[llvm-dev] The Trouble with Triples

Eric Christopher via llvm-dev
Tue, 22 Sep 2015 19:40:55 -0000

On Thu, Sep 17, 2015 at 6:21 AM Daniel Sanders <Daniel.Sanders at
imgtec.com>
wrote:

> I think we need to take a step further back and re-enter from the right
> starting point. The thing that's bothering me about the push back so
far is
> that it's trying to discuss and understand the consequences of
resolving
> the core problem while seemingly ignoring the core problem itself. The
> reason I've been steering everything back to GNU Triple's being
ambiguous
> and inconsistent is because it's the root of all the problems and the
fixes
> to the various issues fall out naturally once this core point has been
> addressed.
>
>
*sigh*


>
>
> Here's the line of thought that I'd like people to start with:
>
> ·         Triples don't describe the target. They look like they
should,
> but they don't. They're really just arbitrary strings.
>

Triples are used as a starting point, but no more.


> ·         LLVM relies on Triple as a description of the target. It
> defines the backend to use, the binary format to use, OS and Vendor
> specific quirks to enable/disable, the default CPU, the default ABI, the
> endian, and countless other details about the target.
>

These two statements aren't necessarily true in whole.

a) We don't use the Triple to fully specify the target.
b) We don't use the Triple to fully specify the ABI.
c) We don't use the Triple to fully specify the CPU.
d) We do use the triple to handle endianness since most, if not all,
triples actually bother to encode endianness.
e) The rest of the "countless details" may or may not be relevant, you
haven't given an example of what you care about.

>From here on your email relies on all of these assumptions being true. So
I'm going to skip past that part and go to where you answer some of my
questions.

> At this point, in the MC layer we have a number of classes that need to
> know the ABI but lack this information. Our TargetMachine has an accurate
> TargetTuple object that describes the invariants of the desired target. The
> desired ABI is an invariant too so why not have it in the TargetTuple which
> is already plumbed in everywhere we need it? After all, it's a property
of
> the target OS/Environment. If we have the ABI in the TargetTuple, then we
> don't need any other means to set the ABI, tools can set it up front in
the
> TargetTuple and we don't need any command-line option handling for it
in
> the backend.
>
>
This isn't sufficient anyways as I don't want to depend on a weird
serialization format to deal with something a simple command line can deal
with (or you've said this in a way that's confused me). I see you saying
you want:

-tuple mips-linux-gnu-abio32-el

to specify on a command line to, say, llvm-mc or a new assembler interface,
or heck, to clang itself, that you want to compile for:

-triple mipsel-linux-gnu -mabi=o32

right? Basically? (Bikeshedding of how to actually serialize things aside?)


> Meanwhile, in clang we have a number of command line options that change
> the desired target. Let's say we've constructed a Triple and
resolved it to
> TargetTuple (more on that below). We're now processing the –EL option.
At
> the moment, we substitute our mips-linux-gnu triple for a mipsel-linux-gnu
> triple, construct a Triple object from it and resolve the new Triple to a
> TargetTuple. But why do we need to bother with that kind of weird hackery
> when we can simply do Obj.setEndian(Little)? This is what Phase 7 of the
> plan is about. We end up with a cleaner way to process target changes that,
> until now, have required weird triple hacking to handle.
>
>
>

This is something else I don't understand. Here is the first time you start
talking about APIs which is what I'm particularly asking about in my
earlier mails. I'd like to see how you plan on changing the TargetMachine
and MC level APIs to deal with this. It seems like the Tuple is going to be
a way to side-load information around to the MC layer and while I agree
that something is necessary there, I don't think that this solution is the
right one. (As I said earlier in the thread)


> I skipped the Triple -> TargetTuple resolution a moment ago and I should
> address that now. We already know that mapping Triple to TargetTuple is a
> many to many mapping. One Triple has many possible TargetTuple's
depending
> on the environment. One TargetTuple can be formed from multiple possible
> Triples. In an ideal world, we'd like to bake in all of these mappings
so
> that one clang binary supports everything. Unfortunately, being a many to
> many mapping, some of these mappings are mutually exclusive. Note that this
> isn't a new problem resulting from this project. The problem has always
> been there but has been ignored until now. To resolve this, we need to
> provide configure-time and possibly run-time controls for how this
> conversion is disambiguated. This resolution is performed as early as
> possible so that the middle/back-ends don't need to know anything about
the
> ambiguity problem.
>
>
>
The minute you start talking about configure time controls we've already
lost. This, for me, is a non-starter. That said, I'd like to see the
examples you think show that things are impossible to deal with in the
current architecture.


