[LLVMdev] [RFC] Embedding command line options in bitcode (PR21471)

Eric Christopher echristo at gmail.com
Tue Dec 2 15:21:15 PST 2014 

On Mon Dec 01 2014 at 4:22:15 PM Bob Wilson <bob.wilson at apple.com> wrote:

> Thanks for your feedback, Eric.
>
> I still think we may be talking past each other a little bit, but rather
> than
>

Might be, sorry if so :(


> delving further into the details right now, I’ve suggested to Akira that
> he look into how we should handle other kinds of options. I’m hoping that
> as we look at more of them, we will gain some insight into the approach
> that we want to take here. This patch really only deals with the easy
> cases, and whether we use Akira’s approach or not, I’m not especially
> worried about this kind of option.
>
> Let’s come back to this in a little while after we’ve looked at what to do
> for some other options…..
>
>
I apologize if this seems a bit rambly, but I'm trying to get a lot of
thoughts on this down:

FWIW I'm looking at some of this at the moment which is why I spoke to a
few of the different options and needing to control TargetMachine creation.
Just to continue with some of the discussion (at least to point some things
the right direction and get feedback) let's think about the -target-abi
option for ARM. It sets the ABI for the backend on the target machine. It
probably shouldn't change on a per function basis (wait, except for the
calling convention which can be changed via an attribute, fun!), so we want
it to control global data layout and valid types which means we need it on
the TargetMachine. The question is how to pass this information into the
backend for such an option - right now, if we wanted, we could punt to
using the command line options that currently exist and the existing
ParseCommandLineOptions call from clang.

That said, it's a pretty awful hack to use them. For some of my particular
needs I've got some code around unifying the DataLayout creation between
the front end and back end, but it's a partial solution for the general
problem and I'll probably run into more issues in this arena.

So, trying to come up with ways to communicate all of this via the API so
that we can set ABIs for targets that need it (ARM, MIPS, others), as well
as probably other options (anything passed via -backend-option is a good
candidate).

One thought is expanding and/or deriving target specific versions of
TargetOptions and populating that on creation from the front end. We'd want
to be careful with the target specific bits and constructing defaults,
probably something off of the bits in Targets.cpp would be appropriate. A
lot of this type of code leads us down the TargetSupport library or
something of the sort - classes to help describe or create backend
constructs.

Anyhow, this is my current thinking on how to do API building of
TargetMachine etc. (Alternately a TargetMachineBuilder? But that sounds
complicated in all of the same ways without any particular gain).

