Re: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation

[Martin Liška <mliska at suse dot cz>]

On 3/25/19 1:36 PM, Moritz Strübe wrote:
> Hi,
> 
> I have an issue with the optimization options. We are on an stm32 and it 
> only has a prefetcher, but no cache. Thus it's nice to have linear 
> default path. For example, we use  __builtin_expect in our asserts. Yet 
> it seems that this does not work when using -Os. I confirmed that this 
> is not an arm issue, but can also be seen on x86.
> I have the following code:
> ----------
> #include <stdint.h>
> #ifdef UN
> #define UNLIKELY(x)     __builtin_expect((x),0)
> #else
> #define UNLIKELY(x) (x)
> #endif
> 
> float a = 66;
> 
> int test (float b, int test) {
>      if(UNLIKELY(test)) {
>          return b / a;
>      } else {
>          return b * a;
>      }
> }
> ----------
> "gcc -O2" reorders the code depending on a passed -DUN, while -Os always 
> produces the same output (see https://godbolt.org/z/cL-Pbg)
> 
> I played around with different options running
> gcc -O{s|2} -Q  --help=optimizers
> , but didn't manage to get -Os to do that optimization.

Hi.

So first we have a misleading documentation for 8.3.0, you're hitting:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87829

So with -Os we enable BB reordering, the only difference is that we set
-freorder-blocks-algorithm=simple with -Os. However, on x86_64, it stays
with -freorder-blocks-algorithm=stv.

Then you can use -fdump-rtl-bbro and see a dump file:
pr-expect.c.300r.bbro

The issues you're seeing is caused by fact that bbro uses
optimize_function_for_size_p functions that return true with -Os.
That's probably why you see the difference. That's my quick analysis.

Maybe Honza can help here?

Martin

> Opposed to what the manual[1] says, this only differs in 
> -finline-functions and -foptimize-strlen for 8.3
> (OT: Especially the info about freorder-blocks-algorithm seems to be 
> outdated for gcc 8.3 (my arm 7.3.1 produces smaller code using stc, too).)
> Since adjusting all those options didn't help I tried
> gcc -O{s|2} -Q  --help={param|common|target|c++}
> but that didn't give me any new insight. (BTW: "-Q --help=param" should 
> probably be documented in the --param-section)
> 
> Cheers
> Moritz
> 
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc-8.3.0/gcc/Optimize-Options.html
>