GNU Compiler Collection (GCC) Internals: Tree SSA passes

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### 9.5 Tree SSA passes

The following briefly describes the Tree optimization passes that are run after gimplification and what source files they are located in.

- Remove useless statements

  This pass is an extremely simple sweep across the gimple code in which we identify obviously dead code and remove it. Here we do things like simplify `if` statements with constant conditions, remove exception handling constructs surrounding code that obviously cannot throw, remove lexical bindings that contain no variables, and other assorted simplistic cleanups. The idea is to get rid of the obvious stuff quickly rather than wait until later when it’s more work to get rid of it. This pass is located in `tree-cfg.cc` and described by `pass_remove_useless_stmts`.

- OpenMP lowering

  If OpenMP generation (`-fopenmp`) is enabled, this pass lowers OpenMP constructs into GIMPLE.

  Lowering of OpenMP constructs involves creating replacement expressions for local variables that have been mapped using data sharing clauses, exposing the control flow of most synchronization directives and adding region markers to facilitate the creation of the control flow graph. The pass is located in `omp-low.cc` and is described by `pass_lower_omp`.

- OpenMP expansion

  If OpenMP generation (`-fopenmp`) is enabled, this pass expands parallel regions into their own functions to be invoked by the thread library. The pass is located in `omp-low.cc` and is described by `pass_expand_omp`.

- Lower control flow

  This pass flattens `if` statements (`COND_EXPR`) and moves lexical bindings (`BIND_EXPR`) out of line. After this pass, all `if` statements will have exactly two `goto` statements in its `then` and `else` arms. Lexical binding information for each statement will be found in `TREE_BLOCK` rather than being inferred from its position under a `BIND_EXPR`. This pass is found in `gimple-low.cc` and is described by `pass_lower_cf`.

- Lower exception handling control flow

  This pass decomposes high-level exception handling constructs (`TRY_FINALLY_EXPR` and `TRY_CATCH_EXPR`) into a form that explicitly represents the control flow involved. After this pass, `lookup_stmt_eh_region` will return a non-negative number for any statement that may have EH control flow semantics; examine `tree_can_throw_internal` or `tree_can_throw_external` for exact semantics. Exact control flow may be extracted from `foreach_reachable_handler`. The EH region nesting tree is defined in `except.h` and built in `except.cc`. The lowering pass itself is in `tree-eh.cc` and is described by `pass_lower_eh`.

- Build the control flow graph

  This pass decomposes a function into basic blocks and creates all of the edges that connect them. It is located in `tree-cfg.cc` and is described by `pass_build_cfg`.

- Find all referenced variables

  This pass walks the entire function and collects an array of all variables referenced in the function, `referenced_vars`. The index at which a variable is found in the array is used as a UID for the variable within this function. This data is needed by the SSA rewriting routines. The pass is located in `tree-dfa.cc` and is described by `pass_referenced_vars`.

- Enter static single assignment form

  This pass rewrites the function such that it is in SSA form. After this pass, all `is_gimple_reg` variables will be referenced by `SSA_NAME`, and all occurrences of other variables will be annotated with `VDEFS` and `VUSES`; PHI nodes will have been inserted as necessary for each basic block. This pass is located in `tree-ssa.cc` and is described by `pass_build_ssa`.

- Warn for uninitialized variables

  This pass scans the function for uses of `SSA_NAME`s that are fed by default definition. For non-parameter variables, such uses are uninitialized. The pass is run twice, before and after optimization (if turned on). In the first pass we only warn for uses that are positively uninitialized; in the second pass we warn for uses that are possibly uninitialized. The pass is located in `tree-ssa.cc` and is defined by `pass_early_warn_uninitialized` and `pass_late_warn_uninitialized`.

- Dead code elimination

  This pass scans the function for statements without side effects whose result is unused. It does not do memory life analysis, so any value that is stored in memory is considered used. The pass is run multiple times throughout the optimization process. It is located in `tree-ssa-dce.cc` and is described by `pass_dce`.

- Dominator optimizations

  This pass performs trivial dominator-based copy and constant propagation, expression simplification, and jump threading. It is run multiple times throughout the optimization process. It is located in `tree-ssa-dom.cc` and is described by `pass_dominator`.