-eric


> On Dec 1, 2014, at 10:53 AM, Eric Christopher <echristo at gmail.com> wrote:
>
> Hi Bob,
>
> I've been giving this some thought, comments inline:
>
> On Wed Nov 19 2014 at 6:24:18 PM Bob Wilson <bob.wilson at apple.com> wrote:
>
>> On Nov 19, 2014, at 4:52 PM, Eric Christopher <echristo at gmail.com> wrote:
>>
>>
>>
>> On Wed Nov 19 2014 at 4:39:42 PM Akira Hatanaka <ahatanak at gmail.com>
>> wrote:
>>
>>> On Wed, Nov 19, 2014 at 3:28 PM, Eric Christopher <echristo at gmail.com>
>>> wrote:
>>>
>>>> So my general concern here is that lots of command line options that
>>>> don't need to be or shouldn't be IR level constructs become oddly named
>>>> string options. It's bad enough that we're doing that with the target cpu
>>>> and target feature string let alone the rest of it.
>>>>
>>>> If we're talking about things that end up changing the subtarget used
>>>> we definitely need to make this part of the lookup key - at which point
>>>> we're going to want a unified interface to all of them. I haven't applied
>>>> your patch to see what the IR changes look like here.
>>>>
>>>>
>>> With the patches applied, the attributes clang embeds in the IR look
>>> like this:
>>>
>>> $ clang -cc1 ... -mllvm -arm-long-calls -mllvm -arm-reserve-r9
>>> -menable-no-nans
>>>
>>> attributes #0 = { nounwind readonly ssp "NoNaNsFPMath"
>>> "arm-long-calls"="1" "arm-reserve-r9"="1" "less-precise-fpmad"="false"
>>> "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
>>> "no-infs-fp-math"="false" "stack-protector-buffer-size"="8"
>>> "target-cpu"="arm7tdmi" "target-features"="-neon,-crypto,+apcs"
>>> "unsafe-fp-math"="false" "use-soft-float"="true" }
>>>
>>> I'm guessing you want the front-end to convert command line options like
>>> "arm-long-calls" or "arm-reserve-r9" to something that doesn't look as odd?
>>>
>>
>> Right, that was part of the idea, the rest is that we define things that
>> actually make sense as front end command line options and use those to
>> communicate via IR constructs or api calls I.e. reserved registers don't
>> need to be a specific back end option, but rather can be passed along when
>> creating a target machine, etc.
>>
>> -eric
>>
>>
>> No, reserved registers (I assume you’re referring to “arm-reserve-r9” in
>> Akira’s example) don’t necessarily need to be backend options. But, those
>> kinds of options do need to be recorded in the IR and the target machine
>> needs to reflect the relevant IR settings. Otherwise, we’ll still be left
>> with a broken LTO model. E.G., I build one LTO object with “arm-reserve-r9”
>> and another without it, but the LTO compilation still needs to respect
>> those options for the code in each object. The details of how a particular
>> option should be handled will vary depending on the option, and we need to
>> discuss each one separately.
>>
>>
> ABI breaking options should probably be considered part of
> TargetMachine/Subtarget creation and handled accordingly (if it's subtarget
> creation it can be on the function as a feature string attribute, if it
> affects global code generation it needs to be at TargetMachine creation
> time).
>
>
>> It’s pretty clear, though, that there is a class of backend options that
>> translate nicely to function attributes. This RFC is about the mechanism
>> that we use to handle those (without specifying which specific options will
>> be handled that way).
>>
>>
> Fair enough. For function attributes I can see a desire to have (as Chris
> posted in an email from someone at synopsys) a generic target specific
> dictionary mapping on functions for backend specific uses. I just haven't
> seen anything in the thread that would be applicable for this rather than
> an actual function attribute. That said, I would like some sort of
> delineation for target function attributes as well. Thoughts on that?
> Perhaps it could look like the string dictionary we've mentioned, maybe not.
>
>
>>
>>
>>> From looking at the patch it seems that the command line option for NaNs
>>>> should be a general attribute on the function rather than a TargetMachine
>>>> option anyhow, so it seems that should be migrated down. Whether or not it
>>>> is exposed as an llvm attribute or a string based key value pair is an
>>>> interesting discussion. Also, would we want to, instead, expose the backend
>>>> options as clang IR level options? Seems to make sense from that
>>>> perspective and then they can add the appropriate attributes to the
>>>> functions themselves. (-mlongcalls for the long calls option for example).
>>>>
>>>> Thanks!
>>>>
>>>> -eric
>>>>
>>>
>> Again, I think we can evaluate specific options on a case-by-case basis.
