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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97553] [missed optimization] constexprness not noticed when
 UBsan enabled
Date: Fri, 23 Oct 2020 16:34:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97553

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Whether the function is constexpr or not doesn't really matter when you
evaluate it in non-constant expression contexts.  In those the ubsan
instrumentation is bypassed (the constant expression evaluation does similar
checking), but otherwise it is a normal function like any other, which
including the instrumentation is inlined etc.  And, the runtime sanitization
intentionally isn't heavily optimized away, because the intent is to detect
when the code is invalid, so it can't e.g. optimize away those checks based=
 on
assumption that undefined behavior will not happen.
If you want a constant via C++ means, use int foo() { constexpr int x =3D
g().length(); return x; }=