- Forward propagation of single-use variables

  This pass attempts to remove redundant computation by substituting variables that are used once into the expression that uses them and seeing if the result can be simplified. It is located in `tree-ssa-forwprop.cc` and is described by `pass_forwprop`.

- Copy Renaming

  This pass attempts to change the name of compiler temporaries involved in copy operations such that SSA->normal can coalesce the copy away. When compiler temporaries are copies of user variables, it also renames the compiler temporary to the user variable resulting in better use of user symbols. It is located in `tree-ssa-copyrename.c` and is described by `pass_copyrename`.

- PHI node optimizations

  This pass recognizes forms of PHI inputs that can be represented as conditional expressions and rewrites them into straight line code. It is located in `tree-ssa-phiopt.cc` and is described by `pass_phiopt`.

- May-alias optimization

  This pass performs a flow sensitive SSA-based points-to analysis. The resulting may-alias, must-alias, and escape analysis information is used to promote variables from in-memory addressable objects to non-aliased variables that can be renamed into SSA form. We also update the `VDEF`/`VUSE` memory tags for non-renameable aggregates so that we get fewer false kills. The pass is located in `tree-ssa-alias.cc` and is described by `pass_may_alias`.

  Interprocedural points-to information is located in `tree-ssa-structalias.cc` and described by `pass_ipa_pta`.

- Profiling

  This pass instruments the function in order to collect runtime block and value profiling data. Such data may be fed back into the compiler on a subsequent run so as to allow optimization based on expected execution frequencies. The pass is located in `tree-profile.cc` and is described by `pass_ipa_tree_profile`.

- Static profile estimation

  This pass implements series of heuristics to guess propababilities of branches. The resulting predictions are turned into edge profile by propagating branches across the control flow graphs. The pass is located in `tree-profile.cc` and is described by `pass_profile`.

- Lower complex arithmetic

  This pass rewrites complex arithmetic operations into their component scalar arithmetic operations. The pass is located in `tree-complex.cc` and is described by `pass_lower_complex`.

- Scalar replacement of aggregates

  This pass rewrites suitable non-aliased local aggregate variables into a set of scalar variables. The resulting scalar variables are rewritten into SSA form, which allows subsequent optimization passes to do a significantly better job with them. The pass is located in `tree-sra.cc` and is described by `pass_sra`.

- Dead store elimination

  This pass eliminates stores to memory that are subsequently overwritten by another store, without any intervening loads. The pass is located in `tree-ssa-dse.cc` and is described by `pass_dse`.

- Tail recursion elimination

  This pass transforms tail recursion into a loop. It is located in `tree-tailcall.cc` and is described by `pass_tail_recursion`.

- Forward store motion

  This pass sinks stores and assignments down the flowgraph closer to their use point. The pass is located in `tree-ssa-sink.cc` and is described by `pass_sink_code`.

- Partial redundancy elimination

  This pass eliminates partially redundant computations, as well as performing load motion. The pass is located in `tree-ssa-pre.cc` and is described by `pass_pre`.

  Just before partial redundancy elimination, if `-funsafe-math-optimizations` is on, GCC tries to convert divisions to multiplications by the reciprocal. The pass is located in `tree-ssa-math-opts.cc` and is described by `pass_cse_reciprocal`.

- Full redundancy elimination

  This is a simpler form of PRE that only eliminates redundancies that occur on all paths. It is located in `tree-ssa-pre.cc` and described by `pass_fre`.

- Loop optimization

  The main driver of the pass is placed in `tree-ssa-loop.cc` and described by `pass_loop`.

  The optimizations performed by this pass are:

  Loop invariant motion. This pass moves only invariants that would be hard to handle on RTL level (function calls, operations that expand to nontrivial sequences of insns). With `-funswitch-loops` it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion. The pass is implemented in `tree-ssa-loop-im.cc`.

  Canonical induction variable creation. This pass creates a simple counter for number of iterations of the loop and replaces the exit condition of the loop using it, in case when a complicated analysis is necessary to determine the number of iterations. Later optimizations then may determine the number easily. The pass is implemented in `tree-ssa-loop-ivcanon.cc`.

  Induction variable optimizations. This pass performs standard induction variable optimizations, including strength reduction, induction variable merging and induction variable elimination. The pass is implemented in `tree-ssa-loop-ivopts.cc`.

