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

From: Taylor Blau <me@ttaylorr.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: Taylor Blau <me@ttaylorr.com>,
	git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Han-Wen Nienhuys <hanwen@google.com>
Subject: Re: [PATCH 2/3] reftable: remove unreachable "return" statements
Date: Wed, 12 Jan 2022 14:19:40 -0500	[thread overview]
Message-ID: <Yd8pzOh1HLvDWSPg@nand.local> (raw)
In-Reply-To: <220112.865yqpxge2.gmgdl@evledraar.gmail.com>

On Wed, Jan 12, 2022 at 01:47:40PM +0100, Ævar Arnfjörð Bjarmason wrote:
> >> 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.

Right, but I'm more concerned about a less advanced compiler that would
complain about the absence of a `return` statement as the last
instruction in a non-void function.

This is probably all academic, anyway, since less advanced compilers
probably have other issues compiling Git as it stands today, but
fundamentally I think that SunCC's warnings here are at the very least
inconsiderate of less advanced compilers.

To me, the safest thing to do would be to leave the code as-is and drop
this patch.

Thanks,
Taylor