The problem occurs earlier in the pipeline than that. The generated C
doesn't have the proper branching present in the original Haskell.
On 3/8/21 7:25 PM, Carter Schonwald wrote:
Isn’t the unregisterized backend a c compiler? You should check what
gcc and clang or whoever we use handles those issues
On Mon, Mar 8, 2021 at 6:56 PM John Ericson
<john.ericson@obsidian.systems> wrote:
Hi everyone,
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4717
<https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4717> fails some
numerical boundary checking tests with the unregisterized backend. In
particular, `minBound / (-1)` and `pred minBound` are *not*
diverging as
expected. This stumped me a few days ago, and no new ideas have
struct
me since. I would very much appreciate some help.
This does seem like something that is very likely to be
backend-dependent, as different instructions/arches handle such edge
cases in different ways. What makes this so peculiar, however, is
that
the boundary condition checking/branching is done *in regular
Haskell*.
I'm thus quite baffled as to what could be going wrong.
If anyone wants to dive in, see my last comment
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4717#note_329840
<https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4717#note_329840>,
which has a minimization to declutter the code without doing enough
constant folding that the problem is avoided entirely. (I went with a
compilation unit trick, but in hindsight NOINLINE should also work
and
be simpler.)
Finally, let my provide some context for why I am hoping to get this
merged soon. To be clear, !4492 was the main MR relating to the
numerics
primops critical to get in for 9.2 and it thankfully already
landed. But
landing this MR too would be nice: It shrinks the intermediate
representations of numerical code probably smaller than it was
originally, whereas now it is perhaps larger than it was before
due to
more size<->native conversions. Also, I think this MR will help get
!3658 over the finish line, which, while again not as critical for
9.2
as !4492 was, would be awfully nice to do to fully realize the new
simplicity of the plan set forth in
https://gitlab.haskell.org/ghc/ghc/-/wikis/Unboxed-Numerics
<https://gitlab.haskell.org/ghc/ghc/-/wikis/Unboxed-Numerics>.
Thanks,
John
N.B. I just rebased and repushed the MR, so CI might be still
running or
failing due to something else, but based on local testing this is
still
the an issue. https://gitlab.haskell.org/ghc/ghc/-/pipelines/31002
<https://gitlab.haskell.org/ghc/ghc/-/pipelines/31002> is an
earlier pipeline run that one can look at until CI finishes again.
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