* GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation
@ 2019-03-25 12:36 Moritz Strübe
2019-03-27 12:27 ` Martin Liška
2019-03-27 13:37 ` __builtin_expect() reduces optimization (Was: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation) Moritz Strübe
0 siblings, 2 replies; 4+ messages in thread
From: Moritz Strübe @ 2019-03-25 12:36 UTC (permalink / raw)
To: gcc
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outdated for gcc 8.3 (my arm 7.3.1 produces smaller code using stc, too).)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
--
Redheads Ltd. Softwaredienstleistungen
Schillerstr. 14
90409 Nürnberg
Telefon: +49 (0)911 180778-50
E-Mail: moritz.struebe@redheads.de | Web: www.redheads.de
Geschäftsführer: Andreas Hanke
Sitz der Gesellschaft: Lauf
Amtsgericht Nürnberg HRB 22681
Ust-ID: DE 249436843
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation
2019-03-25 12:36 GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation Moritz Strübe
@ 2019-03-27 12:27 ` Martin Liška
2019-03-28 8:27 ` Richard Biener
2019-03-27 13:37 ` __builtin_expect() reduces optimization (Was: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation) Moritz Strübe
1 sibling, 1 reply; 4+ messages in thread
From: Martin Liška @ 2019-03-27 12:27 UTC (permalink / raw)
To: Moritz Strübe, gcc; +Cc: Jan Hubicka
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)Â Â Â Â __builtin_expect((x),0)
> #else
> #define UNLIKELY(x) (x)
> #endif
>
> float a = 66;
>
> int test (float b, int test) {
> Â Â Â if(UNLIKELY(test)) {
> Â Â Â Â Â Â Â return b / a;
> Â Â Â } else {
> Â Â Â Â Â Â Â return b * a;
> Â Â Â }
> }
> ----------
> "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} -QÂ --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} -QÂ --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
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation
2019-03-27 12:27 ` Martin Liška
@ 2019-03-28 8:27 ` Richard Biener
0 siblings, 0 replies; 4+ messages in thread
From: Richard Biener @ 2019-03-28 8:27 UTC (permalink / raw)
To: Martin Liška; +Cc: Moritz Strübe, gcc, Jan Hubicka
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
> >
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* __builtin_expect() reduces optimization (Was: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation)
2019-03-25 12:36 GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation Moritz Strübe
2019-03-27 12:27 ` Martin Liška
@ 2019-03-27 13:37 ` Moritz Strübe
1 sibling, 0 replies; 4+ messages in thread
From: Moritz Strübe @ 2019-03-27 13:37 UTC (permalink / raw)
To: gcc
Hey.
Am 25.03.2019 um 13:36 schrieb Moritz Strübe:
> 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 (seehttps://godbolt.org/z/cL-Pbg)
Actually that is wrong. I was diverted by looking for reorderings. When
compiling with -Os -DUN there is an additional, and unnecessary, "movaps
%xmm0, %xmm1". (I seen the same thing on arm, as it is more visible
there: https://bugs.launchpad.net/gcc-arm-embedded/+bug/1821703)
I also had a quick look at the output of -fdump-rtl-bbro, as suggested
by Martin, but I'm afraid I'll need a week of digging into the specifics
before I can interpret it.
Cheers
Morty
--
Redheads Ltd. Softwaredienstleistungen
Schillerstr. 14
90409 Nürnberg
Telefon: +49 (0)911 180778-50
E-Mail: moritz.struebe@redheads.de | Web: www.redheads.de
Geschäftsführer: Andreas Hanke
Sitz der Gesellschaft: Lauf
Amtsgericht Nürnberg HRB 22681
Ust-ID: DE 249436843
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2019-03-28 8:27 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-03-25 12:36 GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation Moritz Strübe
2019-03-27 12:27 ` Martin Liška
2019-03-28 8:27 ` Richard Biener
2019-03-27 13:37 ` __builtin_expect() reduces optimization (Was: GCC-Reordering-Optimization-Options in Os and O2 when using __builtin_expect() and documentation) Moritz Strübe
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).