| Message ID | DB6PR0801MB2053E6F13ED5CA0FA6D0D9F483450@DB6PR0801MB2053.eurprd08.prod.outlook.com |
|---|---|
| State | New |
| Headers | show |
| Series | Change default optimization level to -Og | expand |
On 10/26/17, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote: > GCC's default optimization level is -O0. Unfortunately unlike other > compilers, > GCC generates extremely inefficient code with -O0. It is almost unusable > for > low-level debugging or manual inspection of generated code. So a -O option > is > always required for compilation. -Og not only allows for fast compilation, > but > also produces code that is efficient, readable as well as debuggable. > Therefore -Og makes for a much better default setting. > > Any comments? There are a number of bugs with -Og that I'd want to see fixed before making it the default; I'll follow this message up once I find them all. > > 2017-10-26 Wilco Dijkstra <wdijkstr@arm.com> > > * opts.c (default_options_optimization): Set default to -Og. > > doc/ > * invoke.texi (-O0) Remove default mention. > (-Og): Add mention of default setting. > > -- > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi > index > 3328a3b5fafa6a98007eff52d2a26af520de9128..74c33ea35b9f320b419a3417e6007d2391536f1b > 100644 > --- a/gcc/doc/invoke.texi > +++ b/gcc/doc/invoke.texi > @@ -7343,7 +7343,7 @@ by @option{-O2} and also turns on the following > optimization flags: > @item -O0 > @opindex O0 > Reduce compilation time and make debugging produce the expected > -results. This is the default. > +results. > > @item -Os > @opindex Os > @@ -7371,7 +7371,7 @@ Optimize debugging experience. @option{-Og} enables > optimizations > that do not interfere with debugging. It should be the optimization > level of choice for the standard edit-compile-debug cycle, offering > a reasonable level of optimization while maintaining fast compilation > -and a good debugging experience. > +and a good debugging experience. This is the default. > @end table > > If you use multiple @option{-O} options, with or without level numbers, > diff --git a/gcc/opts.c b/gcc/opts.c > index > dfad955e220870a3250198640f3790c804b191e0..74511215309f11445685db4894be2ab6881695d3 > 100644 > --- a/gcc/opts.c > +++ b/gcc/opts.c > @@ -565,6 +565,12 @@ default_options_optimization (struct gcc_options > *opts, > int opt2; > bool openacc_mode = false; > > + /* Set the default optimization to -Og. */ > + opts->x_optimize_size = 0; > + opts->x_optimize = 1; > + opts->x_optimize_fast = 0; > + opts->x_optimize_debug = 1; > + > /* Scan to see what optimization level has been specified. That will > determine the default value of many flags. */ > for (i = 1; i < decoded_options_count; i++) > >
> GCC's default optimization level is -O0. Unfortunately unlike other > compilers, GCC generates extremely inefficient code with -O0. Agreed (but this can probably be worked on). > It is almost unusable for low-level debugging or manual inspection of > generated code. Likewise. > So a -O option is always required for compilation. For compiler hackers only though. For normal users, -O0 is still the only level where you're guaranteed to be able to do full source debugging.
