[ adding Alexei back to the cc ]
On Fri, Jan 19, 2018 at 9:48 AM, Adam Sampson <ats@xxxxxxxxx> wrote:
> Jann Horn <jannh@xxxxxxxxxx> writes:
>
>>> +/*
>>> + * If idx is negative or if idx > size then bit 63 is set in the mask,
>>> + * and the value of ~(-1L) is zero. When the mask is zero, bounds check
>>> + * failed, array_ptr will return NULL.
>>> + */
>>> +#ifndef array_ptr_mask
>>> +static inline unsigned long array_ptr_mask(unsigned long idx,
>>> unsigned long sz)
>>> +{
>>> + return ~(long)(idx | (sz - 1 - idx)) >> (BITS_PER_LONG - 1);
>>> +}
>>> +#endif
>>
>> Nit: Maybe add a comment saying that this is equivalent to
>> "return ((long)idx >= 0 && idx < sz) ? ULONG_MAX : 0"?
>
> That's only true when sz < LONG_MAX, which is documented below but not
> here; it's also different from the asm version, which doesn't do the idx
> <= LONG_MAX check. So making the constraint explicit would be a good idea.
>
> From a bit of experimentation, when the top bit of sz is set, this
> expression, the C version and the assembler version all have different
> behaviour. For example, with 32-bit unsigned long:
>
> index=00000000 size=80000001: expr=ffffffff c=00000000 asm=ffffffff
> index=80000000 size=80000001: expr=00000000 c=00000000 asm=ffffffff
> index=00000000 size=a0000000: expr=ffffffff c=00000000 asm=ffffffff
> index=00000001 size=a0000000: expr=ffffffff c=00000000 asm=ffffffff
> index=fffffffe size=ffffffff: expr=00000000 c=00000000 asm=ffffffff
>
> It may be worth noting that:
>
> return 0 - ((long) (idx < sz));
>
> causes GCC, on ia32 and amd64, to generate exactly the same cmp/sbb
> sequence as in Linus's asm. Are there architectures where this form
> would allow speculation?
We're operating on the assumption that compilers will not try to
introduce branches where they don't exist in the code, so if this is
producing identical assembly I think we should go with it and drop the
x86 array_ptr_mask.