> ---
>
>
>
> To reply more directly to your email:
>

Thanks :)


> > What can't be done to TargetMachine to avoid this serialization?
>
>
>
> TargetMachine already has the serialization (see
> TargetMachine::TargetTriple). We're not doing anything new here.
We're
> simply replacing one object holding faulty information with a new object
> holding reliable information.
>
>
>

This is side stepping my question and making it about Triple. I've
specifically said that TargetMachine does not and is not completely
dependent upon Triple.


> > And a followup question: What can't be serialized at the function
level
> in the IR to make certain things clear that aren't global? We already
do
> this for a lot of command line options.
>
>
>
> The data I want to fix is global. I think the bit you may be getting hung
> up on here is that small portions of this global data can also be
> overridden at the function level. Those overrides aren't a problem and
> continue to operate in the same way as they do today.
>
>
>
Examples please.


> > And one more: What global options do we need to consider here?
>
>
>
> I'm not certain I understand this question. If you're talking
command line
> options, it's things like –EL, -EB, -mips32, -mips32r[2356], -mips64,
> -mips64r[2356], -mabi=…. If you're talking about Triple ->
TargetTuple
> mappings, there's quite a wide variety but the main ones for Mips are
> endian, architecture, default CPU, and default ABI.
>
>
All of these are representable right now in the TargetMachine as far as I
can tell. What examples are you having problems with?


>
>
> > The goal of the configuration level of the TargetMachine is that it
> controls things that don't change at the object level.
>
> > This is a fairly recently stated goal, but I think it makes sense for
> LLVM in general. TargetSubtargetInfo takes care of
>
> > everything that resides under this (as much as possible, some bits are
> still in transition, e.g. TargetOptions). This is part
>
> > of my suggestion to Daniel about the problems with MCSubtargetInfo and
> the assembler. Targets like Mips and ARM
>
> > were unfortunately designed to change things on the fly during
assembly
> and need to collate or at least change defaults
>
> > as we're processing code. I definitely had to deal with a lot of
the
> pain you're talking about when I was rewriting some
>
> > of the handling there during the TargetSubtargetInfo work.
>
>
>
> I generally agree with this. The key bit I need to draw attention to is
> that the 'defaults' don't change, but are instead overridden.
These
> constant defaults are stored in TargetMachine and particularly
> TargetMachine::TargetTriple. These defaults are wrong for some toolchains
> since the information stored in TargetMachine::TargetTriple are wrong.
It's
> the defaults I'm trying to fix rather than the overrides.
>
>
>

I don't understand what you mean here.


> I think I understand your proposed plan now and it's a few steps ahead
of
> where we are and where we need to be. I agree that overridable state should
> be in TargetSubtargetInfo, however I can't initialize that state
without
> the default values which come from the faulty information in
> TargetMachine::TargetTriple. This triple work is a pre-requisite to your
> plan and at first I don't need to override ABI's.
>
>
>

Can you provide an example of using a tool that you're having problems with?


> > Right now I see TargetTuple as trying to take over all of the various
> arguments to TargetMachine and encapsulate them into a single thing.
>
> > I also don't see this is bad, but I also don't see it taking
all of them
> right now and I'm not sure how it solves some of the existing problems
>
> > with data sharing that we've got which is where the push back
you're
> both getting is coming from here. Ultimately library-wise I can agree
>
> > with some of the directions you're headed - I just don't see
the
> unification and interactions right now.
>
>
>
> I think we'll end up with TargetTuple taking over many arguments to
> TargetMachine but that's not my goal at this stage. My goal is simply
to
> fix the faulty information currently held in Triple and use the
> now-accurate information in TargetTuple to fix various blocking issues that
> prevent a proper Mips toolchain product based on Clang/LLVM. At the end of
> Phase 7, it become possible to fix a number of issues that are impossible
> to fix right now because the available data we can consult at the moment is
> incorrect.
>
>
>

Could you please provide some examples of things that are impossible right
now with command lines, how those interact with the TargetMachine, and how
you see it being impossible to deal with?