  Loop unswitching. This pass moves the conditional jumps that are invariant out of the loops. To achieve this, a duplicate of the loop is created for each possible outcome of conditional jump(s). The pass is implemented in `tree-ssa-loop-unswitch.cc`.

  Loop splitting. If a loop contains a conditional statement that is always true for one part of the iteration space and false for the other this pass splits the loop into two, one dealing with one side the other only with the other, thereby removing one inner-loop conditional. The pass is implemented in `tree-ssa-loop-split.cc`.

  The optimizations also use various utility functions contained in `tree-ssa-loop-manip.cc`, `cfgloop.cc`, `cfgloopanal.cc` and `cfgloopmanip.cc`.

  Vectorization. This pass transforms loops to operate on vector types instead of scalar types. Data parallelism across loop iterations is exploited to group data elements from consecutive iterations into a vector and operate on them in parallel. Depending on available target support the loop is conceptually unrolled by a factor `VF` (vectorization factor), which is the number of elements operated upon in parallel in each iteration, and the `VF` copies of each scalar operation are fused to form a vector operation. Additional loop transformations such as peeling and versioning may take place to align the number of iterations, and to align the memory accesses in the loop. The pass is implemented in `tree-vectorizer.cc` (the main driver), `tree-vect-loop.cc` and `tree-vect-loop-manip.cc` (loop specific parts and general loop utilities), `tree-vect-slp` (loop-aware SLP functionality), `tree-vect-stmts.cc`, `tree-vect-data-refs.cc` and `tree-vect-slp-patterns.cc` containing the SLP pattern matcher. Analysis of data references is in `tree-data-ref.cc`.

  SLP Vectorization. This pass performs vectorization of straight-line code. The pass is implemented in `tree-vectorizer.cc` (the main driver), `tree-vect-slp.cc`, `tree-vect-stmts.cc` and `tree-vect-data-refs.cc`.

  Autoparallelization. This pass splits the loop iteration space to run into several threads. The pass is implemented in `tree-parloops.cc`.

  Graphite is a loop transformation framework based on the polyhedral model. Graphite stands for Gimple Represented as Polyhedra. The internals of this infrastructure are documented in https://gcc.gnu.org/wiki/Graphite. The passes working on this representation are implemented in the various `graphite-*` files.

- Tree level if-conversion for vectorizer

  This pass applies if-conversion to simple loops to help vectorizer. We identify if convertible loops, if-convert statements and merge basic blocks in one big block. The idea is to present loop in such form so that vectorizer can have one to one mapping between statements and available vector operations. This pass is located in `tree-if-conv.cc` and is described by `pass_if_conversion`.

- Conditional constant propagation

  This pass relaxes a lattice of values in order to identify those that must be constant even in the presence of conditional branches. The pass is located in `tree-ssa-ccp.cc` and is described by `pass_ccp`.

  A related pass that works on memory loads and stores, and not just register values, is located in `tree-ssa-ccp.cc` and described by `pass_store_ccp`.

- Conditional copy propagation

  This is similar to constant propagation but the lattice of values is the “copy-of” relation. It eliminates redundant copies from the code. The pass is located in `tree-ssa-copy.cc` and described by `pass_copy_prop`.

  A related pass that works on memory copies, and not just register copies, is located in `tree-ssa-copy.cc` and described by `pass_store_copy_prop`.

- Value range propagation

  This transformation is similar to constant propagation but instead of propagating single constant values, it propagates known value ranges. The implementation is based on Patterson’s range propagation algorithm (Accurate Static Branch Prediction by Value Range Propagation, J. R. C. Patterson, PLDI ’95). In contrast to Patterson’s algorithm, this implementation does not propagate branch probabilities nor it uses more than a single range per SSA name. This means that the current implementation cannot be used for branch prediction (though adapting it would not be difficult). The pass is located in `tree-vrp.cc` and is described by `pass_vrp`.

- Folding built-in functions

  This pass simplifies built-in functions, as applicable, with constant arguments or with inferable string lengths. It is located in `tree-ssa-ccp.cc` and is described by `pass_fold_builtins`.

- Split critical edges

  This pass identifies critical edges and inserts empty basic blocks such that the edge is no longer critical. The pass is located in `tree-cfg.cc` and is described by `pass_split_crit_edges`.

- Control dependence dead code elimination

  This pass is a stronger form of dead code elimination that can eliminate unnecessary control flow statements. It is located in `tree-ssa-dce.cc` and is described by `pass_cd_dce`.

