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

From: Taylor Blau <me@ttaylorr.com>
Date: Jan 11, 2022, 19:16 UTC
Message-ID: <Yd3XpLaZ3qc25PzQ@nand.local>
In-Reply-To: <patch-2.3-7a6a69314b5-20220111T163908Z-avarab@gmail.com>

On Tue, Jan 11, 2022 at 05:40:22PM +0100, Ævar Arnfjörð Bjarmason wrote:

Show 9 quoted lines

> 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...

Show 16 quoted lines

> 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?

Show 11 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.

But even though I'm less skeptical, I'm not sure that I would make the same bet (though in practice this one is probably fine since there are likely plenty of functions which end in the more standard `die()` and do not have a return path).

Can reftable call die()? Or is this the least-common denominator among Git and libgit2 for terminating a running program?

Thanks, Taylor