[cxx-abi-dev] Mangling of string literals versus variadic templates

[Richard Smith]

On Tue, Dec 17, 2013 at 5:47 PM, David Vandevoorde <daveed at edg.com> wrote:

>
> On Dec 17, 2013, at 5:22 PM, John McCall <rjmccall at apple.com> wrote:
>
> On Dec 17, 2013, at 12:01 PM, Sean Hunt <scshunt at csclub.uwaterloo.ca>
> wrote:
>
> On Tue, Dec 17, 2013 at 2:57 PM, John McCall <rjmccall at apple.com> wrote:
>
>> On Dec 17, 2013, at 11:12 AM, David Vandevoorde <daveed at edg.com> wrote:
>> > On Dec 16, 2013, at 8:33 PM, John McCall <rjmccall at apple.com> wrote:
>> >> On Dec 16, 2013, at 5:10 PM, Richard Smith <richardsmith at google.com>
>> wrote:
>> >>> Consider:
>> >>
>> >> Remind me why it’s impossible to go back to the committee and
>> repeatedly weaken any remaining guarantees about string literal addresses
>> until none of this is important?
>> >
>> > I don't know if it's impossible or not, but I suspect it would be
>> controversial.  (I, at least, would be opposed.)
>>
>> Really?  You feel that having really strong guarantees about the address
>> of a string literal is the right thing to do?  Like, it’s worth
>> significantly increasing build times, code size, and launch times over?
>>
>> John.
>
>
> I don't see a situation where baz() below returns false really being
> defensible:
>
>
> Why?  Who cares?  Why is “don’t rely on string literal addresses being
> consistent” actually an unreasonable piece of advice?  Because I’m pretty
> sure that’s the advice that everybody’s been rolling with for over thirty
> years now.
>
>
>
> We put in the C++ rules about 20 years ago because people wanted to have
>
> inline char const* name() { return "SomeName"; }
>
> return the name pointer value in every TU (I think that's reasonable).
> There is certainly still plenty of code relying on that.
>

I find that a little surprising, since GCC does not guarantee this, and it
is the system compiler on a lot of platforms. (Likewise, Clang does not,
and is the system compiler on some platforms). There's a GCC bug on this
with only two duplicates, and no Clang bug has been filed, so I don't get
the impression that this is a big deal for a lot of people on Linux or Mac.

On the other hand, MSVC guarantees something much stronger: it appears to
give *all* string literals a mangled name comprising their type, the first
few (32?) characters of the string, and a checksum of the rest.

It seems like there probably can't be much *portable* code relying on this,
and non-portable code can rely on compiler-provided guarantees all it likes.


> Daveed
>
>
>
> I mean, in practice this is going to work just fine in simple cases,
> because implementations do do some uniquing of string literals, within
> limits.  But to really make a *guarantee* here, you’re talking about taking
> a ubiquitously-used language feature and layering a ton of hidden and
> difficult-to-avoid costs onto it, and for what?  To make the semantics
> slightly prettier in a way that doesn’t really help anybody?
>
> It is extremely difficult to prove that a program does not rely on the
> address of a string literal, because the dominant use of string literals is
> to pass them off to a function, usually one that’s implemented in an
> external library.
>
> John.
>
>
>
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