GNU Compiler Collection (GCC) Internals: Regular IPA passes

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#### 9.4.2 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 IPA).

- 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`.

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