linux-kernel - Re: [PATCH] arm64/cache: fix -Woverride-init compiler warnings

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Message-Id: <06613F4A-3DA7-4FF9-8616-52CB4BB58C48@lca.pw>
Date:   Mon, 5 Aug 2019 23:50:03 -0400
From:   Qian Cai <cai@....pw>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/cache: fix -Woverride-init compiler warnings



> On Aug 5, 2019, at 10:01 AM, Will Deacon <will@...nel.org> wrote:
> 
> On Mon, Aug 05, 2019 at 07:47:37AM -0400, Qian Cai wrote:
>> 
>> 
>>> On Aug 5, 2019, at 5:52 AM, Will Deacon <will@...nel.org> wrote:
>>> 
>>> On Fri, Aug 02, 2019 at 11:32:24AM -0400, Qian Cai wrote:
>>>> The commit 155433cb365e ("arm64: cache: Remove support for ASID-tagged
>>>> VIVT I-caches") introduced some compiation warnings from GCC,
>>>> 
>>>> arch/arm64/kernel/cpuinfo.c:38:26: warning: initialized field
>>>> overwritten [-Woverride-init]
>>>> [ICACHE_POLICY_VIPT]  = "VIPT",
>>>>                         ^~~~~~
>>>> arch/arm64/kernel/cpuinfo.c:38:26: note: (near initialization for
>>>> 'icache_policy_str[2]')
>>>> arch/arm64/kernel/cpuinfo.c:39:26: warning: initialized field
>>>> overwritten [-Woverride-init]
>>>> [ICACHE_POLICY_PIPT]  = "PIPT",
>>>>                         ^~~~~~
>>>> arch/arm64/kernel/cpuinfo.c:39:26: note: (near initialization for
>>>> 'icache_policy_str[3]')
>>>> arch/arm64/kernel/cpuinfo.c:40:27: warning: initialized field
>>>> overwritten [-Woverride-init]
>>>> [ICACHE_POLICY_VPIPT]  = "VPIPT",
>>>>                          ^~~~~~~
>>>> arch/arm64/kernel/cpuinfo.c:40:27: note: (near initialization for
>>>> 'icache_policy_str[0]')
>>>> 
>>>> because it initializes icache_policy_str[0 ... 3] twice.
>>>> 
>>>> Fixes: 155433cb365e ("arm64: cache: Remove support for ASID-tagged VIVT I-caches")
>>>> Signed-off-by: Qian Cai <cai@....pw>
>>>> ---
>>>> arch/arm64/kernel/cpuinfo.c | 4 ++--
>>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>> 
>>>> diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
>>>> index 876055e37352..193b38da8d96 100644
>>>> --- a/arch/arm64/kernel/cpuinfo.c
>>>> +++ b/arch/arm64/kernel/cpuinfo.c
>>>> @@ -34,10 +34,10 @@
>>>> static struct cpuinfo_arm64 boot_cpu_data;
>>>> 
>>>> static char *icache_policy_str[] = {
>>>> -	[0 ... ICACHE_POLICY_PIPT]	= "RESERVED/UNKNOWN",
>>>> +	[ICACHE_POLICY_VPIPT]		= "VPIPT",
>>>> +	[ICACHE_POLICY_VPIPT + 1]	= "RESERVED/UNKNOWN",
>>>> 	[ICACHE_POLICY_VIPT]		= "VIPT",
>>>> 	[ICACHE_POLICY_PIPT]		= "PIPT",
>>>> -	[ICACHE_POLICY_VPIPT]		= "VPIPT",
>>> 
>>> I really don't like this patch. Using "[0 ... MAXIDX] = <default>" is a
>>> useful idiom and I think the code is more error-prone the way you have
>>> restructured it.
>>> 
>>> Why are you passing -Woverride-init to the compiler anyway? There's only
>>> one Makefile that references that option, and it's specific to a pinctrl
>>> driver.
>> 
>> Those extra warnings can be enabled by “make W=1”. “-Woverride-init “ seems to be useful
>> to catch potential developer mistakes with unintented double-initializations. It is normal to
>> start to fix the most of false-positives first before globally enabling the flag by default just like
>> “-Wimplicit-fallthrough” mentioned in,
>> 
>> https://lwn.net/Articles/794944/
> 
> I think this case is completely different to the implicit fallthrough stuff.
> The solution there was simply to add a comment without restructuring the
> surrounding code. What your patch does here is actively make the code harder
> to understand.
> 
> Initialising a static array with a non-zero pattern is a useful idiom and I
> don't think we should throw that away just to appease a silly compiler
> warning that appears only with non-default build options. Have a look at
> the way we use PERF_MAP_ALL_UNSUPPORTED in the Arm PMU code, for example.

Well, both GCC and Clang would generate warnings for those. Clang even enable this by
default,

https://releases.llvm.org/8.0.0/tools/clang/docs/DiagnosticsReference.html#winitializer-overrides

Assume compiler people are sane, I probably not call those are “silly”.