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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/88345] -Os overrides -falign-functions=N on the
 command line
Date: Wed, 22 Nov 2023 13:20:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D88345

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org

--- Comment #17 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
-falign-functions/-falign-jumps/-falign-labels/-falign-loops are originally=
 are
intended for performance tuning.  Starting function entry close to the end =
of
page of code cache may lead to wasted code cache space as well as higher
overhead calling the function when CPU fetches page which contains just lit=
tle
useful information.

As such I would like to keep them affecting only hot code (we should update
documentation for that).  Internally we have FUNCTION_BOUNDARY which specif=
ies
minimal alignment needed by ABI, which is set to 8bits for i386.  My
understanding is that -fpatchable-function-entry requires the alignment to =
be
64bits in order to make it possible to atomically change the instruction.

So perhaps we want to make FUNCTION_BOUNDARY to be 64 for functions where we
output the patchable entry?
I am also OK with extending the flag syntax or adding -fmin-function-alignm=
ent
to specify optional user-defined minimum (increase FUNCTION_BOUNDARY) if th=
at
seems useful, but I think the first one is most consistent way to go with l=
ive
patching?=