On 2018-07-23 20:24:11 +0700, Robert Elz wrote:
> Date: Mon, 23 Jul 2018 15:13:21 +0200
> From: Vincent Lefevre <vincent-o...@vinc17.net>
> Message-ID: <20180723131321.gb12...@zira.vinc17.org>
>
> | No, this is not impossible. The result of the test is 0.
>
> Yes, I know, what I meant was that it is impossible for an
> unsigned value to ever be < 0 - which is what that test is
> based upon.
No, this is just a test. The expression is a valid C expression
with a well-defined result, which may be 0 or 1.
> But because of that, if one writes code like
>
> unsigned x,y,z;
>
> z = x - y;
> if (z < 0) { /* whatever */ }
>
> the code is broken,
This is valid C code. The "if (z < 0) { /* whatever */ }" is just
dead code here. But this is not incorrect. Whether this is what
the user intended to write is another matter.
> and because of that, compilers warn about it
GCC will warn just with -Wtype-limits, which isn't even in -Wall,
thus not in general because such always-true or always-false tests
are common with portability code:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12963
But even if one gets a warning, this doesn't make the code wrong.
Warnings are there just to signal a potential issue, sometimes
with many false positives, like with this kind of tests.
And my code was different: (T) -1 < 0
Here, (T) -1 is a constant expression. Thus GCC shouldn't warn on it,
even with -Wtype-limits, as documented:
-Wtype-limits
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
^^^^^^^^^^^^^^^^^^^^^^^^
expressions. For example, warn if an unsigned variable is compared
^^^^^^^^^^^
against zero with "<" or ">=". This warning is also enabled by
-Wextra.
I get a warning, but this is a bug in the compiler. The goal concerning
constant expressions is to avoid even more false positives.
> (just like, in this meaningless example, they would also warn about
> x and y being used without being set...)
This other warning is useful because this is undefined behavior.
--
Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
