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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/106804] Poor codegen for selecting and incrementing
 value behind a reference
Date: Fri, 02 Sep 2022 07:18:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106804

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

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

--- Comment #8 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #7)
> In fact I'd say the reverse transformation is more profitable?

In the end it depends on the context. It's a trade-off between a conditional
branch and extra data dependencies feeding into the address of a store. If a
branch is perfectly predictable, it's preferable. Otherwise, if there's no
memory dependency via the store, you don't care about delaying it, making t=
he
branchless version preferable if that reduces pipeline flushes. If there is=
 a
dependency, it comes down to how often the branch mispredicts, I guess.

  ________________________________
 /                                \
| People who tinker with compilers |
| need __builtin_branchless_select |
 \                                /
  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D
                                     \
                                      \
                                       \
                                        .--.
                                       |o_o |
                                       | ~  |
                                      //   \ \
                                     (|     | )
                                    /'\_   _/`\
                                    \___)=3D(___/=