V10/cmd/gcc/internals-3
File: internals, Node: Passes, Next: RTL, Prev: Interface, Up: Top
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. The RTL
intermediate code for a function is generated as the function is parsed, a
statement at a time. Each statement is read in as a syntax tree and then
converted to RTL; then the storage for the tree for the statement is
reclaimed. Storage for types (and the expressions for their sizes),
declarations, and a representation of the binding contours and how they
nest, remains until the function is finished being compiled; these are all
needed to output the debugging information.
Each time the parsing pass reads a complete function definition or
top-level declaration, it calls the function `rest_of_compilation' or
`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 (*Note 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 partial syntax trees. This and RTL generation are no
longer truly separate passes (formerly they were), but it is easier to
think of them as separate.
The tree representation does not entirely follow C syntax, because it
is intended to support other languages as well.
C 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.
Constant folding and associative-law simplifications are also done
during this pass.
The source files for parsing are `parse.y', `decl.c', `typecheck.c',
`stor-layout.c', `fold-const.c', and `tree.c'. The last three are
intended to be language-independent. There are also header files
`parse.h', `c-tree.h', `tree.h' and `tree.def'. The last two define
the format of the tree representation.
* RTL generation. This is the conversion of syntax tree into RTL code.
It is actually done statement-by-statement during parsing, but for
most purposes it can be thought of as a separate pass.
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 are `stmt.c', `expr.c',
`explow.c', `expmed.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 files `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 GNU CC.
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.
* 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.)
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'.
* Common subexpression elimination. This pass also does constant
propagation. Its source file is `cse.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.
* Loop optimization. This pass moves constant expressions out of loops.
Its source file is `loop.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.
* Stupid register allocation is performed at this point in a
nonoptimizing compilation. It does a little data flow analysis as
well. When stupid register allocation is in use, the next pass
executed is the reloading pass; the others in between are skipped.
The source file is `stupid.c'.
* 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.
* 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-alloc.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.
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.
* Jump optimization is repeated, this time including cross-jumping and
deletion of no-op move instructions. Machine-specific peephole
optimizations are performed at the same time.
The option `-dJ' causes a debugging dump of the RTL code after this
pass. This dump file's name is made by appending `.jump2' 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. 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 and `symout.c' for GDB's own symbol table format.
Some additional files are used by all or many passes:
* Every pass uses `machmode.def', which defines the machine modes.
* 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.
* 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.
File: internals, Node: RTL, Next: Machine Desc, Prev: Passes, Up: Top
RTL Representation
******************
Most of the work of the compiler is done on an intermediate representation
called register transfer language. In this language, the instructions to
be output are described, pretty much one by one, in an algebraic form that
describes what the instruction does.
RTL is inspired by Lisp lists. It has both an internal form, made up of
structures that point at other structures, and a textual form that is used
in the machine description and in printed debugging dumps. The textual
form uses nested parentheses to indicate the pointers in the internal form.
* Menu:
* RTL Objects:: Expressions vs vectors vs strings vs integers.
* Accessors:: Macros to access expression operands or vector elts.
* Flags:: Other flags in an RTL expression.
* Machine Modes:: Describing the size and format of a datum.
* Constants:: Expressions with constant values.
* Regs and Memory:: Expressions representing register contents or memory.
* Arithmetic:: Expressions representing arithmetic on other expressions.
* Comparisons:: Expressions representing comparison of expressions.
* Bit Fields:: Expressions representing bit-fields in memory or reg.
* Conversions:: Extending, truncating, floating or fixing.
* RTL Declarations:: Declaring volatility, constancy, etc.
* Side Effects:: Expressions for storing in registers, etc.
* Incdec:: Embedded side-effects for autoincrement addressing.
* Assembler:: Representing `asm' with operands.
* Insns:: Expression types for entire insns.
* Calls:: RTL representation of function call insns.
* Sharing:: Some expressions are unique; others *must* be copied.
File: internals, Node: RTL Objects, Next: Accessors, Prev: RTL, Up: RTL
RTL Object Types
================
RTL uses four kinds of objects: expressions, integers, strings and vectors.
Expressions are the most important ones. An RTL expression (``RTX'', for
short) is a C structure, but it is usually referred to with a pointer; a
type that is given the typedef name `rtx'.
