detect empty functions

[Prathamesh Kulkarni]

On Wed, Feb 5, 2014 at 7:33 PM, vijay nag <vijunag@gmail.com> wrote:
> On Wed, Feb 5, 2014 at 7:33 PM, vijay nag <vijunag@gmail.com> wrote:
>> Hello,
>>
>> You can use the below program to find the length of the function
>> compiled for X86 target.
>>
>> Note that assembly instruction for "function()" is immediately
>> followed by assembly for "functionEnd"
>>
>> Key things to note here
>>
>> 1) Compiler still emits epilogue/prologue for a function with empty
>> body. However you could disable this using -fomit-frame-ptr in which
>> case function
>>     body will only have "ret" instruction and in which case difference
>> would be 1.
>>     0x80483a4 <function>            ret
>>
>>                                    │
>>    │0x80483a5 <functionEnd>         ret
>>
>>    So size of the function would be 0x80483a5 - 0x80483a4 = 1
>>
>>  2)In case if you need frame-ptr, there will be a minimum of 4
>> instructions and in this case difference would be 5.
>>     0x80483a4 <function>            push   %ebp
>>
>>                                    │
>>    │0x80483a5 <function+1>          mov    %esp,%ebp
>>
>>                                    │
>>    │0x80483a7 <function+3>          pop    %ebp
>>
>>                                    │
>>    │0x80483a8 <function+4>          ret
>>
>>                                    │
>>    │0x80483a9 <functionEnd>         push   %ebp
>>
>>                                    │
>>    │0x80483aa <functionEnd+1>       mov    %esp,%ebp
>>
>>                                    │
>>    │0x80483ac <functionEnd+3>       pop    %ebp
>>
>>                                    │
>>    │0x80483ad <functionEnd+4>       ret
>>
>>      So size of the function would be 0x80483a9 - 0x80483a4 = 5
>>
>>
>> cat fsize.c
>> #include <stdio.h>
>>
>> int function(){
>> }
>>
>> int functionEnd(){
>> }
>>
>> int main()
>> {
>>   printf("%x\n", &functionEnd - &function);
>> }
>>
>> vijayNag
>>
>> On Wed, Feb 5, 2014 at 7:00 PM, Prathamesh Kulkarni
>> <bilbotheelffriend@gmail.com> wrote:
>>> Hi, is there a way to find if a function has empty body ?
>>>
>>> Thanks and Regards,
>>> Prathamesh
>
> My apologies for top-posting.
I wanted to ask, is there a way to detect if a function has empty body
using GCC API ?
Sorry if I didn't phrase it correctly earlier.