Re: [PATCH 2/3] reftable: remove unreachable "return" statements

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Jan 12, 2022, 12:47 UTC
Message-ID: <220112.865yqpxge2.gmgdl@evledraar.gmail.com>
In-Reply-To: <Yd3XpLaZ3qc25PzQ@nand.local>

On Tue, Jan 11 2022, Taylor Blau wrote:

Show 45 quoted lines

> On Tue, Jan 11, 2022 at 05:40:22PM +0100, Ævar Arnfjörð Bjarmason wrote:
>> Remove unreachable return statements added in acb533440fc (reftable:
>> implement refname validation, 2021-10-07) and f14bd719349 (reftable:
>> write reftable files, 2021-10-07).
>>
>> This avoids the following warnings on SunCC 12.5 on
>> gcc211.fsffrance.org:
>>
>>     "reftable/refname.c", line 135: warning: statement not reached
>>     "reftable/refname.c", line 135: warning: statement not reached
>
> Interesting. From a cursory reading, I agree with the assessment of
> at least my compiler that these return statements are both unnecessary,
> but...
>
>> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
>> ---
>>  reftable/refname.c | 1 -
>>  reftable/writer.c  | 1 -
>>  2 files changed, 2 deletions(-)
>>
>> diff --git a/reftable/refname.c b/reftable/refname.c
>> index 95734969324..136001bc2c7 100644
>> --- a/reftable/refname.c
>> +++ b/reftable/refname.c
>> @@ -132,7 +132,6 @@ static int validate_refname(const char *name)
>>  			return REFTABLE_REFNAME_ERROR;
>>  		name = next + 1;
>>  	}
>> -	return 0;
>>  }
>
> In this case the loop inside of validate_refname() should always
> terminate the function within the loop body. But removing this return
> statement here relies on the compiler to determine that fact.
>
> I could well imagine on the other end of the spectrum there exists a
> compiler which _doesn't_ make this inference pass, and would complain
> about the opposite thing as you're reporting from SunCC (i.e., that this
> function which returns something other than void does not have a return
> statement outside of the loop).
>
> So in that sense, I disagree with the guidance of SunCC's warning. In
> other words: by quelching this warning under one compiler, are we
> introducing a new warning under a different/less advanced compiler?

I'd think that any compiler who'd warn about this sort of thing at all would be able to spot constructs like this one, which are basically:

    while (1) {
    	...
        if (x)
        	return;
	...
    }
    return; /* unreachable */

Where the elided code contains no "break", "goto" or other mechanism for exiting the for-loop.

I.e. GCC and Clang don't bother to note the unreachable code, but I don't think the reverse will be true, that a compiler will say that a "return" is missing there. Having a function be just a loop body that returns an some point is a common pattern.

Show 15 quoted lines

>>  int validate_ref_record_addition(struct reftable_table tab,
>> diff --git a/reftable/writer.c b/reftable/writer.c
>> index 35c8649c9b7..70a7bf142a2 100644
>> --- a/reftable/writer.c
>> +++ b/reftable/writer.c
>> @@ -39,7 +39,6 @@ writer_reftable_block_stats(struct reftable_writer *w, uint8_t typ)
>>  		return &w->stats.log_stats;
>>  	}
>>  	abort();
>> -	return NULL;
>>  }
>
> Here I'm less skeptical, since it's almost certain that any compiler
> would recognize this call to abort() as terminating the whole program.
> So it should be able to infer that anything after it is unreachable.

That's also correct, but in terms of compiler implementations I'd think you'd get basic loop flow analysis first, and the annotation of unreturn-able functions like abort() or a custom die() later.

> ...