An integer is simply an `int', and a string is a `char *'. Within RTL
code, strings appear only inside `symbol_ref' expressions, but they appear
in other contexts in the RTL expressions that make up machine descriptions.
Their written form uses decimal digits.
A string is a sequence of characters. In core it is represented as a `char
*' in usual C fashion, and it is written in C syntax as well. However,
strings in RTL may never be null. If you write an empty string in a
machine description, it is represented in core as a null pointer rather
than as a pointer to a null character. In certain contexts, these null
pointers instead of strings are valid.
A vector contains an arbitrary, specified number of pointers to
expressions. The number of elements in the vector is explicitly present in
the vector. The written form of a vector consists of square brackets
(`[...]') surrounding the elements, in sequence and with whitespace
separating them. Vectors of length zero are not created; null pointers are
used instead.
Expressions are classified by "expression codes" (also called RTX codes).
The expression code is a name defined in `rtl.def', which is also (in upper
case) a C enumeration constant. The possible expression codes and their
meanings are machine-independent. The code of an RTX can be extracted with
the macro `GET_CODE (X)' and altered with `PUT_CODE (X, NEWCODE)'.
The expression code determines how many operands the expression contains,
and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell
by looking at an operand what kind of object it is. Instead, you must know
from its context---from the expression code of the containing expression.
For example, in an expression of code `subreg', the first operand is to be
regarded as an expression and the second operand as an integer. In an
expression of code `plus', there are two operands, both of which are to be
regarded as expressions. In a `symbol_ref' expression, there is one
operand, which is to be regarded as a string.
Expressions are written as parentheses containing the name of the
expression type, its flags and machine mode if any, and then the operands
of the expression (separated by spaces).
Expression code names in the `md' file are written in lower case, but when
they appear in C code they are written in upper case. In this manual, they
are shown as follows: `const_int'.
In a few contexts a null pointer is valid where an expression is normally
wanted. The written form of this is `(nil)'.
File: internals, Node: Accessors, Next: Flags, Prev: RTL Objects, Up: RTL
Access to Operands
==================
For each expression type `rtl.def' specifies the number of contained
objects and their kinds, with four possibilities: `e' for expression
(actually a pointer to an expression), `i' for integer, `s' for string, and
`E' for vector of expressions. The sequence of letters for an expression
code is called its "format". Thus, the format of `subreg' is `ei'.
Two other format characters are used occasionally: `u' and `0'. `u' is
equivalent to `e' except that it is printed differently in debugging dumps,
and `0' means a slot whose contents do not fit any normal category. `0'
slots are not printed at all in dumps, and are often used in special ways
by small parts of the compiler.
There are macros to get the number of operands and the format of an
expression code:
`GET_RTX_LENGTH (CODE)'
Number of operands of an RTX of code CODE.
`GET_RTX_FORMAT (CODE)'
The format of an RTX of code CODE, as a C string.
Operands of expressions are accessed using the macros `XEXP', `XINT' and
`XSTR'. Each of these macros takes two arguments: an expression-pointer
(RTX) and an operand number (counting from zero). Thus,
XEXP (X, 2)
accesses operand 2 of expression X, as an expression.
XINT (X, 2)
accesses the same operand as an integer. `XSTR', used in the same fashion,
would access it as a string.
Any operand can be accessed as an integer, as an expression or as a string.
You must choose the correct method of access for the kind of value
actually stored in the operand. You would do this based on the expression
code of the containing expression. That is also how you would know how
many operands there are.
For example, if X is a `subreg' expression, you know that it has two
operands which can be correctly accessed as `XEXP (X, 0)' and `XINT (X,
1)'. If you did `XINT (X, 0)', you would get the address of the expression
operand but cast as an integer; that might occasionally be useful, but it
would be cleaner to write `(int) XEXP (X, 0)'. `XEXP (X, 1)' would also
compile without error, and would return the second, integer operand cast as
an expression pointer, which would probably result in a crash when
accessed. Nothing stops you from writing `XEXP (X, 28)' either, but this
will access memory past the end of the expression with unpredictable results.
Access to operands which are vectors is more complicated. You can use the
macro `XVEC' to get the vector-pointer itself, or the macros `XVECEXP' and
`XVECLEN' to access the elements and length of a vector.
`XVEC (EXP, IDX)'
Access the vector-pointer which is operand number IDX in EXP.
