On Mon, Nov 13, 2017 at 2:59 PM, Rasmus Villemoes linux@rasmusvillemoes.dk 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