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

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Mar 27, 2019 at 1:27 PM Martin Liška <mliska@suse.cz> wrote:
>
> 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.

Yeah, I think it's use of optimize_function_for_size_p is at least odd.
It seems to be a defensive check in place to do as little as possible
keeping the original order.  I think the only thing that should be disabled
is tracing of cold paths.

But this is all stage1 material.

Richard.

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 outdated for gcc 8.3 (my arm 7.3.1 produces smaller code using stc, too).)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