linux-kernel - Re: [PATCH] drm/amdgpu: Add braces to initialize task

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Message-ID: <CAKwvOdkSy74TKqASdMtWwHKOeAVQn+mEfv0Fi8q+-35dGjXtHA@mail.gmail.com>
Date:   Wed, 12 Sep 2018 16:13:38 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Richard Smith <richardsmith@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        alexander.deucher@....com, christian.koenig@....com,
        David1.Zhou@....com, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/amdgpu: Add braces to initialize task_info subojects

On Wed, Sep 12, 2018 at 1:24 PM Richard Smith <richardsmith@...gle.com> wrote:
>
> On Wed, Sep 12, 2018 at 10:38 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>>
>> On Tue, Sep 11, 2018 at 5:26 PM Nathan Chancellor
>> <natechancellor@...il.com> wrote:
>> >
>> > Clang warns if there are missing braces around a subobject
>> > initializer.
>> >
>> > drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c:1447:41: warning: suggest braces
>> > around initialization of subobject [-Wmissing-braces]
>> >                 struct amdgpu_task_info task_info = { 0 };
>> >                                                       ^
>> >                                                       {}
>> > 1 warning generated.
>> >
>> > drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c:262:41: warning: suggest braces
>> > around initialization of subobject [-Wmissing-braces]
>> >                 struct amdgpu_task_info task_info = { 0 };
>> >                                                       ^
>> >                                                       {}
>> > 1 warning generated.
>> >
>> > Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
>> > ---
>> >  drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c | 2 +-
>> >  drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c | 2 +-
>> >  2 files changed, 2 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c b/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
>> > index 9333109b210d..968cc1b8cdff 100644
>> > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
>> > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
>> > @@ -1444,7 +1444,7 @@ static int gmc_v8_0_process_interrupt(struct amdgpu_device *adev,
>> >                 gmc_v8_0_set_fault_enable_default(adev, false);
>> >
>> >         if (printk_ratelimit()) {
>> > -               struct amdgpu_task_info task_info = { 0 };
>> > +               struct amdgpu_task_info task_info = { { 0 } };
>>
>> Hi Nathan,
>> Thanks for this patch.  I discussed this syntax with our language
>> lawyers.  Turns out, this is not quite correct, as you're now saying
>> "initialize the first subobject to zero, but not the rest of the
>> object."  -Wmissing-field-initializers would highlight this, but it's
>> not part of -Wall.  It would be more correct to zero initialize the
>> full struct, including all of its subobjects with `= {};`.
>
>
> Sorry, I think I've caused some confusion here.
>
> Elements with an omitted initializer get implicitly zero-initialized. In C++, it's idiomatic to write `= {}` to perform aggregate zero-initialization, but in C, that's invalid because at least one initializer is syntactically required within the braces. As a result, `= {0}` is an idiomatic way to perform zero-initialization of an aggregate in C.

That doesn't seem to be the case:
https://godbolt.org/z/TZzfo6 shouldn't Clang warn in the case of bar()?

> Clang intends to suppress the -Wmissing-braces in that case; if the warning is still being produced in a recent version of Clang, that's a bug. However, the warning suppression was added between Clang 5 and Clang 6, so it's very plausible that the compiler being used here is simply older than the warning fix.
>
> (Long story short: the change here seems fine, but should be unnecessary as of Clang 6.)

The warning was identified from clang-8 ToT synced yesterday.

-- 
Thanks,
~Nick Desaulniers