CODEWITHSUNDEEP
c++arm
Foaly
Foaly

Reputation: 93

Remove unused functions from binary

I'm compiling a code for an embedded ARM microcontroller and I'm trying to optimize the binary size. I'm using arm-none-eabi-g++ with options -Os -ffunction-sections -fdata-sections -Wl,--gc-sections in order to achieve this. It looks like this removes some (but not all) unused functions, especially some heavy one.

arm-none-eabi-nm --print-size --size-sort --radix=d gives me :

[...]
00008412 00000084 T _raise_r
00004888 00000088 t restore_non_core_regs
00000172 00000092 t _ZN5USART5writeENS_4PortEc.constprop.22
00005276 00000094 T __gnu_Unwind_RaiseException
00001644 00000094 t _ZN4GPIO16enablePeripheralERKNS_3PinE
536871016 00000096 d impure_data
00004592 00000096 t search_EIT_table
00000624 00000100 t _ZN5Flash9writePageEiPh
00005648 00000112 T __gnu_Unwind_Backtrace
536874336 00000128 b _ZN3USB8_bankEP0E
00000264 00000128 t _ZN5USART9availableENS_4PortE.constprop.20
00002500 00000140 t _Z17usbControlHandlerRN3USB11SetupPacketEPhRi
00004728 00000160 t get_eit_entry
00000464 00000160 t _ZN3USB15ep0SETUPHandlerEv
00005060 00000212 t unwind_phase2_forced
536871424 00000256 b _ZN3USB17_epRAMDescriptorsE
536871688 00000384 b _ZN3USB10_endpointsE
536874496 00000384 b _ZN4Core9isrVectorE
00001972 00000528 t _ZN3USB12ep0INHandlerEv
00006504 00000590 T _Unwind_VRS_Pop
00007612 00000674 T __gnu_unwind_execute
00005760 00000732 t __gnu_unwind_pr_common
00000752 00000892 t _ZN3USB16interruptHandlerEv
00002640 00001848 T main
536872096 00002144 b _ZN5USART5portsE

As you can see, there are quite a lot of room taken by "unwind" functions. These seem to be related to C++ exceptions, but I'm not using them. I looked at the ELF with objdump -d and it looks like no "unwind" function is called from any of my functions, so this would be effectively dead code.

Is there a way to remove it from the ELF? Or are they mandatory for some reason? I could save at least 1/4 of the total binary size.

Upvotes: 6

Views: 1382

Answers (1)

Foaly
Foaly

Reputation: 93

Thanks to @DarkFalcon, @gudok and @jaggedSpire for the answer : the solution was to add -fno-exceptions to remove all unwind-related functions. Now all the code still added in the binary is effectively used.

Upvotes: 2

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