On Thu, Oct 26, 2017 at 05:12:40PM +0000, Wilco Dijkstra wrote: > GCC's default optimization level is -O0. Unfortunately unlike other compilers, > GCC generates extremely inefficient code with -O0. It is almost unusable for > low-level debugging or manual inspection of generated code. So a -O option is > always required for compilation. -Og not only allows for fast compilation, but > also produces code that is efficient, readable as well as debuggable. > Therefore -Og makes for a much better default setting. > > Any comments? > > 2017-10-26 Wilco Dijkstra <wdijkstr@arm.com> > > * opts.c (default_options_optimization): Set default to -Og. > > doc/ > * invoke.texi (-O0) Remove default mention. > (-Og): Add mention of default setting. This would only severely confuse users. -Og has lots of unresolved issues for debugging experience, and changing the default this way is IMHO extremely undesirable. Jakub
On Thu, Oct 26, 2017 at 10:12 AM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote: > GCC's default optimization level is -O0. Unfortunately unlike other compilers, > GCC generates extremely inefficient code with -O0. It is almost unusable for > low-level debugging or manual inspection of generated code. So a -O option is > always required for compilation. -Og not only allows for fast compilation, but > also produces code that is efficient, readable as well as debuggable. > Therefore -Og makes for a much better default setting. > > Any comments? I think this goes against what most folks are used to. I know you are saying most folks are used to a compiler defaulting to optimizations on but I don't think that is true. In fact GCC has been this way since day one. Plus you also missed changing the following part of the documentation: If you are not using some other optimization option, consider using -Og (see Optimize Options) with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. Thanks, Andrew Pinski > > 2017-10-26 Wilco Dijkstra <wdijkstr@arm.com> > > * opts.c (default_options_optimization): Set default to -Og. > > doc/ > * invoke.texi (-O0) Remove default mention. > (-Og): Add mention of default setting. > > -- > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi > index 3328a3b5fafa6a98007eff52d2a26af520de9128..74c33ea35b9f320b419a3417e6007d2391536f1b 100644 > --- a/gcc/doc/invoke.texi > +++ b/gcc/doc/invoke.texi > @@ -7343,7 +7343,7 @@ by @option{-O2} and also turns on the following optimization flags: > @item -O0 > @opindex O0 > Reduce compilation time and make debugging produce the expected > -results. This is the default. > +results. > > @item -Os > @opindex Os > @@ -7371,7 +7371,7 @@ Optimize debugging experience. @option{-Og} enables optimizations > that do not interfere with debugging. It should be the optimization > level of choice for the standard edit-compile-debug cycle, offering > a reasonable level of optimization while maintaining fast compilation > -and a good debugging experience. > +and a good debugging experience. This is the default. > @end table > > If you use multiple @option{-O} options, with or without level numbers, > diff --git a/gcc/opts.c b/gcc/opts.c > index dfad955e220870a3250198640f3790c804b191e0..74511215309f11445685db4894be2ab6881695d3 100644 > --- a/gcc/opts.c > +++ b/gcc/opts.c > @@ -565,6 +565,12 @@ default_options_optimization (struct gcc_options *opts, > int opt2; > bool openacc_mode = false; > > + /* Set the default optimization to -Og. */ > + opts->x_optimize_size = 0; > + opts->x_optimize = 1; > + opts->x_optimize_fast = 0; > + opts->x_optimize_debug = 1; > + > /* Scan to see what optimization level has been specified. That will > determine the default value of many flags. */ > for (i = 1; i < decoded_options_count; i++) >
Andrew Pinski wrote: > I think this goes against what most folks are used to. I know you are > saying most folks are used to a compiler defaulting to optimizations > on but I don't think that is true. In fact GCC has been this way > since day one. Well it may depend which part of the industry you're coming from. GCC certainly has a long history doing it one way, however other compilers took a different approach and have supported optimized debugging for decades. So I don't understand the use of having a "turn every optimization off" option, let alone for it to be the default today... > Plus you also missed changing the following part of the documentation: > If you are not using some other optimization option, consider using > -Og (see Optimize Options) with -g. With no -O option at all, some > compiler passes that collect information useful for debugging do not > run at all, so that -Og may result in a better debugging experience. Sure, the doc part of the patch will need further revision if we agree to change the default. Wilco