`XVECLEN (EXP, IDX)'
Access the length (number of elements) in the vector which is in
operand number IDX in EXP. This value is an `int'.
`XVECEXP (EXP, IDX, ELTNUM)'
Access element number ELTNUM in the vector which is in operand number
IDX in EXP. This value is an RTX.
It is up to you to make sure that ELTNUM is not negative and is less
than `XVECLEN (EXP, IDX)'.
All the macros defined in this section expand into lvalues and therefore
can be used to assign the operands, lengths and vector elements as well as
to access them.
File: internals, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL
Flags in an RTL Expression
==========================
RTL expressions contain several flags (one-bit bit-fields) that are used in
certain types of expression.
`used'
This flag is used only momentarily, at the end of RTL generation for a
function, to count the number of times an expression appears in insns.
Expressions that appear more than once are copied, according to the
rules for shared structure (*Note Sharing::.).
`volatil'
This flag is used in `mem' and `reg' expressions and in insns. In RTL
dump files, it is printed as `/v'.
In a `mem' expression, it is 1 if the memory reference is volatile.
Volatile memory references may not be deleted, reordered or combined.
In a `reg' expression, it is 1 if the value is a user-level variable.
0 indicates an internal compiler temporary.
In an insn, 1 means the insn has been deleted.
`in_struct'
This flag is used in `mem' expressions. It is 1 if the memory datum
referred to is all or part of a structure or array; 0 if it is (or
might be) a scalar variable. A reference through a C pointer has 0
because the pointer might point to a scalar variable.
This information allows the compiler to determine something about
possible cases of aliasing.
In an RTL dump, this flag is represented as `/s'.
`unchanging'
This flag is used in `reg' and `mem' expressions. 1 means that the
value of the expression never changes (at least within the current
function).
In an RTL dump, this flag is represented as `/u'.
File: internals, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL
Machine Modes
=============
A machine mode describes a size of data object and the representation used
for it. In the C code, machine modes are represented by an enumeration
type, `enum machine_mode', defined in `machmode.def'. Each RTL expression
has room for a machine mode and so do certain kinds of tree expressions
(declarations and types, to be precise).
In debugging dumps and machine descriptions, the machine mode of an RTL
expression is written after the expression code with a colon to separate
them. The letters `mode' which appear at the end of each machine mode name
are omitted. For example, `(reg:SI 38)' is a `reg' expression with machine
mode `SImode'. If the mode is `VOIDmode', it is not written at all.
Here is a table of machine modes.
`QImode'
``Quarter-Integer'' mode represents a single byte treated as an integer.
`HImode'
``Half-Integer'' mode represents a two-byte integer.
`SImode'
``Single Integer'' mode represents a four-byte integer.
`DImode'
``Double Integer'' mode represents an eight-byte integer.
`TImode'
``Tetra Integer'' (?) mode represents a sixteen-byte integer.
`SFmode'
``Single Floating'' mode represents a single-precision (four byte)
floating point number.
`DFmode'
``Double Floating'' mode represents a double-precision (eight byte)
floating point number.
`TFmode'
``Tetra Floating'' mode represents a quadruple-precision (sixteen
byte) floating point number.
`BLKmode'
``Block'' mode represents values that are aggregates to which none of
the other modes apply. In RTL, only memory references can have this
mode, and only if they appear in string-move or vector instructions.
On machines which have no such instructions, `BLKmode' will not appear
in RTL.
`VOIDmode'
Void mode means the absence of a mode or an unspecified mode. For
example, RTL expressions of code `const_int' have mode `VOIDmode'
because they can be taken to have whatever mode the context requires.
In debugging dumps of RTL, `VOIDmode' is expressed by the absence of
any mode.
`EPmode'
``Entry Pointer'' mode is intended to be used for function variables
in Pascal and other block structured languages. Such values contain
both a function address and a static chain pointer for access to
automatic variables of outer levels. This mode is only partially
implemented since C does not use it.
`CSImode, ...'
``Complex Single Integer'' mode stands for a complex number
represented as a pair of `SImode' integers. Any of the integer and
floating modes may have `C' prefixed to its name to obtain a complex
number mode. For example, there are `CQImode', `CSFmode', and
`CDFmode'. Since C does not support complex numbers, these machine
modes are only partially implemented.
