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7 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. Routines for emitting
diagnostic messages are defined in diagnostic.c.
The files
pretty-print.h and pretty-print.c provide basic support
for language-independent pretty-printing.
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). (see An
Inline Function is As Fast As a Macro).
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 See Cpplib
internals.
The source files to parse C are found in the toplevel directory,
and
by convention are named c-*.
Some of these are also used by
the other C-like languages: c-common.c,
c-common.def,
c-format.c,
c-opts.c,
c-pragma.c,
c-semantics.c,
c-lex.c,
c-incpath.c,
c-ppoutput.c,
c-cppbuiltin.c,
c-common.h,
c-dump.h,
c.opt,
c-incpath.h
and
c-pragma.h,
Files specific to each language are in subdirectories named
after the
language in question, like ada,
objc, cp (for C++).
- 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.
- Sibling 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.
- 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.
Second loop optimization pass takes care of basic block level
optimizations –
unrolling, peeling and unswitching loops. The source files are
cfgloopanal.c and cfgloopmanip.c containing generic loop
analysis and manipulation code, loop-init.c
with initialization and
finalization code, loop-unswitch.c
for loop unswitching and
loop-unroll.c for loop
unrolling and peeling.
The option -dL
causes a debugging dump of the RTL code after
these passes. The dump file names are made by appending `.loop' and
`.loop2' to the input file name.
- Jump bypassing. This pass is an aggressive form of GCSE that
transforms
the control flow graph of a function by propagating constants into
conditional branch instructions.
The source file for this pass is gcse.c.
The option -dG
causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.bypass'
to the input file name.
- Simple optimization pass that splits independent uses of each
pseudo
increasing effect of other optimizations. This can improve effect of
the
other transformation, such as CSE or register allocation. Its source
files are web.c.
The option -dZ
causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.web' 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 allocation. These passes make sure that all occurrences
of pseudo
registers are eliminated, either by allocating them to a hard register,
replacing them by an equivalent _expression_ (e.g. a constant) or by
placing
them on the stack. This is done in several subpasses:
- 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).
- Graph coloring register allocator. The files ra.c, ra-build.c,
ra-colorize.c, ra-debug.c, ra-rewrite.c
together with
the header ra.h contain another
register allocator, which is used
when the option -fnew-ra is
given. In that case it is run instead
of the above mentioned local and global register allocation passes, and
the
option -dl causes a debugging
dump of its work.
- 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 and information
about processor pipeline characteristics for the instruction
scheduler. It is generated from the machine description by the
program genattrtab.
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