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

Wed Nov 19 18:24:17 PST 2014

> 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 <mailto:ahatanak at gmail.com>> wrote:
> On Wed, Nov 19, 2014 at 3:28 PM, Eric Christopher <echristo at gmail.com <mailto: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.

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).

>  
> 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.

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.

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.

> 
> On Tue Nov 18 2014 at 12:26:49 PM Akira Hatanaka <ahatanak at gmail.com <mailto: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 <mailto:echristo at gmail.com>> wrote:
> 
> 
> On Mon Nov 17 2014 at 9:56:40 AM Bob Wilson <bob.wilson at apple.com <mailto:bob.wilson at apple.com>> wrote:
> 
> > On Nov 14, 2014, at 2:44 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> >
> > +chrisb
> >
> >> On 2014-Nov-13, at 16:33, Akira Hatanaka <ahatanak at gmail.com <mailto: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 <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?
> >
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