Why are C3 allocating constructors never used?

Question

Some of us know that there are several constructors possible for C++ object, C1 and C2. But GCC sources says that there is can be third variant of constructor, the C3 "complete object allocating constructor" (check gcc-4.8/gcc/cp/mangle.c file just before write_special_name_constructor function):

http://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/cp/mangle.c;h=10c2e2beb0c422e4f56e17e7659fbeb4ab3ee31b;hb=refs/tags/gcc-4_8_1-release#l1644

1645 /* Handle constructor productions of non-terminal <special-name>.
1646    CTOR is a constructor FUNCTION_DECL.
1647 
1648      <special-name> ::= C1   # complete object constructor
1649                     ::= C2   # base object constructor
1650                     ::= C3   # complete object allocating constructor
1651 
1652    Currently, allocating constructors are never used.    <<<<<
1653 
1654    We also need to provide mangled names for the maybe-in-charge
1655    constructor, so we treat it here too.  mangle_decl_string will
1656    append *INTERNAL* to that, to make sure we never emit it.  */

Why the C3 may be needed, but not used by GCC? Is there any popular C++ compiler, which generates C3 constructors?

Is the C3 documented in any ABI pdf?

It's mentioned in the Itanium C++ ABI: http://www.swag.uwaterloo.ca/acd/docs/ItaniumC++ABI.htm When and why (or why not) it's used -- no idea :-/ — Nikos C., Jul 18 '13 at 16:23
N.B. the official Itanium ABI URL is http://www.codesourcery.com/cxx-abi/ — Jonathan Wakely, Jul 18 '13 at 23:50

score 4 · Answer 1 · edited Jul 19 '13 at 00:19

4

The idea is that C3 is an optimized version of ::operator new(sizeof(class)) followed by C1, i.e. a pre-inlined version. GCC has to create it in case another compiler uses it. That may obviously depend on inlining decisions, which are generally non-trivial.

edited Jul 19 '13 at 00:19

osgx

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answered Jul 18 '13 at 23:34

MSalters

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Actually, last time I looked gcc *doesn't* create a C3 symbol. – aschepler Jul 18 '13 at 23:44
Hmm, that would be unfunny when linking against an object file which does expect it. The idea of an ABI is that you can link binaries. – MSalters Jul 18 '13 at 23:46
Is there a way to generate also C3 ? – yonutix Sep 08 '17 at 07:12
What is "pre-inlining"? – curiousguy Jun 24 '18 at 18:20

Why are C3 allocating constructors never used?

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