- Tail call elimination

  This pass identifies function calls that may be rewritten into jumps. No code transformation is actually applied here, but the data and control flow problem is solved. The code transformation requires target support, and so is delayed until RTL. In the meantime `CALL_EXPR_TAILCALL` is set indicating the possibility. The pass is located in `tree-tailcall.cc` and is described by `pass_tail_calls`. The RTL transformation is handled by `fixup_tail_calls` in `calls.cc`.

- Warn for function return without value

  For non-void functions, this pass locates return statements that do not specify a value and issues a warning. Such a statement may have been injected by falling off the end of the function. This pass is run last so that we have as much time as possible to prove that the statement is not reachable. It is located in `tree-cfg.cc` and is described by `pass_warn_function_return`.

- Leave static single assignment form

  This pass rewrites the function such that it is in normal form. At the same time, we eliminate as many single-use temporaries as possible, so the intermediate language is no longer GIMPLE, but GENERIC. The pass is located in `tree-outof-ssa.cc` and is described by `pass_del_ssa`.

- Merge PHI nodes that feed into one another

  This is part of the CFG cleanup passes. It attempts to join PHI nodes from a forwarder CFG block into another block with PHI nodes. The pass is located in `tree-cfgcleanup.cc` and is described by `pass_merge_phi`.

- Return value optimization

  If a function always returns the same local variable, and that local variable is an aggregate type, then the variable is replaced with the return value for the function (i.e., the function’s DECL_RESULT). This is equivalent to the C++ named return value optimization applied to GIMPLE. The pass is located in `tree-nrv.cc` and is described by `pass_nrv`.

- Return slot optimization

  If a function returns a memory object and is called as `var = foo()`, this pass tries to change the call so that the address of `var` is sent to the caller to avoid an extra memory copy. This pass is located in `tree-nrv.cc` and is described by `pass_return_slot`.

- Optimize calls to `__builtin_object_size`

  This is a propagation pass similar to CCP that tries to remove calls to `__builtin_object_size` when the size of the object can be computed at compile-time. This pass is located in `tree-object-size.cc` and is described by `pass_object_sizes`.

- Loop invariant motion

  This pass removes expensive loop-invariant computations out of loops. The pass is located in `tree-ssa-loop.cc` and described by `pass_lim`.

- Loop nest optimizations

  This is a family of loop transformations that works on loop nests. It includes loop interchange, scaling, skewing and reversal and they are all geared to the optimization of data locality in array traversals and the removal of dependencies that hamper optimizations such as loop parallelization and vectorization. The pass is located in `tree-loop-linear.c` and described by `pass_linear_transform`.

- Removal of empty loops

  This pass removes loops with no code in them. The pass is located in `tree-ssa-loop-ivcanon.cc` and described by `pass_empty_loop`.

- Unrolling of small loops

  This pass completely unrolls loops with few iterations. The pass is located in `tree-ssa-loop-ivcanon.cc` and described by `pass_complete_unroll`.

- Predictive commoning

  This pass makes the code reuse the computations from the previous iterations of the loops, especially loads and stores to memory. It does so by storing the values of these computations to a bank of temporary variables that are rotated at the end of loop. To avoid the need for this rotation, the loop is then unrolled and the copies of the loop body are rewritten to use the appropriate version of the temporary variable. This pass is located in `tree-predcom.cc` and described by `pass_predcom`.

- Array prefetching

  This pass issues prefetch instructions for array references inside loops. The pass is located in `tree-ssa-loop-prefetch.cc` and described by `pass_loop_prefetch`.

- Reassociation

  This pass rewrites arithmetic expressions to enable optimizations that operate on them, like redundancy elimination and vectorization. The pass is located in `tree-ssa-reassoc.cc` and described by `pass_reassoc`.

- Optimization of `stdarg` functions

  This pass tries to avoid the saving of register arguments into the stack on entry to `stdarg` functions. If the function doesn’t use any `va_start` macros, no registers need to be saved. If `va_start` macros are used, the `va_list` variables don’t escape the function, it is only necessary to save registers that will be used in `va_arg` macros. For instance, if `va_arg` is only used with integral types in the function, floating point registers don’t need to be saved. This pass is located in `tree-stdarg.cc` and described by `pass_stdarg`.

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