[llvm-dev] RFC: On removing magic numbers assuming 8-bit bytes

Jesper Antonsson via llvm-dev llvm-dev at lists.llvm.org
Tue May 14 04:08:49 PDT 2019

Thanks to everyone that have weighed in on this! There have been
remarks along the lines that the proposal lacks consensus to move
forward, and that might very well be true. However, the decision model
is not fully clear to me, so before giving up, I'd like to summarize
the discussion to the best of my ability and ask for clarification on
how the community makes its decisions.

(First, I would like to emphasize that this is merely my
interpretation, which may be flawed. If so, my apologies.) The RFC
tl;dr is "without supporting non-8-bit bytes, we want to abstract the
magic 8's used for bytesize to a DataLayout getter".

It garnered some five supporters, including four companies representing
out-of-tree backends: my own Ericsson, Synopsys, CML Microsystems and
Codasip. I also count James Y Knight's (Google) remarks as positive.

Two commenters I interpret as clearly against, among other things
citing the need for a committment for an in-tree-backend using non-8-
bit bytes to undertake such a change: JF Bastien and Philip Reames.

Nine additional commenters provided side remarks to the discussion, and
my interpretation was that five were in some small regard abstraction-
positive as they provided input on the details of the abstraction,
whereas two were more skeptical, and two were neither.

Now to the decision model: If full consensus is required, we clearly
don't have it, and we can stop here. However, if the abstraction were
in place, we'd have a large majority against replacing them with 8s.
And I guess it would be hard to move forward on very many things if
everyone in a huge community can veto?

Or should we take an insider-outsider perspective, then perhaps the
opposers are in  majority as their in-tree voices are what counts?

Or should we take a company perspective, then there's four companies
backing this, while I'm unsure if e.g. JF is speaking for Apple?

Or should we apply some reputation weights, so some people can veto?

Or is it an informal mix of all of the above? I'm sorry if I'm being
obnoxious, I really don't mean to. I'd just sleep better with clarity
in closure, so that the discussion don't just fizzle out and I give up
while having a sense that the community is leaning toward the positive
on this.


On Thu, 2019-05-02 at 12:20 +0000, Jesper Antonsson via llvm-dev wrote:
>    A. This RFC outlines a proposal regarding non-8-bit-byte support
> that
>       got positive reception at a Round Table at EuroLLVM19. The
> general
>       topic has been brought up several times before and one good
> overview
>       can be found in a FOSDEM 2017 presentation by Jones and Cook:
> In a nutshell, the proposal is for the llvm community to
> allow/encourage interested parties to gradually remove "magic
> numbers",
> e.g. assumptions on the size of bytes from the codebase. Overview,
> rationale and some example refactorings follows.
> Overview:
> LLVM currently assumes 8-bit bytes, while there exist a few out-of-
> tree 
> llvm targets that utilize bytes of other sizes, including our
> (Ericsson's) proprietary target. The main issues are the magic number
> 8
> and "/8" and "*8" all over the place and the use of i8 pointers.
> There's considerable agreement that the use of magic numbers is not
> good coding style, and removing these ones would be of particular
> benefit, even though the effort would not be complete and no in-tree
> target with tests exist to guarantee that all gains are maintained.
> Ericsson is willing to drive this effort. During EuroLLVM19, there
> seemed to be sufficient positive interest from other companies for us
> to expect help with reviewing patch sets. Ericsson has been
> performing
> nightly integration towards top-of-tree with this backend for years,
> catching and fixing new 8-bit-byte continuously. Thus we're able to
> commit to doing similar upstream fixes for the long haul in a no-
> drama
> way.
> Rationale:
> Benefits of moving toward a byte-size agnostic llvm include:
> * Less magic numbers in the codebase.
> * A reduced effort to maintain out-of-tree targets with non-8-bit
> bytes
> as contributors follow the established patterns. (One company has
> told
> us that they created but eventually gave up on a 16-bit byte target
> due
> to too-high integration burden.)
> * A reduction in duplicate efforts as some of the adaptation work
> would
> happen in-tree rather than in several out-of-tree targets.
> * For up-and-coming targets that have non-8-bit-byte sizes, time to
> market using llvm would be far quicker.
> * A higher probability of LLVM being the compiler of choice for such
> targets.
> * Eventually, as the patch set required to make llvm fully byte size
> agnostic becomes small enough, the effort to provide a mock in-tree
> target with some other byte size should be surmountable.
> As cons, one could see a burden for the in-tree community to maintain
> whatever gains that have been had. However the onus should be on
> interested parties to mend any bit-rot. The impact of not having as
> much magic numbers and such should if anything make the code more
> easy
> to understand. The permission to go ahead would be under the
> condition
> that significant added complexities are avoided. Another con would be
> added compilation time e.g. in cases where the byte size is a run-
> time
> variable rather than a constant. However, this cost seems negligible
> in
> practice.
> Refactoring examples:
> https://reviews.llvm.org/D61432
> Best Regards,
> Jesper
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