[llvm-dev] [RFC] Coding Standards: "prefer `int` for, regular arithmetic, use `unsigned` only for bitmask and when you, intend to rely on wrapping behavior."

[via llvm-dev]

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of JF
> Bastien via llvm-dev
> Sent: Thursday, June 13, 2019 12:25 PM
> To: John Reagan
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] [RFC] Coding Standards: "prefer `int` for, regular
> arithmetic, use `unsigned` only for bitmask and when you, intend to rely
> on wrapping behavior."
> 
> 
> 
> > On Jun 13, 2019, at 9:19 AM, John Reagan <john.reagan at vmssoftware.com>
> wrote:
> >
> > Yes.  We currently build LLVM 3.4.2 on our OpenVMS Itanium box with an
> > older EDG/Intel C++03 compiler to create legacy cross-compilers to our
> > OpenVMS x86 box (well, VirtualBox).  We do have a few tweaks to the
> > relocations to access static data always through the GOT (including
> > CodeGen's static data).  Our linker sees references to code (which might
> > be in 64-bit space) and creates trampolines in 32-bit space.  That lets
> > any legacy code from the VAX-days to continue to take the address of a
> > routine and save it into some INTEGER*4 Fortran variable.
> >
> > So mostly a small memory model with a few things from medium/large.
> >
> > I've been unable to build the companion clang 3.4.2 however as its use
> > of templates seems to push our old compiler over the edge of sanity.
> >
> > We're working on bootstrapping 8.0.0.  Compile 8.0.0 on Linux, move
> > objects to our OpenVMS Itanium box to use the cross-linker (which can
> > handle Linux objects as well as our own), move the resulting image to
> > our OpenVMS x86 box...  (and the same thing for libcxx, compiler-rt,
> > libcxxabi, etc.) and with a wave of a magic wand, we end up with a
> > native clang.  And with a little more waving, our other legacy compilers
> > (our C, BLISS, Pascal, COBOL, BASIC, and Fortran)
> >
> > I'm planning on submitting a lightning talk for this fall on "When 3
> > memory models isn't enough.”
> 
> This is a rare occurrence… but you leave me speechless. I don’t even know
> where to start.

The word-size migration is rare but not unique.  The HP/Tandem NonStop
line has moved from 16-bit to 32-bit to 64-bit over the decades, and we
needed to handle mixed-size pointers and "int" weirdness at each stage.  
OpenVMS is a special snowflake in terms of its LLVM bootstrapping process, 
and the mind just boggles at what John and his team have had to contend 
with. But the size part is certainly familiar to me, and actually simpler
than what NonStop had to deal with. (We weren't using LLVM, clearly, but
container size etc. is not a particularly LLVM-specific issue.)
--paulr

> 
> 
> > On 6/13/19 12:00 PM, JF Bastien wrote:
> >>
> >>
> >>> On Jun 12, 2019, at 2:01 PM, John Reagan via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> >>>
> >>> On Tue, Jun 11, 2019 at 12:26 PM Michael Kruse <llvmdev at meinersbur.de>
> >>> wrote:
> >>>
> >>> vector.size() returns a size_t, which on 64-bit platforms can
> represent
> >>> types values larger than those that can fit into an int64_t.  So to
> turn
> >>> your argument around, since it's theoretically possible to have a
> vector
> >>> with more items than an int64_t can represent, isn't it already worth
> it
> >>> to use size_t, which is an unsigned type?
> >>>
> >>>
> >>> That's not true on my platform.  I have 64-bit pointers so intptr_t is
> >>> 64-bits, but the largest thing you can allocate is only 32-bits big so
> >>> size_t (and ptrdiff_t) are 32-bits.
> >>
> >> It runs LLVM?
> >>
> >
> 
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