gccint.info: Passes

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Passes and Files of the Compiler

   The overall control structure of the compiler is in `toplev.c'.  This
file is responsible for initialization, decoding arguments, opening and
closing files, and sequencing the passes.
   The parsing pass is invoked only once, to parse the entire input.  A
high level tree representation is then generated from the input, one
function at a time.  This tree code is then transformed into RTL
intermediate code, and processed.  The files involved in transforming
the trees into RTL are `expr.c', `expmed.c', and `stmt.c'.  The order
of trees that are processed, is not necessarily the same order they are
generated from the input, due to deferred inlining, and other
considerations.
   Each time the parsing pass reads a complete function definition or
top-level declaration, it calls either the function
`rest_of_compilation', or the function `rest_of_decl_compilation' in
`toplev.c', which are responsible for all further processing necessary,
ending with output of the assembler language.  All other compiler
passes run, in sequence, within `rest_of_compilation'.  When that
function returns from compiling a function definition, the storage used
for that function definition's compilation is entirely freed, unless it
is an inline function, or was deferred for some reason (this can occur
in templates, for example).  (*note An Inline Function is As Fast As a
Macro: (gcc)Inline.).
   Here is a list of all the passes of the compiler and their source
files.  Also included is a description of where debugging dumps can be
requested with `-d' options.
   * Parsing.  This pass reads the entire text of a function definition,
     constructing a high level tree representation.  (Because of the
     semantic analysis that takes place during this pass, it does more
     than is formally considered to be parsing.)
     The tree representation does not entirely follow C syntax, because
     it is intended to support other languages as well.
     Language-specific data type analysis is also done in this pass,
     and every tree node that represents an expression has a data type
     attached.  Variables are represented as declaration nodes.
     The language-independent source files for parsing are `tree.c',
     `fold-const.c', and `stor-layout.c'.  There are also header files
     `tree.h' and `tree.def' which define the format of the tree
     representation.
     C preprocessing, for language front ends, that want or require it,
     is performed by cpplib, which is covered in separate
     documentation.  In particular, the internals are covered in *Note
     Cpplib internals: (cppinternals)Top.
     The source files to parse C are `c-convert.c', `c-decl.c',
     `c-errors.c', `c-lang.c', `c-objc-common.c', `c-parse.in',
     `c-aux-info.c', and `c-typeck.c', along with a header file
     `c-tree.h' and some files shared with Objective-C and C++.
     The source files for parsing C++ are in `cp/'.  They are `parse.y',
     `class.c', `cvt.c', `decl.c', `decl2.c', `except.c', `expr.c',
     `init.c', `lex.c', `method.c', `ptree.c', `search.c', `spew.c',
     `semantics.c', `tree.c', `typeck2.c', and `typeck.c', along with
     header files `cp-tree.def', `cp-tree.h', and `decl.h'.
     The special source files for parsing Objective-C are in `objc/'.
     They are `objc-act.c', `objc-tree.def', and `objc-act.h'.  Certain
     C-specific files are used for this as well.
     The files `c-common.c', `c-common.def', `c-format.c', `c-pragma.c',
     `c-semantics.c', and `c-lex.c', along with header files
     `c-common.h', `c-dump.h', `c-lex.h', and `c-pragma.h', are also
     used for all of the above languages.
   * Tree optimization.   This is the optimization of the tree
     representation, before converting into RTL code.
     Currently, the main optimization performed here is tree-based
     inlining.  This is implemented in `tree-inline.c' and used by both
     C and C++.  Note that tree based inlining turns off rtx based
     inlining (since it's more powerful, it would be a waste of time to
     do rtx based inlining in addition).
     Constant folding and some arithmetic simplifications are also done
     during this pass, on the tree representation.  The routines that
     perform these tasks are located in `fold-const.c'.
   * RTL generation.  This is the conversion of syntax tree into RTL
     code.
     This is where the bulk of target-parameter-dependent code is found,
     since often it is necessary for strategies to apply only when
     certain standard kinds of instructions are available.  The purpose
     of named instruction patterns is to provide this information to
     the RTL generation pass.
     Optimization is done in this pass for `if'-conditions that are
     comparisons, boolean operations or conditional expressions.  Tail
     recursion is detected at this time also.  Decisions are made about
     how best to arrange loops and how to output `switch' statements.
