Re: [Tinycc-devel] Call for testing

[Thomas Preud'homme]

Le jeudi 17 janvier 2013 01:48:01, Michael Matz a écrit :
> Hi,
> 
> 
> Pointer to integer conversions (and back) are implementation defined, the
> only requirement being that if the integer type is (u)intptr_t that a
> pointer converted to that one and back to a pointer shall compare equal to
> the original pointer.
> 
> So, depending on how the compiler implements this conversion (and
> 
> documents this) one might get different results:
> > $ gcc -o long_long long_long.c && ./long_long
> > [SNIP gcc warnings]
> > -66 -66 -66 -66 -66 -66
> 
> This is a correct result for ILP32 (i.e. 32bit code).  GCC sign extends
> pointers when the pointer representation needs fewer bits than the target
> type.
> 
> > $ clang -o long_long long_long.c && ./long_long
> > -66 -66 -66 -66 4294967230 4294967230
> 
> And this is also a correct result for 32bit code, when the compiler
> defines pointer to integer conversion to be zero-extending when the target
> type has more bits than a pointer.  IMO sign-extending is more useful, and
> as GCC set the precedent a long time ago I would declare clang to at least
> have an QoI issue with compatibility with GCC.  In other words: tcc has no
> bug here.

Thanks a lot for your explaination Michael. There is also the p -= 
0x700000000042; assignment which is compiler-specific for C11 (not sure for 
C99) I think because nothing in the standard say there should be overflow. The 
compiler can do crazy things with it, as explained in 
http://lwn.net/Articles/511259/.

Ok so no bug on tcc here, good :)

> 
> 
> Ciao,
> Michael.

Best regards,

Thomas

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