Re: gcc x64 linux code generation (passing pointer var-args) bug

[Reuben Hawkins <reubenhwk@gmail.com>]

On Fri, Jan 23, 2015 at 1:40 PM, Andrew Pinski <pinskia@gcc.gnu.org> wrote:
> On Fri, Jan 23, 2015 at 1:36 PM, Reuben Hawkins <reubenhwk@gmail.com> wrote:
>> Hi Overseers,
>>
>> I ran into an issue with all versions of gcc which support x64 which
>> *could* be considered a bug.  At the very least, it's a pitfall.  I'm
>> not really sure to whom I should bring this problem to.  Bugzilla?
>> Mailing list?  Not sure...
>
>
> It is not a bug because ...
>
>>
>> Anyway, the gist of the bug is this...
>>
>> printf("%p %p %p %p %p %p\n", 0, 0, 0, 0, 0, 0);
>
> You are passing 32bit values where 64bit values are expected.  If you
> used -Wformat, it would have warned you about this issue.

printf is just an example.  I noticed it when using
newtFormAddComponents, which has no warning.

>
>>
>> The first 5 zero ints are copied into the esi, edx, ecx, r8d and r9d,
>> (as the linux x64 calling convention mandates) with the movl
>> instruction.  The movl instruction will zero out the upper 32-bits of
>> those registers.  The last zero int, however is copied to (%rsp) with
>> movl, which does *not* zero out the upper 32 bits because (%rsp) is
>> not a register, so the last 0 is not promoted to a 64-bit zero, but
>> the rest of the zeros are.  If I were to add another zero, that zero
>> would be copied to 8(%rsp), so the upper 32-bits of (%rsp) are skipped
>> and whatever garbage happens to be there is passed to the called
>> function.
>>
>> printf("%p %p %p %p %p %p\n", 0, 0, 0, 0, 0, (void*)0);
>>
>> ...works because the (void*) causes gcc to emit a movq instruction.
>>
>> I'm wondering if there's a possibility to change this unexpected
>> behavior in gcc such that it always uses movq on stack args.
>
>
> it is not unexpected because the ABI says something different from
> what you are trying to work with.
>
>>
>> I realize all the zeros are technically wrong, they should be either
>> NULL or (void*) casts, but it's a huge pain that '0' works for the
>> first 6 args, then doesn't on the 7th when the args start going on the
>> stack.
>
>
> So this is undefined in C and will never work on most other targets too.

Dang.  :(

Which is preferred, NULL or (void*)0?  Are there cases where NULL will
be simply defined as 0?

(sorry if this question take this into off-topic territory).

>
> Thanks,
> Andrew
>
>>
>> Thanks in advance,
>> Reuben Hawkins