dead code remover under gcc -O and higher(Is there a flag to do this)

dead code remover under gcc -O and higher(Is there a flag to do this)

[chengjian (D)]

I have written a simple code like this

```c
#include <stdlib.h>
#include <stdio.h>

//#define CONFIG_TARGET_X86_64

#ifdef CONFIG_TARGET_X86_64
static void A( )
{
     printf("A\n");
}
#else
void A( );
#endif

static void B( )
{
     printf("B\n");
}


static int xx( )
{
#ifdef CONFIG_TARGET_X86_64
   return 1;
#else
   return 0;
#endif
}

int main(void)
{
   if (xx( ))   /* define CONFIG_TARGET_X86_64 */
     A( );
   else
     B( );
}
```

If we don't define the CONFIG_TARGET_X86_64, xx( ) will always return 
FALSE, so functiopn A which is only declared, but not implemented will 
never be called(dead code).

compile it with gcc -O0

```cpp
/tmp/cctSpgGk.o: In function `main':
1.c:(.text+0x34): undefined reference to `A'
collect2: error: ld returned 1 exit status
```

But it can be compiled by -O1 or higher.

use GCC V6.1.0.

I can declare A as weak:

```cpp
void A (void) __attribute__ ((weak));
```

Then the linker will ignore the undefined symbol reference, but a call 
to this function will lead to a crash


So my question is :
It seems that one of the optimization options in the -O1 option 
eliminates the dead code, I have seen the optimize doccument about GCC

https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Optimize-Options.html

but I can't find it.

So if I just want to compile this code under the -O0 option, Is it 
possible ? Are there some optimization flags help me to do this?

Thanks.

Re: dead code remover under gcc -O and higher(Is there a flag to do this)

[carl hansen]

line 12 , you say "void A( );"

say instead:
void A(){};

That solved it for me, using gcc7.2

On Sun, Dec 3, 2017 at 4:56 PM, chengjian (D) <cj.chengjian@huawei.com> wrote:
> I have written a simple code like this
>
> ```c
> #include <stdlib.h>
> #include <stdio.h>
>
> //#define CONFIG_TARGET_X86_64
>
> #ifdef CONFIG_TARGET_X86_64
> static void A( )
> {
>     printf("A\n");
> }
> #else
> void A( );
> #endif
>
> static void B( )
> {
>     printf("B\n");
> }
>
>
> static int xx( )
> {
> #ifdef CONFIG_TARGET_X86_64
>   return 1;
> #else
>   return 0;
> #endif
> }
>
> int main(void)
> {
>   if (xx( ))   /* define CONFIG_TARGET_X86_64 */
>     A( );
>   else
>     B( );
> }
> ```
>
> If we don't define the CONFIG_TARGET_X86_64, xx( ) will always return FALSE,
> so functiopn A which is only declared, but not implemented will never be
> called(dead code).
>
> compile it with gcc -O0
>
> ```cpp
> /tmp/cctSpgGk.o: In function `main':
> 1.c:(.text+0x34): undefined reference to `A'
> collect2: error: ld returned 1 exit status
> ```
>
> But it can be compiled by -O1 or higher.
>
> use GCC V6.1.0.
>
> I can declare A as weak:
>
> ```cpp
> void A (void) __attribute__ ((weak));
> ```
>
> Then the linker will ignore the undefined symbol reference, but a call to
> this function will lead to a crash
>
>
> So my question is :
> It seems that one of the optimization options in the -O1 option eliminates
> the dead code, I have seen the optimize doccument about GCC
>
> https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Optimize-Options.html
>
> but I can't find it.
>
> So if I just want to compile this code under the -O0 option, Is it possible
> ? Are there some optimization flags help me to do this?
>
> Thanks.
>

Re: dead code remover under gcc -O and higher(Is there a flag to do this)

[chengjian (D)]

On 2017/12/4 10:25, carl hansen wrote:
> line 12 , you say "void A( );"
>
> say instead:
> void A(){};
>
> That solved it for me, using gcc7.2
>
>

Yeah, I miss the implementation, but I did this on purpose

I'm curious is there any options to eliminate this.
like gcc -O2 do.

thanks.

Re: dead code remover under gcc -O and higher(Is there a flag to do this)

[Jakub Jelinek]

On Mon, Dec 04, 2017 at 08:56:05AM +0800, chengjian (D) wrote:
> So my question is :
> It seems that one of the optimization options in the -O1 option eliminates
> the dead code, I have seen the optimize doccument about GCC
> 
> https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Optimize-Options.html
> 
> but I can't find it.

-ftree-dce, -fdce (but actually many other passes do that, if some condition
is folded to a constant, any cfg cleanup will remove the dead code).

> So if I just want to compile this code under the -O0 option, Is it possible
> ? Are there some optimization flags help me to do this?

No.  The thing is, -O0 vs. -Og vs. -O1+/-Os are using different sets of
passes, and various passes have gates flag_foo_bar_baz && optimize or
similar, so even if you enable -ffoo-bar-baz, at -O0 it will not do
anything.  In your testcase, you need either inlining (which is at -O0 done
only for __attribute__((always_inline)) functions), or some IPA analysis on
the return value range (I think that is still unimplemented), but even if it
would be, it certainly wouldn't be done at -O0.

So, make it always_inline and it might happen to "work" at -O0, otherwise,
no, don't try that at -O0.

	Jakub