Horton

Re: shadow variables - pg15 edition

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, pgsql-hackers@postgresql.org, Tomas Vondra <tomas.vondra@postgresql.org>, Peter Smith <smithpb2250@gmail.com>
Date: 2022-10-06T07:32:36Z
Lists: pgsql-hackers 
On 2022-Oct-06, David Rowley wrote:

> On Thu, 6 Oct 2022 at 02:34, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> > A simpler idea might be to just remove the inner declaration, and have
> > that block set the outer var.  There's no damage, since the block is
> > going to end and not access the previous value anymore.
> >
> > diff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.c
> > index aa1a3541fe..91a067859b 100644
> > --- a/src/bin/pgbench/pgbench.c
> > +++ b/src/bin/pgbench/pgbench.c
> > @@ -7506,7 +7506,7 @@ threadRun(void *arg)
> >                 /* progress report is made by thread 0 for all threads */
> >                 if (progress && thread->tid == 0)
> >                 {
> > -                       pg_time_usec_t now = pg_time_now();
> > +                       now = pg_time_now();    /* not lazy; clobbers outer value */
> 
> I didn't want to do it that way because all this code is in a while
> loop and the outer "now" will be reused after it's set by the code
> above.  It's not really immediately obvious to me what repercussions
> that would have, but it didn't seem worth taking any risks.

No, it's re-initialized to zero every time through the loop, so setting
it to something else at the bottom doesn't have any further effect.

If it were *not* reinitialized every time through the loop, then what
would happen is that every iteration in the loop (and each operation
within) would see exactly the same value of "now", because it's only set
"lazily" (meaning, if already set, don't change it.)

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/

Commits

  1. Fix shadow variable in postgres.c

  2. Add -Wshadow=compatible-local to the standard compilation flags

  3. Fix final warnings produced by -Wshadow=compatible-local

  4. Fix final compiler warning produced by -Wshadow=compatible-local

  5. Add optional parameter to PG_TRY() macros

  6. Small refactor to get rid of -Wshadow=compatible-local warning

  7. More -Wshadow=compatible-local warning fixes

  8. Further -Wshadow=compatible-local warning fixes

  9. Further reduce warnings with -Wshadow=compatible-local

  10. Reduce warnings with -Wshadow=compatible-local builds

  11. Remove shadowed local variables that are new in v15