     The source files for RTL generation include `stmt.c', `calls.c',
     `expr.c', `explow.c', `expmed.c', `function.c', `optabs.c' and
     `emit-rtl.c'.  Also, the file `insn-emit.c', generated from the
     machine description by the program `genemit', is used in this
     pass.  The header file `expr.h' is used for communication within
     this pass.
     The header files `insn-flags.h' and `insn-codes.h', generated from
     the machine description by the programs `genflags' and `gencodes',
     tell this pass which standard names are available for use and
     which patterns correspond to them.
     Aside from debugging information output, none of the following
     passes refers to the tree structure representation of the function
     (only part of which is saved).
     The decision of whether the function can and should be expanded
     inline in its subsequent callers is made at the end of rtl
     generation.  The function must meet certain criteria, currently
     related to the size of the function and the types and number of
     parameters it has.  Note that this function may contain loops,
     recursive calls to itself (tail-recursive functions can be
     inlined!), gotos, in short, all constructs supported by GCC.  The
     file `integrate.c' contains the code to save a function's rtl for
     later inlining and to inline that rtl when the function is called.
     The header file `integrate.h' is also used for this purpose.
     The option `-dr' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.rtl' to
     the input file name.
   * Sibiling call optimization.   This pass performs tail recursion
     elimination, and tail and sibling call optimizations.  The purpose
     of these optimizations is to reduce the overhead of function calls,
     whenever possible.
     The source file of this pass is `sibcall.c'
     The option `-di' causes a debugging dump of the RTL code after
     this pass is run.  This dump file's name is made by appending
     `.sibling' to the input file name.
   * Jump optimization.  This pass simplifies jumps to the following
     instruction, jumps across jumps, and jumps to jumps.  It deletes
     unreferenced labels and unreachable code, except that unreachable
     code that contains a loop is not recognized as unreachable in this
     pass.  (Such loops are deleted later in the basic block analysis.)
     It also converts some code originally written with jumps into
     sequences of instructions that directly set values from the
     results of comparisons, if the machine has such instructions.
     Jump optimization is performed two or three times.  The first time
     is immediately following RTL generation.  The second time is after
     CSE, but only if CSE says repeated jump optimization is needed.
     The last time is right before the final pass.  That time,
     cross-jumping and deletion of no-op move instructions are done
     together with the optimizations described above.
     The source file of this pass is `jump.c'.
     The option `-dj' causes a debugging dump of the RTL code after
     this pass is run for the first time.  This dump file's name is
     made by appending `.jump' to the input file name.
   * Register scan.  This pass finds the first and last use of each
     register, as a guide for common subexpression elimination.  Its
     source is in `regclass.c'.
   * Jump threading.  This pass detects a condition jump that branches
     to an identical or inverse test.  Such jumps can be `threaded'
     through the second conditional test.  The source code for this
     pass is in `jump.c'.  This optimization is only performed if
     `-fthread-jumps' is enabled.
   * Static Single Assignment (SSA) based optimization passes.  The SSA
     conversion passes (to/from) are turned on by the `-fssa' option
     (it is also done automatically if you enable an SSA optimization
     pass).  These passes utilize a form called Static Single
     Assignment.  In SSA form, each variable (pseudo register) is only
     set once, giving you def-use and use-def chains for free, and
     enabling a lot more optimization passes to be run in linear time.
     Conversion to and from SSA form is handled by functions in `ssa.c'.
     The option `-de' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.ssa' to
     the input file name.
        * SSA Conditional Constant Propagation.  Turned on by the
          `-fssa-ccp' option.  This pass performs conditional constant
          propagation to simplify instructions including conditional
          branches.  This pass is more aggressive than the constant
          propagation done by the CSE and GCSE passes, but operates in
          linear time.
          The option `-dW' causes a debugging dump of the RTL code after
          this pass.  This dump file's name is made by appending
          `.ssaccp' to the input file name.
        * SSA Aggressive Dead Code Elimination.  Turned on by the
          `-fssa-dce' option.  This pass performs elimination of code
          considered unnecessary because it has no externally visible
          effects on the program.  It operates in linear time.
          The option `-dX' causes a debugging dump of the RTL code after
          this pass.  This dump file's name is made by appending
          `.ssadce' to the input file name.
   * Common subexpression elimination.  This pass also does constant
     propagation.  Its source files are `cse.c', and `cselib.c'.  If
     constant  propagation causes conditional jumps to become
     unconditional or to become no-ops, jump optimization is run again
     when CSE is finished.
     The option `-ds' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.cse' to
     the input file name.