>> For some of them, it might make sense to expose them as clang options. Like
>> you, I had previously assumed we would teach the front-end about all the
>> backend options. I was a little surprised by the approach Akira suggested
>> here, but after giving it some thought, I think it’s a great idea. There
>> are a LOT of target-specific backend options. Some of them do not need to
>> be IR, but many do. Teaching clang about all of them would be a lot of
>> work, and I don’t see any clear benefit from that. This approach lets clang
>> remain relatively independent of the backend options. We still need to
>> think carefully about which backend options use this mechanism, of course.
>>
>>
> I'm unconvinced here...
>
>
>> Are there other reasons why you think we should add all those options to
>> clang? I just don’t see much to be gained by that.
>>
>>
> .... I think that anything you'd like to expose should probably be a front
> end option. If nothing else I don't see how you plan on annotating the IR.
> Basically you're constructing a parallel option hierarchy and that seems...
> redundant. I agree that it's nice for tools to be able to override things
> in the backend - for testing if nothing else. We do this a lot by checking
> options and values in the code. A way of streamlining this would be nice -
> but I don't think that's the goal of these patches no?
>
>
>> Do you have objections to the use of string-based key/value pairs? I
>> believe the intention all along, going back to Bill’s work on this a few
>> years ago, was that we would add new attributes for target-independent
>> things and use strings for the target-specific options. That seems like a
>> reasonable starting point to me.
>>
>>
> Only some as I mentioned above - I haven't seen anything that needs to be
> target specific in this way, but I'm open to the idea :)
>
> As a note, this is all for function level things. We need a solution for
> non-function/module level attributes and I think that's going to look like
> API calls into the backend for TargetMachine creation - there may be a list
> of things that would turn into module level attributes, but that is a
> direction fraught with danger and I'd like to avoid that as much as
> possible.
>
> (As a note, the particular patch that Akira has is unacceptable because of
> the changes to the subtarget lookup, but that's a separate issue :)
>
> -eric
>
>
>>
>>>> On Tue Nov 18 2014 at 12:26:49 PM Akira Hatanaka <ahatanak at gmail.com>
>>>> wrote:
>>>>
>>>>> Updated patch is attached. Note this is just a work-in-progress patch
>>>>> and I plan to address the feedback comments later if this patch is in the
>>>>> right direction.
>>>>>
>>>>> This is how the command line options are parsed and used by the
>>>>> backend passes:
>>>>>
>>>>> 1. Tools such as clang and llc call cl::ParseCommandLineOptions. Any
>>>>> of the options that should be written to the bitcode as function or module
>>>>> attributes (these options are declared as CodeGenOpt, which is a subclass
>>>>> of cl::opt) are saved to Function or ModuleIntAttrs in CodeGenOptions.cpp.
>>>>> 2. setFunctionAttribute is called to move the function and module
>>>>> options saved in step 1 to the bitcode. Also, options stored in
>>>>> TargetOptions are moved to the bitcode too. Pre-existing attributes in the
>>>>> bitcode are overwritten by the options in the command line.
>>>>> 3. The backend passes and subtarget classes call getFunctionAttribute
>>>>> to read the attributes stored to the bitcode in step 2.
>>>>> Function::hasFnAttribute can be called directly (for example, NoNaNsFPMath
>>>>> in the patch), if the default value is known to be "false".
>>>>>
>>>>> I also made the following changes in the patch:
>>>>>
>>>>> 1. In the constructor of MachineFunction, call the version of
>>>>> getSubtargetImpl that takes a Function parameter, so that it gets the
>>>>> Subtarget specific to the Function being compiled.
>>>>> 2. Change ARMTargetMachine::SubtargetMap to be a DenseMap<Function*,
>>>>> unique_ptr<ARMSubtarget>>. This is just a temporary change to ease
>>>>> debugging and should be reverted to a StringMap or changed to another type
>>>>> of map later.
>>>>>
>>>>>
>>>>> On Mon, Nov 17, 2014 at 11:40 AM, Eric Christopher <echristo at gmail.com
>>>>> > wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon Nov 17 2014 at 9:56:40 AM Bob Wilson <bob.wilson at apple.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>> > On Nov 14, 2014, at 2:44 PM, Duncan P. N. Exon Smith <
>>>>>>> dexonsmith at apple.com> wrote:
>>>>>>> >
>>>>>>> > +chrisb
>>>>>>> >
