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Subject: Re: [PATCH v4 4/8] rt2800: fix registers init for MT7620
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Tom Psyborg <pozega.tomislav@gmail.com>,
        linux-wireless@vger.kernel.org,
        Daniel Golle <daniel@makrotopia.org>,
        Mathias Kresin <dev@kresin.me>
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From:   Felix Fietkau <nbd@nbd.name>
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Message-ID: <84d794ee-6889-db6c-0e23-bf4df4b16368@nbd.name>
Date:   Tue, 16 Oct 2018 13:25:45 +0200
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On 2018-10-16 13:21, Stanislaw Gruszka wrote:
> On Tue, Oct 16, 2018 at 01:19:52PM +0200, Felix Fietkau wrote:
>> > I have no idea how this could be related. But I think I found
>> > somewhat reasonable explenation where the problem is.
>> > I think below code :
>> > 
>> > 	if (a || b || c) {
>> > 		CODE1();
>> > 	} else if (c) {
>> > 		CODE2();
>> > 	}
>> > 
>> > can not be deterministic and can be compiled differently depending
>> > on compiler version and used options. Sometimes it could result
>> > in this 
>> > 
>> > 	if (a || b || c) {
>> > 		CODE1();
>> > 	}
>> > 
>> > and sometimes in this:
>> > 
>> > 	if (a || b) {
>> > 		CODE1();
>> > 	} else if (c) {
>> > 		CODE2();
>> > 	}
>> > 
>> > So that would explain the problems you see. And indeed patch
>> > could cause regression on systems where second variant of
>> > initalizing RT6352 registers was used.
>> I don't see how that can be non-deterministic at all. The 'else if' part
>> can only be hit if the first if did not match.
> 
> I meant non-deterministic during compilation process, when compiler
> do or do not some optimizations or if compiler version differs.
In my opinion, this is not C undefined behavior territory. The compiler
is not allowed to change the behavior here based on optimization settings.

- Felix