Re: bit tweaks [was: Re: [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11]

[Linus Torvalds]

On Mon, Nov 13, 2017 at 2:59 PM, Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> wrote:
>> Sadly, gcc makes a mess of it and actually generates code that looks
>> like the original C.[...]
>
> Actually, new enough gcc (7.1, I think) does contain a pattern that does
> this, but unfortunately only if one spells it
>
>   y |= (x & BIT) ? OTHER_BIT : 0;

Ahh, I should have recognized that, I think that's what we ended up
doing with the VM_READ -> PROT_READ translation in a few places,
exactly because gcc would then recognize it and do the much better
code generation.

> which is half-way to doing it by hand, I suppose.

Yeah, but it is at least acceptable, and the code is still legible C.
The alternatives of doing it _entirely_ by hand tend to be much worse
(ie you end up using a macro from hell that checks which of the two
bits are bigger and shifting in the right direction by using
multiplication or division).

So let's just rewrite that mnt_flags conversion that way, justr to get
gcc to generate the obvious code.

It's a bit sad how gcc didn't pick up on the original code, especially
since it had already done the much more complicated translation of
doing the if-conversion.

Thanks for pointing out the gcc pattern.

          Linus