svn commit: r312977 - head/sys/dev/adb

[Mark Millard]

> Author: jhibbits
> Date: Mon Jan 30 02:32:33 2017
> New Revision: 312977
> URL: 
> https://svnweb.freebsd.org/changeset/base/312977
> 
> 
> Log:
>   Force the setting of bit 7 in the sysmouse packet byte 1 to be unsigned.
>   
>   Clang complains about the shift of (1 << 7) into a int8_t changing the value:
>   
>   warning: implicit conversion from 'int' to 'int8_t' (aka 'signed char') changes
>   value from 128 to -128 [-Wconstant-conversion]
>   
>   Squash this warning by forcing clang to see it as an unsigned bit.
>   
>   This seems odd, given that it's still a conversion of 128->-128, but I'm
>   guessing the explicit unsigned attribute notifies clang that sign really doesn't
>   matter in this case.

[The following is based just on the C standard, not POSIX
or other standards that may also be involved from FreeBSD's
point of view.]

An FYI/explanation of sorts. . .

In the C11 standard (e.g., since I have it handy) having the
new type be signed has the rule for signed and unsigned integer
implicit conversions between the types:

(After the cases of value-representable-so-value-is-unchanged
and new-type-is-unsigned, quoting:)

> Otherwise, the new type is signed and the value cannot be represented
> in it; either the result is implementation-defined or an
> implementation-defined signal is raised.

So while 1U use may make the compiler(s) tested be quiet it still leaves
the code in implementation-defined territory where different starting
types for the same value are allowed to have different results. But
they are not required to and compiler releases could change the
classification --and if there are messages from the compiler or not.
Bit patterns need not be preserved for the sign-bit and/or
value-carrying bits in the new type vs. the old type.

(By contrast a new type being unsigned is defined with a mathematically
specific/unique result and so a specific bit pattern for the
value-carrying bits, ignoring trap representations and other pad bits
if they exist.)


>   Reported by:	Mark Millard <markmi AT dsl-only DOT net>
>   MFC after:	2 weeks
> 
> Modified:
>   head/sys/dev/adb/adb_mouse.c
> 
> Modified: head/sys/dev/adb/adb_mouse.c
> ==============================================================================
> --- head/sys/dev/adb/adb_mouse.c	Mon Jan 30 02:25:55 2017	(r312976)
> +++ head/sys/dev/adb/adb_mouse.c	Mon Jan 30 02:32:33 2017	(r312977)
> @@ -520,7 +520,7 @@ ams_read(struct cdev *dev, struct uio *u
>  			}
>  		}
>  
> -		sc->packet[0] = 1 << 7;
> +		sc->packet[0] = 1U << 7;
>  		sc->packet[0] |= (!(sc->buttons & 1)) << 2;
>  		sc->packet[0] |= (!(sc->buttons & 4)) << 1;
>  		sc->packet[0] |= (!(sc->buttons & 2));


===
Mark Millard
markmi at dsl-only.net