Thanks

-eric


>
>
> *From:* Eric Christopher [mailto:echristo at gmail.com]
> *Sent:* 16 September 2015 23:52
> *To:* Renato Golin; Jim Grosbach
> *Cc:* Daniel Sanders; llvm-dev at lists.llvm.org
>
>
> *Subject:* Re: The Trouble with Triples
>
>
>
> Let's take a step back here.
>
>
>
> It appears that you and Daniel are trying to solve some problems. I think
> solving problems is good, I just want to make sure that we're solving
them
> in a way that gets us a decent API at the end. I also want to make sure
> we're solving the right problems.
>
>
>
> TargetTuple appears to be related to the TargetParser as you bring up in
> this mail. They're two separate parts of similar problems - people
trying
> to both serialize command line options and communication from the front end
> to the backend with respect to target information.
>
>
>
> This leads me to a question: What can't be done to TargetMachine to
avoid
> this serialization?
>
> And a followup question: What can't be serialized at the function level
in
> the IR to make certain things clear that aren't global? We already do
this
> for a lot of command line options.
>
> And one more: What global options do we need to consider here?
>
>
>
> The goal of the configuration level of the TargetMachine is that it
> controls things that don't change at the object level. This is a fairly
> recently stated goal, but I think it makes sense for LLVM in general.
> TargetSubtargetInfo takes care of everything that resides under this (as
> much as possible, some bits are still in transition, e.g. TargetOptions).
> This is part of my suggestion to Daniel about the problems with
> MCSubtargetInfo and the assembler. Targets like Mips and ARM were
> unfortunately designed to change things on the fly during assembly and need
> to collate or at least change defaults as we're processing code. I
> definitely had to deal with a lot of the pain you're talking about when
I
> was rewriting some of the handling there during the TargetSubtargetInfo
> work.
>
>
>
> Now a bit more on TargetParser + TargetTuple:
>
>
>
> TargetParser appears to be trying to solve the parsing in Triple in a nice
> way for ARM and also some of the "what kind of subtarget feature
> canonicalization can we do in llvm that makes sense to communicate to the
> front end". I like this particular idea and have often wanted a
library of
> feature handling, but it seems to have stabilized at an ARM specific set of
> code with no defined interface. I can't even figure out how I'd use
it in
> lib/Basic right now for any target other than ARM. This isn't a
> condemnation of TargetParser, but I think it's something that needs to
be
> thought through a bit more. It's been hooked up well before I'd
expected it
> to and right now if we moved it to the ARM backend from Support it'd
make
> just as much sense as it does where it is now other than making clang
> depend on the ARM backend as well as the X86 backend :)
>
>
>
> Right now I see TargetTuple as trying to take over all of the various
> arguments to TargetMachine and encapsulate them into a single thing. I also
> don't see this is bad, but I also don't see it taking all of them
right now
> and I'm not sure how it solves some of the existing problems with data
> sharing that we've got which is where the push back you're both
getting is
> coming from here. Ultimately library-wise I can agree with some of the
> directions you're headed - I just don't see the unification and
> interactions right now.
>
>
>
> As a suggestion as a way forward here let's see if we can get my
questions
> above answered and also show some of how the interactions between
llvm's
> libraries are going to get fixed, moved to a better place, etc here.
>
>
>
> Thanks!
>
>
>
> -eric
>
>
>
>
>
> On Wed, Sep 16, 2015 at 3:02 PM Renato Golin <renato.golin at
linaro.org>
> wrote:
>
> On 16 September 2015 at 21:56, Jim Grosbach <grosbach at apple.com>
wrote:
> > Why do we care about GAS? We have an assembler.
>
> It's not that simple.
>
> There are a lot of old code out there, including the Linux kernel
> which we do care a lot, that only compiles with GAS. We're slowly
> moving the legacy code up to modern standards, and specifically some
> kernel folks are happy to move up not only the asm syntax, but the C
> standard and move away from GNU-specific behaviour. But we're not
> quite there yet, and might not be for a few more years. so, yes, we
> still care about GAS.
>
> But this is not just about GAS.
>
> As I said on my previous email, this is about clearing the bloat in
> target descriptions by both: removing the need for adding numerous CPU
> names, target features, architecture names (xscale, strongarm, etc),
> AND making sure all parties (front/middle/back-ends) speak the same
> language, produced from the same source.
>
> The TargetTuple is that common language, and the TargetParser created
> from the TableGen files is the common source. The Triple becomes a
> legacy constructor value for the Tuple. All other target information
> classes are already (or should be) generated from the TableGen files,
> so the ultimate source becomes the TableGen description, which I think
> it what you were aiming to on your comment.
>
> For simple architectures, like x86, you don't even need a
> TargetParser. You can easily construct the Tuple from a triple and use
> the Tuple as you've always used the triple. No harm done. But for the
> complex ones like ARM and MIPS, having a common interface generated
> from the same place the other interfaces are is important to avoid
> more bridges between front and middle and back end interpretations of
> the same target. Whatever legacy ARM or MIPS carry can be isolated in
> their own implementation, leaving the rest of the targets with a clean
> and simple interface.
>
> cheers,
> --renato
>
>
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