dead function elimination

dead function elimination

[jimmy]

Can gcc or ld perform dead procedure elimination apart from using the 
combination of --gc-sections/-ffunction-sections?


-jb
-- 
mathematician, n.:
         Some one who believes imaginary things appear right before your 
i's.

Re: dead function elimination

[Nick Clifton]

Hi Jimmy,

> Can gcc or ld perform dead procedure elimination apart from using the 
> combination of --gc-sections/-ffunction-sections?

(Why are you excluding the --gc-sections / -ffunction-sections pairing 
which were designed precisely for this purpose ?)

The short answer is yes.  Gcc can and will eliminate unused local 
procedures, and with its --whole-program option it can even eliminate 
unused non-local procedures.

But your question implies that you are asking if between them the linker 
and compiler can eliminate unused procedures without recompilation of 
all the sources, and in this case the answer is no.

Cheers
   Nick

Re: dead function elimination

[jimmy]

Nick Clifton wrote:
> Hi Jimmy,
> 
>> Can gcc or ld perform dead procedure elimination apart from using the 
>> combination of --gc-sections/-ffunction-sections?
> 
> (Why are you excluding the --gc-sections / -ffunction-sections pairing 
> which were designed precisely for this purpose ?)
> 
> The short answer is yes.  Gcc can and will eliminate unused local 
> procedures, and with its --whole-program option it can even eliminate 
> unused non-local procedures.
> 
> But your question implies that you are asking if between them the linker 
> and compiler can eliminate unused procedures without recompilation of 
> all the sources, and in this case the answer is no.
> 
> Cheers
>   Nick
> 
I thght the linker alone was supposed to be able to do 
dead-procedure-elimination. After all can't the linker determine which 
procedures are unreferenced in the final executable?

If i'm not mistaken the diablo linker can do this?


Thanks!
-jb
-- 
mathematician, n.:
         Some one who believes imaginary things appear right before your 
i's.

Re: dead function elimination

[Andrew Haley]

jimmy writes:
 > Nick Clifton wrote:
 > > Hi Jimmy,
 > > 
 > >> Can gcc or ld perform dead procedure elimination apart from using the 
 > >> combination of --gc-sections/-ffunction-sections?
 > > 
 > > (Why are you excluding the --gc-sections / -ffunction-sections pairing 
 > > which were designed precisely for this purpose ?)
 > > 
 > > The short answer is yes.  Gcc can and will eliminate unused local 
 > > procedures, and with its --whole-program option it can even eliminate 
 > > unused non-local procedures.
 > > 
 > > But your question implies that you are asking if between them the linker 
 > > and compiler can eliminate unused procedures without recompilation of 
 > > all the sources, and in this case the answer is no.
 >
 > I thght the linker alone was supposed to be able to do 
 > dead-procedure-elimination. After all can't the linker determine which 
 > procedures are unreferenced in the final executable?
 > 
 > If i'm not mistaken the diablo linker can do this?

GNU ld can do this too.  It does it if you compile with
-ffunction-sections.

Andrew.

Re: dead function elimination

[Nick Clifton]

Hi Jimmy,

> I thght the linker alone was supposed to be able to do 
> dead-procedure-elimination.

No.  It is a nice feature, but it is not a requirement of a linker.

> After all can't the linker determine which 
> procedures are unreferenced in the final executable?

Which is exactly what the --ffunction-sections and --gc-sections 
switches do.

> If i'm not mistaken the diablo linker can do this?

It probably can.

Cheers
   Nick

Re: dead function elimination

[jimmy]

Nick Clifton wrote:
> Hi Jimmy,
> 
>> I thght the linker alone was supposed to be able to do 
>> dead-procedure-elimination.
> 
> No.  It is a nice feature, but it is not a requirement of a linker.
> 
>> After all can't the linker determine which procedures are unreferenced 
>> in the final executable?
> 
> Which is exactly what the --ffunction-sections and --gc-sections 
> switches do.
> 
>> If i'm not mistaken the diablo linker can do this?
> 
> It probably can.
> 
> Cheers
>   Nick
> 

If i understand things correctly, there is a restriction on 
'--gc-sections' that it doesn't work if the link is dynamic. What if my 
scenario is like this:

	libsomething.a - some static library built with -ffunction-sections & 
-fdata-sections

	src.o - my source code also built with -ffucntion-sections

final step:

	ld -o exec --gc-sections src.o -lsomething

In the above step, ld simply says

'/usr/bin/ld: Warning: gc-sections option ignored'

My aim here was to remove unreferenced functions from both my code & the 
static library, but at the same time using libc as an '.so' (obviously 
that makes sense, cos it's shared across many apps & probably not built 
with -ffunction-sections to use a static link).

Is there any way to achieve this?


-jb
-- 
mathematician, n.:
         Some one who believes imaginary things appear right before your 
i's.

Re: dead function elimination

[Nick Clifton]

Hi Jimmy,

> If i understand things correctly, there is a restriction on 
> '--gc-sections' that it doesn't work if the link is dynamic. What if my 
> scenario is like this:
> 
>     libsomething.a - some static library built with -ffunction-sections 
> & -fdata-sections
> 
>     src.o - my source code also built with -ffucntion-sections
> 
> final step:
> 
>     ld -o exec --gc-sections src.o -lsomething
> 
> In the above step, ld simply says
> 
> '/usr/bin/ld: Warning: gc-sections option ignored'

Hmm, interesting.  I had thought that this would work.  I will have to 
look into it when I get from vacation.  Are you using the most up to 
date version of the linker sources ?

Cheers
   Nick

Re: dead function elimination

[jimmy]

Nick Clifton wrote:
> Hi Jimmy,
> 
>> If i understand things correctly, there is a restriction on 
>> '--gc-sections' that it doesn't work if the link is dynamic. What if 
>> my scenario is like this:
>>
>>     libsomething.a - some static library built with 
>> -ffunction-sections & -fdata-sections
>>
>>     src.o - my source code also built with -ffucntion-sections
>>
>> final step:
>>
>>     ld -o exec --gc-sections src.o -lsomething
>>
>> In the above step, ld simply says
>>
>> '/usr/bin/ld: Warning: gc-sections option ignored'
> 
> Hmm, interesting.  I had thought that this would work.  I will have to 
> look into it when I get from vacation.  Are you using the most up to 
> date version of the linker sources ?
> 
> Cheers
>   Nick

binutils-2.17 is able to do it! thank you for the help.


-jb
-- 
mathematician, n.:
         Some one who believes imaginary things appear right before your 
i's.