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

[Moritz Strübe <moritz dot struebe at redheads dot de>]

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.
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outdated for gcc 8.3 (my arm 7.3.1 produces smaller code using stc, too).)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

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