linux-kernel - Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

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Message-ID: <CAHk-=wj=Zv+mMuqJQJptH9zGFhPXqku9YKyR7Vo4f0O0HEcbxw@mail.gmail.com>
Date: Sun, 28 Jul 2024 10:24:30 -0700
From: Linus Torvalds <torvalds@...uxfoundation.org>
To: David Laight <David.Laight@...lab.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Christoph Hellwig <hch@...radead.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Dan Carpenter <dan.carpenter@...aro.org>, Arnd Bergmann <arnd@...nel.org>, 
	"Jason@...c4.com" <Jason@...c4.com>, "pedro.falcato@...il.com" <pedro.falcato@...il.com>, 
	Mateusz Guzik <mjguzik@...il.com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Subject: Re: [PATCH v2 1/8] minmax: Put all the clamp() definitions together

On Sun, 28 Jul 2024 at 07:18, David Laight <David.Laight@...lab.com> wrote:
>
> +#define min_t(type, x, y)      __careful_cmp(min, (type)(x), (type)(y))
> +#define max_t(type, x, y)      __careful_cmp(max, (type)(x), (type)(y))

This is unrelated to your patch, but since it moves things around and
touches these, I reacted to it..

We should *not* use __careful_cmp() here.

Why? Because part of __careful_cmp() is the "only use arguments once".

But *another* part of __careful_cmp() is "be careful about the types"
in __cmp_once().

And being careful about the types is what causes horrendous expansion,
and is pointless when we just forced things to be the same type.

So we should split __careful_cmp() into one that does just the "do
once" and one that then also does the type checking.

But I think even if we don't do that, I wonder if we can just do this:

  #define __cmp_once(op, x, y, unique_x, unique_y) ({     \
          typeof(x) unique_x = (x);                       \
          typeof(y) unique_y = (y);                       \
          static_assert(__types_ok(x, y),                 \
          ...

and change it to

  #define __cmp_once(op, x, y, unique_x, unique_y) ({     \
          __auto_type unique_x = (x);                     \
          __auto_type unique_y = (y);                     \
          static_assert(__types_ok(unique_x, unique_y),   \
          ...

because while that may screw up the "constant integer" case (because
it now goes through that "unique_XY" variable, maybe it doesn't? At
least gcc has been known to deal with things like arguments to inline
functions well enough (ie a constant argument means that the arguments
shows as __builtin_constant_p(), and we already depend on that).

That single change would cut down on duplication of 'x' and 'y'
_enormously_. No?

(You already did the __auto_type part elsewhere)

Note that this would require the more relaxed "__is_noneg_int()" that
I suggested that allows for any expression, not just C constant
expressions)

           Linus