At some point, the C++ standard changed to cause volatile nonstatic data
members to make a generated copy/move constructor nontrivial.
Unfortunately, that would change the parameter passing mechanism if we stuck to
letter of the ABI; see 3.1.1/1:
1. In the special case where the parameter type has a non-trivial copy
constructor or destructor, the caller must allocate space
for a temporary copy, and pass the resulting copy by reference (below).
Specifically, ...
AFAICT, recent versions of GCC and Clang do implement the language aspects of
nontriviality of copy/move constructors in such cases (e.g., causing union
constructors to become deleted), but not this ABI aspect of it. For example:
typedef struct { int value; } TypeA;
typedef struct { TypeA volatile value; } TypeB;
typedef struct { TypeA value; } TypeC;
int foo(TypeB p) { return p.value.value; }
int foo(TypeC p) { return p.value.value; }
Identical code is being generated for these two definitions of foo, even though
TypeB has a nontrivial copy constructor and TypeC has a trivial copy
constructor.
If that is right, should the 3.1.1/1 words above be edited to read:
1. In the special case where the parameter type has a non-trivial copy
constructor (with the exception of a generated copy constructor that is
nontrivial only because one or more nonstatic data member are trivial) or
destructor, the caller must allocate space for a temporary copy,
and pass the resulting copy by reference (below). Specifically, ...
?
Daveed
P.S.: I sent this e-mail from a different account a few days ago. Sorry if
anyone sees it twice.
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