[llvm-dev] RFC: On non 8-bit bytes and the target for it

[Jorg Brown via llvm-dev]

On Fri, Nov 1, 2019 at 8:40 AM Adrian Prantl via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> > On Nov 1, 2019, at 3:41 AM, Dmitriy Borisenkov via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > It seems that there are two possible solutions on how to move forward
> with non 8 bits byte:
> >
> > 1. Commit changes without tests. Chris Lattner, Mikael Holmen, Jeroen
> Dobbelaere, Jesper Antonsson support this idea.
> > James Y Knight says that at least magic numbers should be removed "at
> least where it arguably helps code clarity". This might be not exactly the
> scope of the changes discussed, but it's probably worth do discuss code
> clarity having concrete patches.
> > GCC (according to James Y Knight) has the same practice meaning non-8
> bits byte is supported but there are no tests in upstream and we have
> downstream contributors who will fix the bugs if they appear in the LLVM
> core.
> > David Chisnall raised a question about what to count as a byte (which
> defines the scope of the changes) and we suggest to use all 5 criteria he
> granted:
> > > - The smallest unit that can be loaded / stored at a time.
> > > - The smallest unit that can be addressed with a raw pointer in a
> specific address space.
> > > - The largest unit whose encoding is opaque to anything above the ISA.
> > > - The type used to represent `char` in C.
> > > - The type that has a size that all other types are a multiple of.
> > But if DSPs are less restrictive about byte, some of the criteria could
> be removed.
> >
> > 2. Use an iconic target. PDP10 was suggested as a candidate. This
> opinion found support from Tim Northover, Joerg Sonenberger, Mehdi AMINI,
> Philip Reames. It's not clear though does this opinion oppose upstreaming
> non-8-bits byte without tests or just a dummy and TVM targets options.
> >
> > So if there is no strong opposition to the solution 1 from the people
> supporting an iconic target option, we could probably move to the patches.
>
> I'm in camp (2). Any changes that are not tested are an invitation to
> upstream developers to "simplify" the code, not knowing that those changes
> are important. Anyone who commits untested changes to LLVM will inevitably
> face an uphill battle against benevolent NFC refactorings that break these
> changes because the expectation of how the code is supposed to behave is
> not codified in a test. In the short term option (1) sounds more appealing
> because they can start right away, but I'm going to predict that it will be
> more expensive for the downstream maintainers of non 8-bit targets in the
> long term.
>

I've worked on multiple codebases where an option existed in order to
satisfy an extremely small userbase, with little or no testing, and as
such, I'm adamantly opposed to repeating it.

In addition to what Adrian said, where the weird option exists but is
constantly being incidentally broken, I've seen the opposite problem:
people become afraid to refactor a section of code because it might break
the weird option.  You might say "if there aren't any tests, people should
feel free to refactor the code; their only responsibility is to make sure
the tests will still work", but honestly, I've seen the opposite: without
tests, it's just presumed that touching certain parts of code is likely to
break things, and so after a time, an aura of untouchability starts to
surround regions of the code.  And then, the more time goes by, the more
that code becomes unfamiliar to everyone (because no one is actively
maintaining it).  In the long run, the cost of an unmaintained option may
be far more than the cost of a maintained one.

In short: please don't commit changes without tests.  Even if the test is
nothing but making sure this works:

int main(int argc, char *argv[]) {
  return argv[argc - 1][0];
}

That at least would give some freedom from the guilt of breaking something
important.

-- Jorg
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