clang mis-compiles "x"[2]

clang mis-compiles "x"[2]

[David Laight]

Clang mis-compiles:

int foo(void) { return "a"[2]; }

to be just a 'ret' instruction.

While clearly non-sensical returning the input value of %rax
doesn't make any sense.

gcc generates a real string and then returns the byte after
the '\0' - which is (sort of) what the code asks for.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: clang mis-compiles "x"[2]

[Nick Desaulniers]

On Mon, Aug 28, 2023 at 9:44 AM David Laight <David.Laight@aculab.com> wrote:
>
> Clang mis-compiles:
>
> int foo(void) { return "a"[2]; }
>
> to be just a 'ret' instruction.
>
> While clearly non-sensical returning the input value of %rax
> doesn't make any sense.
>
> gcc generates a real string and then returns the byte after
> the '\0' - which is (sort of) what the code asks for.

Reports of what code is generated when there is very clearly UB are
inappropriate for this list.

I encourage you to inspect the output of -fsanitize=undefined before
making such reports to compiler vendors.

https://godbolt.org/z/o4E6o33fP

Perhaps you're conflating the address of one past the end of an object
being defined, vs the value one past the end of an object which is
not.

>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
>


-- 
Thanks,
~Nick Desaulniers