Inter-procedural optimization passes#

The inter-procedural optimization (IPA) passes use call graph information to perform transformations across function boundaries. IPA is a critical part of link-time optimization (LTO) and whole-program (WHOPR) optimization, and these passes are structured with the needs of LTO and WHOPR in mind by dividing their operations into stages. For detailed discussion of the LTO/WHOPR IPA pass stages and interfaces, see Using summary information in IPA passes.

The following briefly describes the inter-procedural optimization (IPA) passes, which are split into small IPA passes, regular IPA passes, and late IPA passes, according to the LTO/WHOPR processing model.

Small IPA passes#

A small IPA pass is a pass derived from simple_ipa_opt_pass. As described in Using summary information in IPA passes, it does everything at once and defines only the Execute stage. During this stage it accesses and modifies the function bodies. No generate_summary, read_summary, or write_summary hooks are defined.

  • IPA free lang data

    This pass frees resources that are used by the front end but are not needed once it is done. It is located in tree.cc and is described by pass_ipa_free_lang_data.

  • IPA function and variable visibility

    This is a local function pass handling visibilities of all symbols. This happens before LTO streaming, so -fwhole-program should be ignored at this level. It is located in ipa-visibility.cc and is described by pass_ipa_function_and_variable_visibility.

  • IPA remove symbols

    This pass performs reachability analysis and reclaims all unreachable nodes. It is located in passes.cc and is described by pass_ipa_remove_symbols.

  • IPA OpenACC

    This is a pass group for OpenACC processing. It is located in tree-ssa-loop.cc and is described by pass_ipa_oacc.

  • IPA points-to analysis

    This is a tree-based points-to analysis pass. The idea behind this analyzer is to generate set constraints from the program, then solve the resulting constraints in order to generate the points-to sets. It is located in tree-ssa-structalias.cc and is described by pass_ipa_pta.

  • IPA OpenACC kernels

    This is a pass group for processing OpenACC kernels regions. It is a subpass of the IPA OpenACC pass group that runs on offloaded functions containing OpenACC kernels loops. It is located in tree-ssa-loop.cc and is described by pass_ipa_oacc_kernels.

  • Target clone

    This is a pass for parsing functions with multiple target attributes. It is located in multiple_target.cc and is described by pass_target_clone.

  • IPA auto profile

    This pass uses AutoFDO profiling data to annotate the control flow graph. It is located in auto-profile.cc and is described by pass_ipa_auto_profile.

  • IPA tree profile

    This pass does profiling for all functions in the call graph. It calculates branch probabilities and basic block execution counts. It is located in tree-profile.cc and is described by pass_ipa_tree_profile.

  • IPA free function summary

    This pass is a small IPA pass when argument small_p is true. It releases inline function summaries and call summaries. It is located in ipa-fnsummary.cc and is described by pass_ipa_free_free_fn_summary.

  • IPA increase alignment

    This pass increases the alignment of global arrays to improve vectorization. It is located in tree-vectorizer.cc and is described by pass_ipa_increase_alignment.

  • IPA transactional memory

    This pass is for transactional memory support. It is located in trans-mem.cc and is described by pass_ipa_tm.

  • IPA lower emulated TLS

    This pass lowers thread-local storage (TLS) operations to emulation functions provided by libgcc. It is located in tree-emutls.cc and is described by pass_ipa_lower_emutls.

Regular IPA passes#

A regular IPA pass is a pass derived from ipa_opt_pass_d that is executed in WHOPR compilation. Regular IPA passes may have summary hooks implemented in any of the LGEN, WPA or LTRANS stages (see Using summary information in IPA passes).

  • IPA whole program visibility

    This pass performs various optimizations involving symbol visibility with -fwhole-program, including symbol privatization, discovering local functions, and dismantling comdat groups. It is located in ipa-visibility.cc and is described by pass_ipa_whole_program_visibility.

  • IPA profile

    The IPA profile pass propagates profiling frequencies across the call graph. It is located in ipa-profile.cc and is described by pass_ipa_profile.

  • IPA identical code folding

    This is the inter-procedural identical code folding pass. The goal of this transformation is to discover functions and read-only variables that have exactly the same semantics. It is located in ipa-icf.cc and is described by pass_ipa_icf.

  • IPA devirtualization

    This pass performs speculative devirtualization based on the type inheritance graph. When a polymorphic call has only one likely target in the unit, it is turned into a speculative call. It is located in ipa-devirt.cc and is described by pass_ipa_devirt.

  • IPA constant propagation

    The goal of this pass is to discover functions that are always invoked with some arguments with the same known constant values and to modify the functions accordingly. It can also do partial specialization and type-based devirtualization. It is located in ipa-cp.cc and is described by pass_ipa_cp.

  • IPA scalar replacement of aggregates

    This pass can replace an aggregate parameter with a set of other parameters representing part of the original, turning those passed by reference into new ones which pass the value directly. It also removes unused function return values and unused function parameters. This pass is located in ipa-sra.cc and is described by pass_ipa_sra.

  • IPA constructor/destructor merge

    This pass merges multiple constructors and destructors for static objects into single functions. It’s only run at LTO time unless the target doesn’t support constructors and destructors natively. The pass is located in ipa.cc and is described by pass_ipa_cdtor_merge.

  • IPA function summary

    This pass provides function analysis for inter-procedural passes. It collects estimates of function body size, execution time, and frame size for each function. It also estimates information about function calls: call statement size, time and how often the parameters change for each call. It is located in ipa-fnsummary.cc and is described by pass_ipa_fn_summary.

  • IPA inline

    The IPA inline pass handles function inlining with whole-program knowledge. Small functions that are candidates for inlining are ordered in increasing badness, bounded by unit growth parameters. Unreachable functions are removed from the call graph. Functions called once and not exported from the unit are inlined. This pass is located in ipa-inline.cc and is described by pass_ipa_inline.

  • IPA pure/const analysis

    This pass marks functions as being either const (TREE_READONLY) or pure (DECL_PURE_P). The per-function information is produced by pure_const_generate_summary, then the global information is computed by performing a transitive closure over the call graph. It is located in ipa-pure-const.cc and is described by pass_ipa_pure_const.

  • IPA free function summary

    This pass is a regular IPA pass when argument small_p is false. It releases inline function summaries and call summaries. It is located in ipa-fnsummary.cc and is described by pass_ipa_free_fn_summary.

  • IPA reference

    This pass gathers information about how variables whose scope is confined to the compilation unit are used. It is located in ipa-reference.cc and is described by pass_ipa_reference.

  • IPA single use

    This pass checks whether variables are used by a single function. It is located in ipa.cc and is described by pass_ipa_single_use.

  • IPA comdats

    This pass looks for static symbols that are used exclusively within one comdat group, and moves them into that comdat group. It is located in ipa-comdats.cc and is described by pass_ipa_comdats.

Late IPA passes#

Late IPA passes are simple IPA passes executed after the regular passes. In WHOPR mode the passes are executed after partitioning and thus see just parts of the compiled unit.

  • Materialize all clones

    Once all functions from compilation unit are in memory, produce all clones and update all calls. It is located in ipa.cc and is described by pass_materialize_all_clones.

  • IPA points-to analysis

    Points-to analysis; this is the same as the points-to-analysis pass run with the small IPA passes (see Small IPA passes).

  • OpenMP simd clone

    This is the OpenMP constructs’ SIMD clone pass. It creates the appropriate SIMD clones for functions tagged as elemental SIMD functions. It is located in omp-simd-clone.cc and is described by pass_omp_simd_clone.