On 08/26/2013 11:21 AM, Jan Hubicka wrote:
Our default behaviour special case inline functions that are always
AVAIL_AVAILABLE and, via decl_replaceable_p, also any COMDAT (that may be for
functions since all COMDATs are also inlines, but makes difference for
variables I think).
Not all COMDAT functions are inlines; template instantiations are also
COMDAT.
My understanding of C++ One Definition Rule, in a strict sense, does not a
allow in to define two functions of the same name and different semantics in a
valid program.
Strictly speaking, the same is true of C:
"If an identifier declared with external linkage is used in an
expression (other than as part of the operand of a sizeof or _Alignof
operator whose result is an integer constant), somewhere in the entire
program there shall be exactly one external definition for the
identifier; otherwise, there shall be no more than one."
I also think that all DSOs eventually linked together or
dlopenned are part of the same program. So theoretically, for C++ produced
code, we may go with AVAIL_AVAILABLE everywhere.
And for C code as well.
ELF symbol replacement is an extension to C/C++ semantics, which does
not include DSOs; it seems to me that it is up to us to define what
assumptions we want the compiler to be able to make.
On IRC we got into an agreement that we may disallow interposition for
virtuals, since replacing these seems fishy - they don't even have address well
defined and compiler can freely duplicate and/or unify them. I think same
apply for C++ constructors and destructors now.
But it would be simple to create a DSO which replaces a virtual function
or a constructor, by simply providing a new definition. Since
interposition happens outside the language, I don't think reasoning
based on things that happen within the language makes much sense.
Of course I would be happier with a stronger rule - for instance allowing
interposition only on plain functions not on methods.
Existing uses of interposition apply to plain functions, but that
doesn't mean someone might not want to use it for member functions as well.
I would be happy with an even stronger default that optimizes on the
assumption that no interposition occurs; typically interposition is
overriding a symbol found in a dynamic library (i.e. malloc) rather than
a symbol defined in the same translation unit as the use.
Jason
