On Wed, Oct 07, 2020 at 06:03:16PM -0400, Tom Lane wrote:
> I wrote:
> > I'm tempted to propose the attached small code rearrangement, which
> > might dissuade the compiler from thinking it can get away with this.
That patch does get hornet's -O2 build to again pass "make check". It doesn't
harm the code, so we may as well use it.
> > While I concur with your point that an old xlc version might not be
> > that exciting, there could be other compilers doing the same thing
> > in the future.
>
> After thinking about it a bit more, I'm not even convinced that what
> xlc seems to be doing is illegal per C spec. There are no sequence
> points within
>
> return list_make2(list_concat(directargs, orderedargs),
> makeInteger(ndirectargs));
There is, however, a sequence point between list_length(directargs) and
list_concat(), and the problem arises because xlc reorders those two. It's
true that makeInteger() could run before or after list_concat(), but that
alone would not have been a problem.