svn commit: r357349 - in head/sys: conf modules/tpm

[Ian Lepore]

On Fri, 2020-01-31 at 19:36 +0000, Dimitry Andric wrote:
> Author: dim
> Date: Fri Jan 31 19:36:14 2020
> New Revision: 357349
> URL: https://svnweb.freebsd.org/changeset/base/357349
> 
> Log:
>   Merge r357348 from the clang 10.0.0 import branch:
>   
>   Disable new clang 10.0.0 warnings about converting the result of shift
>   operations to a boolean in tpm(4):
>   
>   sys/dev/tpm/tpm_crb.c:301:32: error: converting the result of '<<' to a boolean; did you mean '(1 << (0)) != 0'? [-Werror,-Wint-in-bool-context]
>           WR4(sc, TPM_CRB_CTRL_CANCEL, !TPM_CRB_CTRL_CANCEL_CMD);
>                                         ^
>   sys/dev/tpm/tpm_crb.c:73:34: note: expanded from macro 'TPM_CRB_CTRL_CANCEL_CMD'
>   #define TPM_CRB_CTRL_CANCEL_CMD         BIT(0)
>                                           ^
>   sys/dev/tpm/tpm20.h:60:19: note: expanded from macro 'BIT'
>   #define BIT(x) (1 << (x))
>                     ^
>   
>   Such warnings can be useful in C++ contexts, but not so much in kernel
>   drivers, where this type of bit twiddling is commonplace.  So disable it
>   for this case.
>   

I think the point of the compiler warning about shift in a boolean
context is the same as warning about assignment in a boolean
context.  I.e,

   if (a << 3)

might be a typo for 

   if (a < 3)

in the same way as "a = 3" might have been intended to be "a == 3".

When this type of bit twiddling is used in drivers, it's almost always
combined with an & or | operator, which I assume the compiler then
won't complain about (or you would have seen thousands of warnings
while compiling in dev/*).

-- Ian