The docs say...
@item -Os
@opindex Os
Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.
@option{-Os} disables the following optimization flags:
@gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
-falign-labels -freorder-blocks -freorder-blocks-and-partition @gol
-fprefetch-loop-arrays -ftree-vect-loop-version}
But in reality, the only thing -Os does beyond -O2, aside from a few
niche special cases, is enable inlining, and maybe scheduling, which
for some cases may be the wrong thing to do.
Is this what we want?
flag_schedule_insns = opt2 && ! optimize_size;
if (optimize_size)
{
/* Inlining of functions reducing size is a good idea regardless of them
being declared inline. */
flag_inline_functions = 1;
/* Basic optimization options. */
optimize_size = 1;
if (optimize > 2)
optimize = 2;
/* We want to crossjump as much as possible. */
set_param_value ("min-crossjump-insns", 1);
}
else
set_param_value ("min-crossjump-insns", initial_min_crossjump_insns);
$ grep optimize_size *.c
genconditions.c: { "! optimize_size && ! TARGET_READ_MODIFY_WRITE",
genconditions.c: __builtin_constant_p (! optimize_size && !
TARGET_READ_MODIFY_WRITE)
genconditions.c: ? (int) (! optimize_size && ! TARGET_READ_MODIFY_WRITE)
opts.c: optimize_size = 0;
opts.c: optimize_size = 0;
opts.c: optimize_size = 1;
opts.c: optimize_size = 0;
opts.c: flag_schedule_insns = opt2 && ! optimize_size;
opts.c: if (optimize_size)
opts.c: optimize_size = 1;
opts.c: OPTIMIZATION_OPTIONS (optimize, optimize_size);
predict.c: if (optimize_size)
predict.c: return (optimize_size
toplev.c: The only valid values are zero and nonzero. When optimize_size is
toplev.c:int optimize_size = 0;
toplev.c: if (flag_prefetch_loop_arrays > 0 && optimize_size)
tree-inline.c: if (size < 0 || size > MOVE_MAX_PIECES * MOVE_RATIO
(!optimize_size))
tree-inline.c: || (caller_opt->optimize_size != callee_opt->optimize_size))
