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Re: [cxx-abi-dev] Re: C++ ABI: Reuse of tail padding


Mark Mitchell wrote:


--On Wednesday, August 14, 2002 10:15:07 PM +0100 Nathan Sidwell <nathan@codesourcery.com> wrote:

Mark Mitchell wrote:

We have a problem here.

Intel's compiler does not pack things into the tail padding.
Neither, I expect, does HP's.
I would like to get hard data from more than the two compilers
you've verified.
Looks like the HP compiler does make use of the tail padding.
For example it reports "sizeof(S2)" to be 12. I think c2
is placed in the "tail padding".

So this would favor g++'s implementation.

Kerch


There aren't many more to find.  HP, Intel, and GCC are the compilers
that have implemented the ABI to date.

Others are in the process, which is part of why this is important.

I think I would prefer to change G++, and drop this idea from the spec,
even though it is an optimization.

Although using tail padding is neat, how many bytes would it
save in real programs? You only get it with structs of the form
    int i;
    char c;
or (on some non-i86 machines)
    double d;
    int i;

Exactly.

Well, that sounds like two votes for changing G++ (and clarifying the
spec to drop any hint of using tail padding).




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