Re: [PATCH] Change default optimization level to -Og

[Eric Gallager <egall at gwmail dot gwu dot edu>]

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++)
>>
>>