Overhauling the command-line processing/codegen options code

Is anyone currently working on overhauling the command-line processing code?

We’re currently having some design issues with this component, and I’d like to start a larger conversation on it. For context, I am talking from an “LLVM as a library” perspective instead of an “LLVM as a set of tools” perspective. In a nut-shell, the problems we are facing are as follows:

  1. As per bug 11944, the command-line processing logic accounts for a non-trivial amount of global constructors and leads to memory that is allocated when the containing library is loaded, and not freed until the containing library is unloaded. Granted, this is not a lot of data…

  2. Command-line options are currently used for codegen options in inherently non-thread-safe ways.

(1) is mostly self-explanatory and has a bug about it, but (2) requires a bit more explanation. Take for example the “-unroll-threshold” option in LoopUnroll. If I’m setting up a pass pipeline programmatically, I add the LoopUnroll pass to the PassManager, but I cannot set the unroll threshold without calling cl::ParseCommandLineOptions() [or perhaps some ugly hacks that involve getting at and modifying the global state]. In addition to being awkward, this is not thread safe! I cannot run two PassManager pipelines concurrently with different unroll threshold values. In this case, I am singling out the LoopUnroll pass, but this design is very prevalent within the LLVM analysis/transformation/codegen infrastructure. This has no effect on users of opt/llc as tools, but library users can be greatly affected by this.

Ideally, what I would like to see is a separation between the definition of analysis/transformation/codegen options and their value storage. To get the conversation started, I would like to propose the following:

Requirements

  • Make it easy for passes to declare arbitrary options, just like they do now

  • Let different pass pipelines have different sets of option values

  • Keep the option registry dynamic, so plugins loaded with “-load” can register new options

  • Let option values be parseable from the command-line (for opt, llc, …)

Implementation

As a first design draft, I propose that cl::opt and friends be extended to support argument storage in an LLVMContext. Instead of storing the value directly in a cl::opt instance or specifying a global variable, this new storage would utilize a StringMap stored in an LLVMContext. As a consequence, parsing would be delayed until the option is read.

  • Command Line Parsing

The cl::ParseCommandLineOptions() call would take an additional parameter, an LLVMContext reference. Global options would be written directly to their cl::opt instances as they are now, but per-context options will be copied in the LLVMContext instance. Tools would use something like:

LLVMContext &Ctx = getGlobalContext();

cl::ParseCommandLineOptions(argc, argv, “my tool”, Ctx);

In addition, library users can use a new API:

LLVMContext MyCtx;

cl::ParseContextOptions(ArgList, MyCtx);

which will only parse per-context options (and not overwrite any global state).

  • Reading Option Values

For per-context options, an LLVMContext would be required when reading back the option values. For example, in the LoopUnroll example, you could write:

unsigned Threshold = UnrollThreshold(Mod.getContext());

This would get the “-unroll-threshold” option value for the context for the current module. Parsing of the value would be delayed until the value is read, since the value would need to be stored as a string in the context. Global options can be used as they currently are. It would be a run-time error to read a per-context option without a context parameter.

Open Questions

  • Can we just make all options per-context, and assign tool options to the global context? May require special handling for “-debug”, “-time-passes”, and friends.

  • Alternatively, we could try to just eliminate the codegen options altogether and rely on per-module data, like the new attributes functionality. But this seems like it may be less flexible and would require the Module to know what options to use. Supporting command-line options would then require changes to the Module instance.

This is just a first-pass idea for making options more scalable to library users. Please let me know if you have any other ideas, or if someone is already working on this.

Is anyone currently working on overhauling the command-line processing code?

Bill's attributes work is quite related, it will hopefully define away a number of the TargetOptions concepts, replacing them with per-function attributes. It should also subsume a number of target-independent concepts like "omit frame pointers" and other codegen flags.

We're currently having some design issues with this component, and I'd like to start a larger conversation on it. For context, I am talking from an "LLVM as a library" perspective

My favorite perspective :slight_smile:

In a nut-shell, the problems we are facing are as follows:

1. As per bug 11944, the command-line processing logic accounts for a non-trivial amount of global constructors and leads to memory that is allocated when the containing library is loaded, and not freed until the containing library is unloaded. Granted, this is not a lot of data…

Yep, this sucks. It has been a long-known problem. In my ideal world, the only cl::opt's that would exist would be in leaf tools (e.g. in tools/opt, but not in optimization passes themselves). There is some major design work required to make this happen though, and I'm not aware of anyone tackling it.

2. Command-line options are currently used for codegen options in inherently non-thread-safe ways.

Hopefully Bill's work will help address the majority of this.

(1) is mostly self-explanatory and has a bug about it, but (2) requires a bit more explanation. Take for example the "-unroll-threshold" option in LoopUnroll. If I'm setting up a pass pipeline programmatically, I add the LoopUnroll pass to the PassManager, but I cannot set the unroll threshold without calling cl::ParseCommandLineOptions() [or perhaps some ugly hacks that involve getting at and modifying the global state]. In addition to being awkward, this is not thread safe! I cannot run two PassManager pipelines concurrently with different unroll threshold values. In this case, I am singling out the LoopUnroll pass, but this design is very prevalent within the LLVM analysis/transformation/codegen infrastructure. This has no effect on users of opt/llc as tools, but library users can be greatly affected by this.

This is a very interesting one that is orthogonal to Bill's work. The preferred approach for this sort of thing is to change the LoopUnroll pass to take the unroll threshold as a constructor argument or a struct that wraps up all of the configuration settings. When the pass is *default* constructed (e.g. from opt -loop-unroll), it is acceptable to have the default ctor read cl::opt variables, but the optimization pipeline should not depend on cl::opts to configure the pass.

There is currently some gray area here for debug settings, and for things that are being staged in but are not on by default, but the default optimization pipeline should not be looking to cl::opt's for their settings (in an ideal world).

Requirements

- Make it easy for passes to declare arbitrary options, just like they do now
- Let different pass pipelines have different sets of option values
- Keep the option registry dynamic, so plugins loaded with "-load" can register new options
- Let option values be parseable from the command-line (for opt, llc, …)

All good requirements.

Implementation

As a first design draft, I propose that cl::opt and friends be extended to support argument storage in an LLVMContext. Instead of storing the value directly in a cl::opt instance or specifying a global variable, this new storage would utilize a StringMap stored in an LLVMContext. As a consequence, parsing would be delayed until the option is read.

I'd really like to avoid going here. With the direction sketched out above, is enough of your problem solved? Taking loop unroll as an example, I'd much rather you refactor the code to have its constructor take a configuration struct setting its various settings, than have the various settings pulled out to llvmcontext.

-Chris

Reply to list as I had originally intended…

It is generally convenient to decouple pass configuration from construction. CodeGen does this using the TargetPassConfig analysis. It allows pass instantiation to be configured using only Pass IDs.

Right now it’s only used for subtarget options. But it would be nice to unify all codegen options for transparency and discoverability. e.g. I need to know why clang/opt/llc is not generating the same code for some test case. -print-options should tell me everything that might have affected code generation.

-Andy

So the plan is to fold all options into TargetPassConfig (and an equivalent pass for target-independent optimizations)? If so, it would be nice to design this in such a way that you could pass arbitrary options to passes unknown at compile-time. For example, I may have a separate library of passes for my compiler that LLVMCodeGen may not know about. Right now I can add those passes just fine to TargetPassConfig, but I have no way of setting options on them with the current design (I could just configure the pass during construction, but this seems to fly against what you are proposing). Perhaps TargetPassConfig can hold a String->String map of options that a pass can query…