Re: [Tinycc-devel] Call for testing

[Michael Matz]

Hi,

On Wed, 16 Jan 2013, Thomas Preud'homme wrote:

> $ cat long_long.c
> #include <stdio.h>
>
> int main(void)
> {
>        char *p = NULL;
>
>        p -= 0x700000000042;
>
>        /* from pointer to integer types */
>        printf("%d %d %ld %ld %lld %lld\n",
>               (int)p, (unsigned int)p,
>               (long)p, (unsigned long)p,
>               (long long)p, (unsigned long long)p);
>        return 0;
> }

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.


Ciao,
Michael.