On Thu, Oct 26, 2017 at 10:12 AM, Wilco Dijkstra <wilco.dijks...@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 <wdijk...@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++)
>
