Re: [PATCH v2 2/6] configure: avoid new clang 11+ warnings

[Thomas Huth]

On 23/10/2020 22.06, Daniele Buono wrote:
> Clang 11 finds a couple of spots in the code that trigger new warnings:
> 
> ../qemu-base/hw/usb/dev-uas.c:157:31: error: field 'status' with variable 
> sized type 'uas_iu' not at the end of a struct or class is a GNU extension 
> [-Werror,-Wgnu-variable-sized-type-not-at-end]
>     uas_iu                    status;
>                               ^
> 1 error generated.
> 
> The data structure is UASStatus, which must end with a QTAILQ_ENTRY, so
> I believe we cannot have uas_iu at the end. Since this is a gnu
> extension but CLANG supports it, just add
> -Wno-gnu-variable-sized-type-not-at-end
> to remove the warning.
> 
> ../qemu-base/target/s390x/cpu_models.c:985:21: error: cast to smaller integer 
> type 'S390Feat' from 'void *' [-Werror,-Wvoid-pointer-to-enum-cast]
>     S390Feat feat = (S390Feat) opaque;
>                     ^~~~~~~~~~~~~~~~~
> ../qemu-base/target/s390x/cpu_models.c:1002:21: error: cast to smaller 
> integer type 'S390Feat' from 'void *' [-Werror,-Wvoid-pointer-to-enum-cast]
>     S390Feat feat = (S390Feat) opaque;
>                     ^~~~~~~~~~~~~~~~~
> ../qemu-base/target/s390x/cpu_models.c:1036:27: error: cast to smaller 
> integer type 'S390FeatGroup' from 'void *' 
> [-Werror,-Wvoid-pointer-to-enum-cast]
>     S390FeatGroup group = (S390FeatGroup) opaque;
>                           ^~~~~~~~~~~~~~~~~~~~~~
> ../qemu-base/target/s390x/cpu_models.c:1057:27: error: cast to smaller 
> integer type 'S390FeatGroup' from 'void *' 
> [-Werror,-Wvoid-pointer-to-enum-cast]
>     S390FeatGroup group = (S390FeatGroup) opaque;
>                           ^~~~~~~~~~~~~~~~~~~~~~
> 4 errors generated.
> 
> These are void * that get casted to enums, which are (or can be)
> smaller than a 64bit pointer.
> A code reorg may be better on the long term, but for now will
> fix this adding
> -Wno-void-pointer-to-enum-cast

Compiling all code with -Wno-void-pointer-to-enum-cast sounds like the wrong
approach to me, since this might hide some real bugs in other spots instead.

Could you please try to cast the value through (uintptr_t) first, e.g. :

    S390Feat feat = (S390Feat)(uintptr_t) opaque;

It's a little bit ugly, but still better than to disable the warning
globally, I think.

 Thomas


> Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
> ---
>  configure | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/configure b/configure
> index e6754c1e87..9dc05cfb8a 100755
> --- a/configure
> +++ b/configure
> @@ -2000,6 +2000,8 @@ add_to nowarn_flags -Wno-shift-negative-value
>  add_to nowarn_flags -Wno-string-plus-int
>  add_to nowarn_flags -Wno-typedef-redefinition
>  add_to nowarn_flags -Wno-tautological-type-limit-compare
> +add_to nowarn_flags -Wno-gnu-variable-sized-type-not-at-end
> +add_to nowarn_flags -Wno-void-pointer-to-enum-cast
>  add_to nowarn_flags -Wno-psabi
>  
>  gcc_flags="$warn_flags $nowarn_flags"
>