RE: [PATCH v4 2/5] m68k/bitops: use __builtin_{clz,ctzl,ffs} to evaluate constant expressions

[David Laight <David.Laight@ACULAB.COM>]

From: Vincent MAILHOL
> Sent: 28 January 2024 06:27
> 
> On Sun. 28 Jan. 2024 at 14:39, Finn Thain <fthain@linux-m68k.org> wrote:
> > On Sun, 28 Jan 2024, Vincent Mailhol wrote:
> >
> > > The compiler is not able to do constant folding on "asm volatile" code.
> > >
> > > Evaluate whether or not the function argument is a constant expression
> > > and if this is the case, return an equivalent builtin expression.
> > >
...
> > If the builtin has the desired behaviour, why do we reimplement it in asm?
> > Shouldn't we abandon one or the other to avoid having to prove (and
> > maintain) their equivalence?
> 
> The asm is meant to produce better results when the argument is not a
> constant expression. Below commit is a good illustration of why we
> want both the asm and the built:
> 
>   https://git.kernel.org/torvalds/c/146034fed6ee
> 
> I say "is meant", because I did not assert whether this is still true.
> Note that there are some cases in which the asm is not better anymore,
> for example, see this thread:
> 
>   https://lore.kernel.org/lkml/20221106095106.849154-2-mailhol.vincent@wanadoo.fr/
> 
> but I did not receive more answers, so I stopped trying to investigate
> the subject.
> 
> If you want, you can check the produced assembly of both the asm and
> the builtin for both clang and gcc, and if the builtin is always
> either better or equivalent, then the asm can be removed. That said, I
> am not spending more effort there after being ghosted once (c.f. above
> thread).

I don't see any example there of why the __builtin_xxx() versions
shouldn't be used all the time.
(The x86-64 asm blocks contain unrelated call instructions and objdump
wasn't passed -d to show what they were.
One even has the 'return thunk pessimisation showing.)

I actually suspect the asm versions predate the builtins.

Does (or can) the outer common header use the __builtin functions
if no asm version exists?

	David

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