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Re: -O2 inliner retuning 3/n: make inline hints more systematic
On Fri, 4 Oct 2019, Jan Hubicka wrote:
I don't really understand the purpose of having 2 params where one is used
for -O2 and the other for -O3 (I didn't check -Os), instead of a single
param whose default value depends on -On (what we had before?). Is it so
that we can more easily mix some functions compiled at -O3 with other
functions compiled at -O2 and thus using a different param?
The point is that the auto-inlining we want to do at -O3 is too much for
what we want to do at -O2. For C++ codebases it is really important to
auto-inline and thus I have enabled it by default for -O2+ while previously
we auto-inlined only with -O3 unless the resulting code was expected to
shrink. Making all -O2 code suddenly 30-50% bigger is not a good idea.
So for inliner (and some other optimizations) we really want to have
conservative and agressive tuning options.
I completely agree with all that, I was just wondering about the technical
way you achieve it. You end up writing essentially:
val = (opt >= 3) ? param : param_for_O2;
and I was wondering why not just
val = param;
but making sure that the default value of param is different for O2
(conservative) and O3 (aggressive). Basically moving the conditional one
step earlier, so the param has a single name for users. But that's not
very important, feel free to ignore.
By the way, did you mean to close the parenthesis so late
if (opt_for_fn (n->decl, optimize >= 3))
or earlier
if (opt_for_fn (n->decl, optimize) >= 3)
? It doesn't matter, just looks surprising.
--
Marc Glisse