Re: A note about recent ecpg buildfarm failures

On Tue, Feb 26, 2019 at 1:25 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Since my commits 9e138a401 et al on Saturday, buildfarm members
> blobfish, brotula, and wunderpus have been showing core dumps
> in the ecpg preprocessor. This seemed inexplicable given what
> the commits changed, and even more so seeing that only HEAD is
> failing, while the change was back-patched into all branches.
>
> Mark Wong and I poked into this off-list, and what we find is that
> this seems to be a compiler bug. Those animals are all running
> nearly the same version of clang (3.8.x / ppc64le). Looking into
> the assembly code for preproc.y, the crash is occurring at a branch
> that is supposed to jump forward exactly 32768 bytes, but according
> to gdb's disassembler it's jumping backwards exactly -32768 bytes,
> into invalid memory. It will come as no surprise to hear that the
> branch displacement field in PPC conditional branches is 16 bits
> wide, so that positive 32768 doesn't fit but negative 32768 does.
> Evidently what is happening is that either the compiler or the
> assembler is failing to detect the edge-case field overflow and
> switch to different coding. So the apparent dependency on 9e138a401
> is because that happened to insert exactly the right number of
> instructions in-between to trigger this scenario. It's pure luck we
> didn't trip over it before, although none of those buildfarm animals
> have been around for all that long.
>
> Moral: don't use clang 3.8.x on ppc64. I think Mark is going
> to upgrade those animals to some more recent compiler version.

Wow, that's some pretty impressive debugging!

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company