From: Willy Tarreau

Sent: 06 December 2024 19:39
On Fri, Dec 06, 2024 at 11:15:20AM -0800, Linus Torvalds wrote:

quoted

On Fri, 6 Dec 2024 at 11:07, David Laight [off-list ref] wrote:

quoted

I'm missing the compiler version and options to generate the error.

Just -Wall with most recent gcc versions seems to do it. At least I
can repro it with gcc-14.2.1 and something silly like this:

  $ cat t.c
  int fn(int a) { return (a<<2)?1:2; }
  $ gcc -Wall -S t.c
  t.c: In function 'fn':
  t.c:1:26: warning: '<<' in boolean context, did you mean '<'?
[-Wint-in-bool-context]

quoted

Does a '+ 0' help?  "(var << 2) + 0 ? 0 : 0"

Yeah, that actually works.

And "+0" is nice in that it should work in any context.

I've already used "+0" to shut certain warnings, I don't really remember
which one, but also remember it was OK everywhere I needed.

I've often used +0u when -Wsign-compare is enabled.
Much safer than a cast.

Another trick I've been using to shut up the compiler is a cast via typeof
and an intermediary variable:

  #define shut_up(expr)                           \
          ({                                      \
                  typeof(expr) _expr_ = expr;     \
                  _expr_;                         \
                  })

That is like OPTIMISER_HIDE_VAR() and can't be used in a 'constant integer expression'.

I suspect it also has the same nasty habit of adding an extra register move.

	David

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