

BT_CTLR_LTO:
Memory region         Used Size  Region Size  %age Used
           FLASH:      221484 B       256 KB     84.49%
             RAM:       60120 B        64 KB     91.74%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

Author: cvinayak

subsys/bluetooth/controller/CMakeLists.txt

@@ -212,6 +212,11 @@ add_subdirectory_ifdef(
   ll_sw/openisa
   )
 
+zephyr_library_compile_options_ifdef(
+  CONFIG_BT_CTLR_OPTIMIZE_FOR_SIZE
+  ${OPTIMIZE_FOR_SIZE_FLAG}
+  )
+
 zephyr_library_compile_options_ifdef(
   CONFIG_BT_CTLR_OPTIMIZE_FOR_SPEED
   ${OPTIMIZE_FOR_SPEED_FLAG}

subsys/bluetooth/controller/Kconfig.ll_sw_split

@@ -598,12 +598,34 @@ config BT_CTLR_DYNAMIC_INTERRUPTS
 	  permit use of SoC's peripheral for custom use when Bluetooth is not
 	  enabled.
 
+choice
+	prompt "Optimization options"
+	depends on !LTO
+	default BT_CTLR_OPTIMIZE_FOR_SPEED
+
+config BT_CTLR_OPTIMIZE_FOR_APP_DEFAULT
+	bool "Optimize for application defaults"
+	help
+	  Optimize compilation of Controller for application's default.
+
+config BT_CTLR_OPTIMIZE_FOR_SIZE
+	bool "Optimize for space"
+	help
+	  Optimize compilation of Controller for code size.
+
+	  This is the Zephyr project default, but is overridden to use optimize
+	  for speed when compiling the Controller to reduce application's ISR
+	  and thread latencies.
+
 config BT_CTLR_OPTIMIZE_FOR_SPEED
-	prompt "Optimize for Speed"
-	bool
-	default y
+	bool "Optimize for Speed"
 	help
-	  Optimize compilation of controller for execution speed.
+	  Optimize compilation of Controller for execution speed. As the
+	  Controller's Upper Link Layer and Lower Link Layer execute in ISRs,
+	  use of speed optimizations reduces application's ISR and thread
+	  latencies.
+
+endchoice
 
 config BT_CTLR_XTAL_ADVANCED
 	bool "Advanced event preparation"