   * Global common subexpression elimination.  This pass performs two
     different types of GCSE  depending on whether you are optimizing
     for size or not (LCM based GCSE tends to increase code size for a
     gain in speed, while Morel-Renvoise based GCSE does not).  When
     optimizing for size, GCSE is done using Morel-Renvoise Partial
     Redundancy Elimination, with the exception that it does not try to
     move invariants out of loops--that is left to  the loop
     optimization pass.  If MR PRE GCSE is done, code hoisting (aka
     unification) is also done, as well as load motion.  If you are
     optimizing for speed, LCM (lazy code motion) based GCSE is done.
     LCM is based on the work of Knoop, Ruthing, and Steffen.  LCM
     based GCSE also does loop invariant code motion.  We also perform
     load and store motion when optimizing for speed.  Regardless of
     which type of GCSE is used, the GCSE pass also performs global
     constant and  copy propagation.
     The source file for this pass is `gcse.c', and the LCM routines
     are in `lcm.c'.
     The option `-dG' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.gcse' to
     the input file name.
   * Loop optimization.  This pass moves constant expressions out of
     loops, and optionally does strength-reduction and loop unrolling
     as well.  Its source files are `loop.c' and `unroll.c', plus the
     header `loop.h' used for communication between them.  Loop
     unrolling uses some functions in `integrate.c' and the header
     `integrate.h'.  Loop dependency analysis routines are contained in
     `dependence.c'.
     The option `-dL' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.loop' to
     the input file name.
   * If `-frerun-cse-after-loop' was enabled, a second common
     subexpression elimination pass is performed after the loop
     optimization pass.  Jump threading is also done again at this time
     if it was specified.
     The option `-dt' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.cse2' to
     the input file name.
   * Data flow analysis (`flow.c').  This pass divides the program into
     basic blocks (and in the process deletes unreachable loops); then
     it computes which pseudo-registers are live at each point in the
     program, and makes the first instruction that uses a value point at
     the instruction that computed the value.
     This pass also deletes computations whose results are never used,
     and combines memory references with add or subtract instructions
     to make autoincrement or autodecrement addressing.
     The option `-df' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.flow' to
     the input file name.  If stupid register allocation is in use, this
     dump file reflects the full results of such allocation.
   * Instruction combination (`combine.c').  This pass attempts to
     combine groups of two or three instructions that are related by
     data flow into single instructions.  It combines the RTL
     expressions for the instructions by substitution, simplifies the
     result using algebra, and then attempts to match the result
     against the machine description.
     The option `-dc' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.combine'
     to the input file name.
   * If-conversion is a transformation that transforms control
     dependencies into data dependencies (IE it transforms conditional
     code into a single control stream).  It is implemented in the file
     `ifcvt.c'.
     The option `-dE' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.ce' to
     the input file name.
   * Register movement (`regmove.c').  This pass looks for cases where
     matching constraints would force an instruction to need a reload,
     and this reload would be a register-to-register move.  It then
     attempts to change the registers used by the instruction to avoid
     the move instruction.
     The option `-dN' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.regmove'
     to the input file name.
   * Instruction scheduling (`sched.c').  This pass looks for
     instructions whose output will not be available by the time that
     it is used in subsequent instructions.  (Memory loads and floating
     point instructions often have this behavior on RISC machines).  It
     re-orders instructions within a basic block to try to separate the
     definition and use of items that otherwise would cause pipeline
     stalls.
     Instruction scheduling is performed twice.  The first time is
     immediately after instruction combination and the second is
     immediately after reload.
     The option `-dS' causes a debugging dump of the RTL code after this
     pass is run for the first time.  The dump file's name is made by
     appending `.sched' to the input file name.
   * Register class preferencing.  The RTL code is scanned to find out
     which register class is best for each pseudo register.  The source
     file is `regclass.c'.
   * Local register allocation (`local-alloc.c').  This pass allocates
     hard registers to pseudo registers that are used only within one
     basic block.  Because the basic block is linear, it can use fast
     and powerful techniques to do a very good job.
     The option `-dl' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.lreg' to
     the input file name.
   * Global register allocation (`global.c').  This pass allocates hard
     registers for the remaining pseudo registers (those whose life
     spans are not contained in one basic block).
   * Reloading.  This pass renumbers pseudo registers with the hardware
     registers numbers they were allocated.  Pseudo registers that did
     not get hard registers are replaced with stack slots.  Then it
     finds instructions that are invalid because a value has failed to
     end up in a register, or has ended up in a register of the wrong
     kind.  It fixes up these instructions by reloading the
     problematical values temporarily into registers.  Additional
     instructions are generated to do the copying.