>>>>>>> >> On 2014-Nov-13, at 16:33, Akira Hatanaka <ahatanak at gmail.com>
>>>>>>> wrote:
>>>>>>> >>
>>>>>>> >> I'm working on fixing PR21471, which is about embedding codegen
>>>>>>> command line options into the bitcode as function or module-level
>>>>>>> attributes so that they don't get ignored when doing LTO.
>>>>>>> >>
>>>>>>> >> http://llvm.org/bugs/show_bug.cgi?id=21471
>>>>>>> >>
>>>>>>> >> I have an initial patch (attached to this email) which enables
>>>>>>> clang/llvm to recognize one command line option, write it to the IR, and
>>>>>>> read it out in a backend pass. I'm looking to get feedback from the
>>>>>>> community on whether I'm headed in the right direction or whether there are
>>>>>>> alternate ideas before I go all the way on fixing the PR. Specifically, I'd
>>>>>>> like to know the answers to the following questions:
>>>>>>> >>
>>>>>>> >> 1. How do we make sure we continue to be able to use the command
>>>>>>> line options we've been using for llc and other tools?
>>>>>>> >> 2. How to handle cases where two functions in a module have
>>>>>>> different sets of command line options?
>>>>>>> >> 3. Where should the command line options or module/function
>>>>>>> attributes be stored once they are read out from the IR?
>>>>>>> >>
>>>>>>> >> The short description of the approach I took in my patch is that
>>>>>>> command line options that are important to codegen are collected by
>>>>>>> cl::ParseCommandLineOptions, written to the bitcode as function or module
>>>>>>> attributes, and read out directly by the optimization passes that need
>>>>>>> them. cl::opt options are replaced with CodeGenOpt options (subclass of
>>>>>>> cl::opt), which are needed only to parse the command line and provide the
>>>>>>> default value when the corresponding options are not in the bitcode.
>>>>>>> >
>>>>>>> > I like this approach, since it means the frontend doesn't have to
>>>>>>> understand
>>>>>>> > options in order to pass them on to the backend.
>>>>>>>
>>>>>>> I agree. It’s not the approach that was I was expecting, but after
>>>>>>> reading the patches, I like it.
>>>>>>>
>>>>>>> If we can get consensus on the approach, I suggest that we stage the
>>>>>>> work to first adopt Akira’s patches and then we can incrementally convert
>>>>>>> various options over to use it. There may be complications for some
>>>>>>> options, so I’d like to see separate patches for each one (possibly with a
>>>>>>> few similar options combined in one patch).
>>>>>>>
>>>>>>>
>>>>>> I'm not sold quite yet :)
>>>>>>
>>>>>> Akira: Can you show an example of how you expect this to work in the
>>>>>> IR and how the backend is going to process?
>>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>> -eric
>>>>>>
>>>>>>
>>>>>>> >
>>>>>>> > The static variables should be straightforward to migrate to an
>>>>>>> LLVMContext
>>>>>>> > once ParseCommandLineOptions stores things there instead of in
>>>>>>> globals.
>>>>>>> >
>>>>>>> >> diff --git a/lib/CodeGen/CodeGenOption.cpp
>>>>>>> b/lib/CodeGen/CodeGenOption.cpp
>>>>>>> >> new file mode 100644
>>>>>>> >> index 0000000..2d74c2f
>>>>>>> >> --- /dev/null
>>>>>>> >> +++ b/lib/CodeGen/CodeGenOption.cpp
>>>>>>> >> @@ -0,0 +1,59 @@
>>>>>>> >> +//===- CodeGen/CodeGenOptions.cpp - Code-gen option.
>>>>>>>  --*- C++ -*-===//
>>>>>>> >> +//
>>>>>>> >> +//                     The LLVM Compiler Infrastructure
>>>>>>> >> +//
>>>>>>> >> +// This file is distributed under the University of Illinois
>>>>>>> Open Source
>>>>>>> >> +// License. See LICENSE.TXT for details.
>>>>>>> >> +//
>>>>>>> >> +//===------------------------------------------------------
>>>>>>> ----------------===//
>>>>>>> >> +//
>>>>>>> >> +//===------------------------------------------------------
>>>>>>> ----------------===//
>>>>>>> >> +
>>>>>>> >> +#include "llvm/CodeGen/CodeGenOption.h"
>>>>>>> >> +#include "llvm/IR/Attributes.h"
>>>>>>> >> +#include "llvm/IR/Module.h"
>>>>>>> >> +
>>>>>>> >> +using namespace llvm;
>>>>>>> >> +
>>>>>>> >> +static std::map<std::string, bool> FunctionBoolAttrs;
>>>>>>> >> +static std::map<std::string, bool> ModuleBoolAttrs;
>>>>>>> >> +
>>>>>>> >
>>>>>>> > @Chris, should these be using ManagedStatic?
>>>>>>> >
>>>>>>> > _______________________________________________
>>>>>>> > LLVM Developers mailing list
>>>>>>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>>>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>>>>
>>>>>>>
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>>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
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>>>>>
>>>>>
>>
>
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