linux-kernel - Re: [PATCH v5 1/8] x86/resctrl: Prepare for new domain scope

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Date:   Thu, 31 Aug 2023 15:02:16 +0200
From:   Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
CC:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        "Jonathan Corbet" <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        "Babu Moger" <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH v5 1/8] x86/resctrl: Prepare for new domain scope

On 2023-08-30 at 14:11:14 +0000, Luck, Tony wrote:
>> >+static int get_domain_id_from_scope(int cpu, enum resctrl_scope scope)
>> >+{
>> >+    switch (scope) {
>> >+    case RESCTRL_L3_CACHE:
>> >+            return get_cpu_cacheinfo_id(cpu, 3);
>> >+    case RESCTRL_L2_CACHE:
>> >+            return get_cpu_cacheinfo_id(cpu, 2);
>> >+    default:
>> >+            WARN_ON_ONCE(1);
>> >+            break;
>> >+    }
>> >+
>> >+    return -1;
>> >+}
>>
>> Is there some reason the "return -1" is outside of the default switch
>> case?
>>
>> Other switch statements in this patch do have returns inside the default
>> case, is this one different in some way?
>
>I've sometimes had compilers complain about code written:
>
>static int get_domain_id_from_scope(int cpu, enum resctrl_scope scope)
>{
>        switch (scope) {
>        case RESCTRL_L3_CACHE:
>                return get_cpu_cacheinfo_id(cpu, 3);
>        case RESCTRL_L2_CACHE:
>                return get_cpu_cacheinfo_id(cpu, 2);
>        default:
>                WARN_ON_ONCE(1);
>                return -1;
>        }
>}
>
>because they failed to notice that every path in the switch does a "return and they
>issue a warning that the function has no return value because they don't realize
>that the end of the function can't be reached.
>
>So it's defensive programming against possible complier issues.

I recall getting that error somewhere while playing around with a
language server protocol for neovim a while ago but I tried to cause
it today with gcc and clang and with some different flags and coulnd't.
Are there some particular compilers or compiler flags that trigger
that?

-- 
Kind regards
Maciej Wieczór-Retman