     The reload pass also optionally eliminates the frame pointer and
     inserts instructions to save and restore call-clobbered registers
     around calls.
     Source files are `reload.c' and `reload1.c', plus the header
     `reload.h' used for communication between them.
     The option `-dg' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.greg' to
     the input file name.
   * Instruction scheduling is repeated here to try to avoid pipeline
     stalls due to memory loads generated for spilled pseudo registers.
     The option `-dR' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.sched2'
     to the input file name.
   * Basic block reordering.  This pass implements profile guided code
     positioning.  If profile information is not available, various
     types of static analysis are performed to make the predictions
     normally coming from the profile feedback (IE execution frequency,
     branch probability, etc).  It is implemented in the file
     `bb-reorder.c', and the various prediction routines are in
     `predict.c'.
     The option `-dB' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.bbro' to
     the input file name.
   * Delayed branch scheduling.  This optional pass attempts to find
     instructions that can go into the delay slots of other
     instructions, usually jumps and calls.  The source file name is
     `reorg.c'.
     The option `-dd' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.dbr' to
     the input file name.
   * Branch shortening.  On many RISC machines, branch instructions
     have a limited range.  Thus, longer sequences of instructions must
     be used for long branches.  In this pass, the compiler figures out
     what how far each instruction will be from each other instruction,
     and therefore whether the usual instructions, or the longer
     sequences, must be used for each branch.
   * Conversion from usage of some hard registers to usage of a register
     stack may be done at this point.  Currently, this is supported only
     for the floating-point registers of the Intel 80387 coprocessor.
     The source file name is `reg-stack.c'.
     The options `-dk' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.stack' to
     the input file name.
   * Final.  This pass outputs the assembler code for the function.  It
     is also responsible for identifying spurious test and compare
     instructions.  Machine-specific peephole optimizations are
     performed at the same time.  The function entry and exit sequences
     are generated directly as assembler code in this pass; they never
     exist as RTL.
     The source files are `final.c' plus `insn-output.c'; the latter is
     generated automatically from the machine description by the tool
     `genoutput'.  The header file `conditions.h' is used for
     communication between these files.
   * Debugging information output.  This is run after final because it
     must output the stack slot offsets for pseudo registers that did
     not get hard registers.  Source files are `dbxout.c' for DBX
     symbol table format, `sdbout.c' for SDB symbol table format,
     `dwarfout.c' for DWARF symbol table format, files `dwarf2out.c' and
     `dwarf2asm.c' for DWARF2 symbol table format, and `vmsdbgout.c'
     for VMS debug symbol table format.
   Some additional files are used by all or many passes:
   * Every pass uses `machmode.def' and `machmode.h' which define the
     machine modes.
   * Several passes use `real.h', which defines the default
     representation of floating point constants and how to operate on
     them.
   * All the passes that work with RTL use the header files `rtl.h' and
     `rtl.def', and subroutines in file `rtl.c'.  The tools `gen*' also
     use these files to read and work with the machine description RTL.
   * All the tools that read the machine description use support
     routines found in `gensupport.c', `errors.c', and `read-rtl.c'.
   * Several passes refer to the header file `insn-config.h' which
     contains a few parameters (C macro definitions) generated
     automatically from the machine description RTL by the tool
     `genconfig'.
   * Several passes use the instruction recognizer, which consists of
     `recog.c' and `recog.h', plus the files `insn-recog.c' and
     `insn-extract.c' that are generated automatically from the machine
     description by the tools `genrecog' and `genextract'.
   * Several passes use the header files `regs.h' which defines the
     information recorded about pseudo register usage, and
     `basic-block.h' which defines the information recorded about basic
     blocks.
   * `hard-reg-set.h' defines the type `HARD_REG_SET', a bit-vector
     with a bit for each hard register, and some macros to manipulate
     it.  This type is just `int' if the machine has few enough hard
     registers; otherwise it is an array of `int' and some of the
     macros expand into loops.
   * Several passes use instruction attributes.  A definition of the
     attributes defined for a particular machine is in file
     `insn-attr.h', which is generated from the machine description by
     the program `genattr'.  The file `insn-attrtab.c' contains
     subroutines to obtain the attribute values for insns.  It is
     generated from the machine description by the program `genattrtab'.