Optimization passes

LLVM gives you the opportunity to fine-tune optimization passes. Optimization passes are managed by a pass manager. There are two kinds of pass managers:

• FunctionPassManager, for optimizations that work on single functions.

• ModulePassManager, for optimizations that work on whole modules.

llvmlite provides bindings for LLVM’s New and Legacy pass managers, which have slightly different APIs and behaviour. The differences between them and the motivations for the New Pass Manager are outlined in the LLVM Blog post on the New Pass Manager.

In a future version of llvmlite, likely coinciding with a minimum LLVM version requirement of 17, support for the Legacy Pass Manager will be removed. It is recommended that new code using llvmlite uses the New Pass Manager, and existing code using the Legacy Pass Manager be updated to use the New Pass Manager.

New Pass Manager APIs

To manage the optimization attributes we first need to instantiate a PipelineTuningOptions instance:

class llvmlite.binding.PipelineTuningOptions(speed_level=2, size_level=0)

Creates a new PipelineTuningOptions object.

The following writable attributes are available, whose default values depend on the initial setting of the speed and size optimization levels:

• loop_interleaving

  Enable loop interleaving.

• loop_vectorization

  Enable loop vectorization.

• slp_vectorization

  Enable SLP vectorization, which uses a different algorithm to loop vectorization. Both may be enabled at the same time.

• loop_unrolling

  Enable loop unrolling.

• speed_level

  The level of optimization for speed, as an integer between 0 and 3.

• size_level

  The level of optimization for size, as an integer between 0 and 2.

We also need a PassBuilder object to manage the respective function and module pass managers:

class llvmlite.binding.PassBuilder(target_machine, pipeline_tuning_options)

A pass builder that uses the given TargetMachine and PipelineTuningOptions instances.

getModulePassManager()

Return a populated ModulePassManager object based on PTO settings.

getFunctionPassManager()

Return a populated FunctionPassManager object based on PTO settings.

The ModulePassManager and FunctionPassManager classes implement the module and function pass managers:

class llvmlite.binding.ModulePassManager

A pass manager for running optimization passes on an LLVM module.

add_verifier()

Add the Module Verifier pass.

run(module, passbuilder)

Run optimization passes on module, a ModuleRef instance.

class llvmlite.binding.FunctionPassManager

A pass manager for running optimization passes on an LLVM function.

run(function, passbuilder)

Run optimization passes on function, a ValueRef instance.

These can be created with passes populated by using the PassBuilder.getModulePassManager() and PassBuilder.getFunctionPassManager() methods, or they can be instantiated unpopulated, then passes can be added using the add_* methods.

To instantiate the unpopulated instances, use:

llvmlite.binding.create_new_module_pass_manager()

Create an unpopulated ModulePassManager instance.

and

llvmlite.binding.create_new_function_pass_manager()

Create an unpopulated FunctionPassManager instance.

The add_* methods supported by both pass manager classes are:

add_aa_eval_pass()

Add the Exhaustive Alias Analysis Precision Evaluator pass.

add_loop_unroll_pass()

Add the Loop Unroll pass.

add_loop_rotate_pass()

Add the Loop Rotate pass.

add_instruction_combine_pass()

Add the Combine Redundant Instructions pass.

add_jump_threading_pass()

Add the Jump Threading pass.

add_simplify_cfg_pass()

Add the Simplify CFG pass.

add_refprune_pass()

Add the Reference pruning pass.

Legacy Pass Manager APIs

To instantiate either of these pass managers, you first need to create and configure a PassManagerBuilder.

class llvmlite.binding.PassManagerBuilder

Create a new pass manager builder. This object centralizes optimization settings.

The populate method is available:

populate(pm)

Populate the pass manager pm with the optimization passes configured in this pass manager builder.

The following writable attributes are available:

• disable_unroll_loops

  If True, disable loop unrolling.

• inlining_threshold

  The integer threshold for inlining one function into another. The higher the number, the more likely that inlining will occur. This attribute is write-only.

• loop_vectorize

  If True, allow vectorizing loops.

• opt_level

  The general optimization level, as an integer between 0 and 3.

• size_level

  Whether and how much to optimize for size, as an integer between 0 and 2.

• slp_vectorize

  If True, enable the SLP vectorizer, which uses a different algorithm than the loop vectorizer. Both may be enabled at the same time.

class llvmlite.binding.PassManager

The base class for pass managers. Use individual add_* methods or PassManagerBuilder.populate() to add optimization passes.

• add_constant_merge_pass()

  See constmerge pass documentation.

• add_dead_arg_elimination_pass()

  See deadargelim pass documentation.

• add_function_attrs_pass()

  See functionattrs pass documentation.

• add_function_inlining_pass(self)

  See inline pass documentation.

• add_global_dce_pass()

  See globaldce pass documentation.

• add_global_optimizer_pass()

  See globalopt pass documentation.

• add_ipsccp_pass()

  See ipsccp pass documentation.

• add_dead_code_elimination_pass()

  See dce pass documentation.

• add_cfg_simplification_pass()

  See simplifycfg pass documentation.

• add_gvn_pass()

  See gvn pass documentation.

• add_instruction_combining_pass()

  See instcombine pass documentation.

• add_licm_pass()

  See licm pass documentation.

• add_sccp_pass()

  See sccp pass documentation.

• add_sroa_pass()

  See scalarrepl pass documentation.

  While the scalarrepl pass documentation describes the transformation performed by the pass added by this function, the pass corresponds to the opt -sroa command-line option and not to opt -scalarrepl.

• add_type_based_alias_analysis_pass()

  See tbaa metadata documentation.

• add_basic_alias_analysis_pass()

  See basicaa pass documentation.

• add_instruction_namer_pass()

  See instnamer pass documentation.

• add_refprune_pass()

  Add the Reference pruning pass.

class llvmlite.binding.ModulePassManager

Create a new pass manager to run optimization passes on a module.

The run method is available:

run(module)

Run optimization passes on the module, a ModuleRef instance.

Returns True if the optimizations made any modification to the module. Otherwise returns False.

class llvmlite.binding.FunctionPassManager(module)

Create a new pass manager to run optimization passes on a function of the given module, a ModuleRef instance.

The following methods are available:

• finalize()

  Run all the finalizers of the optimization passes.

• initialize()

  Run all the initializers of the optimization passes.

• run(function)

  Run optimization passes on function, a ValueRef instance.

  Returns True if the optimizations made any modification to the module. Otherwise returns False.