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outdated for gcc 8.3 (my arm 7.3.1 produces smaller code using stc, too).)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