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 outdated for gcc 8.3 (my arm 7.3.1 produces smaller code using stc, too).)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