[Bug c++/98801] Request for a conditional move built-in function

["jeffhurchalla at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98801

--- Comment #2 from Jeff Hurchalla <jeffhurchalla at gmail dot com> ---
Relying on improved codegen, I believe we have the current problem that there
are times that a programmer knows a conditional is unpredictable, yet it would
be impossible for a compiler to know.  There's no documented way for a
programmer to inform the compiler.  There might also be cases (beyond my realm)
for security where execution time can't be allowed to vary due to a
mispredicted branch - again with no way to tell the compiler to use a
conditional move, or branchless code.  Barring a built-in function for
conditional move, I'd alternatively request a documented canonical form to use
in C that provides conditional moves when the ISA supports it (hypothetically,
the earlier ugly bit-hack example could work for this due to its clear intent,
whereas the ternary operator example could not due to unclear intent).  Without
a canonical form, could a compiler anticipate the near infinite number of ways
a programmer might express it in C, even when the intent is clear?
I'd certainly prefer a conditional move built-in function, if it's a
possibility.  It's clear and simple from a user's point of view.  The
translation to a machine instruction also seems pretty direct and limited in
scope.  Most ISAs support the instruction - ARM aarch64/A64/A32, x86_64, PPC. 
The one I know of that doesn't automatically is RISC-V, which needs the bit
manipulation extension.