linux-kernel - Re: [PATCH next v4 0/5] minmax: Relax type checks in min() and max().

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Message-ID: <CAHk-=wjvM5KiQFpbPMPXH-DcvheNcPGj+ThNEJVm+QL6n05A8A@mail.gmail.com>
Date: Mon, 8 Jan 2024 10:19:07 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Jiri Slaby <jirislaby@...il.com>
Cc: David Laight <David.Laight@...lab.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Christoph Hellwig <hch@...radead.org>, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH next v4 0/5] minmax: Relax type checks in min() and max().

On Mon, 8 Jan 2024 at 03:46, Jiri Slaby <jirislaby@...il.com> wrote:
>
>    CPP [M] drivers/media/pci/solo6x10/solo6x10-p2m.i
> real    0m45,002s
>
> $ git revert 867046cc7027703f60a46339ffde91a1970f2901
>    CPP [M] drivers/media/pci/solo6x10/solo6x10-p2m.i
> real    0m11,132s
>
> $ git revert 4ead534fba42fc4fd41163297528d2aa731cd121
>    CPP [M] drivers/media/pci/solo6x10/solo6x10-p2m.i
> real    0m3,711s

Ouch. Yeah, that's unfortunate. There's a lot of nested nasty macro
expansion there, but that timing is excessive.

Sparse actually complains about that file:

  drivers/media/pci/solo6x10/solo6x10-p2m.c:309:13: error: too long
token expansion
  drivers/media/pci/solo6x10/solo6x10-p2m.c:310:17: error: too long
token expansion

and while that is a sparse limitation, it's still interesting. Having
that file expand to 122M is not ok.

In this case, I suspect the right thing to do is to simply not use
min()/max() in that header at all, but do something like

  --- a/drivers/media/pci/solo6x10/solo6x10-offsets.h
  +++ b/drivers/media/pci/solo6x10/solo6x10-offsets.h
  @@ -56,2 +56,5 @@

  +#define MIN(X, Y) ((X) < (Y) ? (X) : (Y))
  +#define MAX(X, Y) ((X) > (Y) ? (X) : (Y))
  +
   #define SOLO_MP4E_EXT_ADDR(__solo) \
  @@ -59,4 +62,4 @@
   #define SOLO_MP4E_EXT_SIZE(__solo) \
  -     max((..),                               \
  -         min(((..) - \
  +     MAX((..),                               \
  +         MIN(((..) - \
                 ..), 0x00ff0000))
  @@ -67,4 +70,4 @@
   #define SOLO_JPEG_EXT_SIZE(__solo) \
  -     max(..,                         \
  -         min(..)
  +     MAX(..,                         \
  +         MIN(..)

and avoid this issue.

That said, I'm sure this thing exists to a smaller degree elsewhere. I
wonder if we could simplify our min/max type tests..

             Linus