On 10/26/2017 01:50 PM, Jakub Jelinek wrote: > On Thu, Oct 26, 2017 at 05:12:40PM +0000, Wilco Dijkstra wrote: >> GCC's default optimization level is -O0. Unfortunately unlike other compilers, >> GCC generates extremely inefficient code with -O0. It is almost unusable for >> low-level debugging or manual inspection of generated code. So a -O option is >> always required for compilation. -Og not only allows for fast compilation, but >> also produces code that is efficient, readable as well as debuggable. >> Therefore -Og makes for a much better default setting. >> >> Any comments? >> >> 2017-10-26 Wilco Dijkstra <wdijkstr@arm.com> >> >> * opts.c (default_options_optimization): Set default to -Og. >> >> doc/ >> * invoke.texi (-O0) Remove default mention. >> (-Og): Add mention of default setting. > > This would only severely confuse users. -Og has lots of unresolved issues > for debugging experience, and changing the default this way is IMHO > extremely undesirable. And changing a default that has been in place for 30 years just seems unwise at this point. jeff
On Thu, Oct 26, 2017 at 2:16 PM, Eric Gallager <egall@gwmail.gwu.edu> wrote: > On 10/26/17, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote: >> GCC's default optimization level is -O0. Unfortunately unlike other >> compilers, >> GCC generates extremely inefficient code with -O0. It is almost unusable >> for >> low-level debugging or manual inspection of generated code. So a -O option >> is >> always required for compilation. -Og not only allows for fast compilation, >> but >> also produces code that is efficient, readable as well as debuggable. >> Therefore -Og makes for a much better default setting. >> >> Any comments? > > There are a number of bugs with -Og that I'd want to see fixed before > making it the default; I'll follow this message up once I find them > all. update: I've filed bug 82738 to track them all: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82738 https://gcc.gnu.org/ml/gcc-bugs/2017-10/msg03058.html > >> >> 2017-10-26 Wilco Dijkstra <wdijkstr@arm.com> >> >> * opts.c (default_options_optimization): Set default to -Og. >> >> doc/ >> * invoke.texi (-O0) Remove default mention. >> (-Og): Add mention of default setting. >> >> -- >> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi >> index >> 3328a3b5fafa6a98007eff52d2a26af520de9128..74c33ea35b9f320b419a3417e6007d2391536f1b >> 100644 >> --- a/gcc/doc/invoke.texi >> +++ b/gcc/doc/invoke.texi >> @@ -7343,7 +7343,7 @@ by @option{-O2} and also turns on the following >> optimization flags: >> @item -O0 >> @opindex O0 >> Reduce compilation time and make debugging produce the expected >> -results. This is the default. >> +results. >> >> @item -Os >> @opindex Os >> @@ -7371,7 +7371,7 @@ Optimize debugging experience. @option{-Og} enables >> optimizations >> that do not interfere with debugging. It should be the optimization >> level of choice for the standard edit-compile-debug cycle, offering >> a reasonable level of optimization while maintaining fast compilation >> -and a good debugging experience. >> +and a good debugging experience. This is the default. >> @end table >> >> If you use multiple @option{-O} options, with or without level numbers, >> diff --git a/gcc/opts.c b/gcc/opts.c >> index >> dfad955e220870a3250198640f3790c804b191e0..74511215309f11445685db4894be2ab6881695d3 >> 100644 >> --- a/gcc/opts.c >> +++ b/gcc/opts.c >> @@ -565,6 +565,12 @@ default_options_optimization (struct gcc_options >> *opts, >> int opt2; >> bool openacc_mode = false; >> >> + /* Set the default optimization to -Og. */ >> + opts->x_optimize_size = 0; >> + opts->x_optimize = 1; >> + opts->x_optimize_fast = 0; >> + opts->x_optimize_debug = 1; >> + >> /* Scan to see what optimization level has been specified. That will >> determine the default value of many flags. */ >> for (i = 1; i < decoded_options_count; i++) >> >>
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index 3328a3b5fafa6a98007eff52d2a26af520de9128..74c33ea35b9f320b419a3417e6007d2391536f1b 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -7343,7 +7343,7 @@ by @option{-O2} and also turns on the following optimization flags: @item -O0 @opindex O0 Reduce compilation time and make debugging produce the expected -results. This is the default. +results. @item -Os @opindex Os @@ -7371,7 +7371,7 @@ Optimize debugging experience. @option{-Og} enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation -and a good debugging experience. +and a good debugging experience. This is the default. @end table If you use multiple @option{-O} options, with or without level numbers, diff --git a/gcc/opts.c b/gcc/opts.c index dfad955e220870a3250198640f3790c804b191e0..74511215309f11445685db4894be2ab6881695d3 100644 --- a/gcc/opts.c +++ b/gcc/opts.c @@ -565,6 +565,12 @@ default_options_optimization (struct gcc_options *opts, int opt2; bool openacc_mode = false; + /* Set the default optimization to -Og. */ + opts->x_optimize_size = 0; + opts->x_optimize = 1; + opts->x_optimize_fast = 0; + opts->x_optimize_debug = 1; + /* Scan to see what optimization level has been specified. That will determine the default value of many flags. */ for (i = 1; i < decoded_options_count; i++)