[cfe-dev] [llvm-dev] [RFC] New Feature Proposal: De-Optimizing Cold Functions using PGO Info

[Arthur O'Dwyer via cfe-dev]

On Wed, Sep 9, 2020 at 3:10 PM Renato Golin via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Wed, 9 Sep 2020 at 19:26, Nemanja Ivanovic <nemanja.i.ibm at gmail.com>
> wrote:
>
>> - Default (i.e. no -profile-deopt-cold): do nothing
>> - Option with no arg (i.e. -profile-deopt-cold): add attribute only to
>> functions that have an execution count of zero
>> - Option with an arg (i.e. -profile-deopt-cold=<N>): add attribute to
>> functions that account for <N>% of total execution counts
>>
>
> I see. This looks confusing to me, but perhaps it's just me.
>

It's not just you. :)  Assuming "account for <N>% of total execution
counts" means "account for <N>% *or less* of total execution counts," then
it seems like the proposed -profile-deopt-cold does the same thing as
-profile-deopt-cold=0.

Also, for build-system-friendliness, IMHO every positive option should have
a negative option — i.e., the default behavior should be regainable via an
option such as -profile-no-deopt-cold.  (Or -fno-profile-deopt-cold, if
there was a missing `f` in all of the above.)  That seems easier to do if
the whole thing is controlled by just one option instead of two.

my $.02,
–Arthur
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