`BImode'
This is the machine mode of a bit-field in a structure. It is used
only in the syntax tree, never in RTL, and in the syntax tree it
appears only in declaration nodes. In C, it appears only in
`FIELD_DECL' nodes for structure fields defined with a bit size.
The machine description defines `Pmode' as a C macro which expands into the
machine mode used for addresses. Normally this is `SImode'.
The only modes which a machine description must support are `QImode',
`SImode', `SFmode' and `DFmode'. The compiler will attempt to use `DImode'
for two-word structures and unions, but it would not be hard to program it
to avoid this. Likewise, you can arrange for the C type `short int' to
avoid using `HImode'. In the long term it would be desirable to make the
set of available machine modes machine-dependent and eliminate all
assumptions about specific machine modes or their uses from the
machine-independent code of the compiler.
Here are some C macros that relate to machine modes:
`GET_MODE (X)'
Returns the machine mode of the RTX X.
`PUT_MODE (X, NEWMODE)'
Alters the machine mode of the RTX X to be NEWMODE.
`GET_MODE_SIZE (M)'
Returns the size in bytes of a datum of mode M.
`GET_MODE_BITSIZE (M)'
Returns the size in bits of a datum of mode M.
`GET_MODE_UNIT_SIZE (M)'
Returns the size in bits of the subunits of a datum of mode M. This
is the same as `GET_MODE_SIZE' except in the case of complex modes and
`EPmode'. For them, the unit size is the size of the real or
imaginary part, or the size of the function pointer or the context
pointer.
File: internals, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL
Constant Expression Types
=========================
The simplest RTL expressions are those that represent constant values.
`(const_int I)'
This type of expression represents the integer value I. I is
customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)',
which is equivalent to `XINT (EXP, 0)'.
There is only one expression object for the integer value zero; it is
the value of the variable `const0_rtx'. Likewise, the only expression
for integer value one is found in `const1_rtx'. Any attempt to create
an expression of code `const_int' and value zero or one will return
`const0_rtx' or `const1_rtx' as appropriate.
`(const_double:M I0 I1)'
Represents a floating point constant value of mode M. The two
inteGERS I0 and I1 together contain the bits of a `double' value. To
convert them to a `double', do
union { double d; int i[2];} u;
u.i[0] = XINT (x, 0);
u.i[1] = XINT (x, 1);
and then refer to `u.d'. The value of the constant is represented as
a double in this fashion even if the value represented is
single-precision.
The global variables `dconst0_rtx' and `fconst0_rtx' hold
`const_double' expressions with value 0, in modes `DFmode' and
`SFmode', respectively.
`(symbol_ref SYMBOL)'
Represents the value of an assembler label for data. SYMBOL is a
string that describes the name of the assembler label. If it starts
with a `*', the label is the rest of SYMBOL not including the `*'.
Otherwise, the label is SYMBOL, prefixed with `_'.
`(label_ref LABEL)'
Represents the value of an assembler label for code. It contains one
operand, an expression, which must be a `code_label' that appears in
the instruction sequence to identify the place where the label should
go.
The reason for using a distinct expression type for code label
references is so that jump optimization can distinguish them.
`(const EXP)'
Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, EXP, is an expression that
contains only constants (`const_int', `symbol_ref' and `label_ref'
expressions) combined with `plus' and `minus'. However, not all
combinations are valid, since the assembler cannot do arbitrary
arithmetic on relocatable symbols.
File: internals, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL
Registers and Memory
====================
Here are the RTL expression types for describing access to machine
registers and to main memory.
`(reg:M N)'
For small values of the integer N (less than `FIRST_PSEUDO_REGISTER'),
this stands for a reference to machine register number N: a "hard
register". For larger values of N, it stands for a temporary value or
"pseudo register". The compiler's strategy is to generate code
assuming an unlimited number of such pseudo registers, and later
convert them into hard registers or into memory references.
The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine
description, since the number of hard registers on the machine is an
invariant characteristic of the machine. Note, however, that not all
of the machine registers must be general registers. All the machine
registers that can be used for storage of data are given hard register
numbers, even those that can be used only in certain instructions or
can hold only certain types of data.
Each pseudo register number used in a function's RTL code is
represented by a unique `reg' expression.
M is the machine mode of the reference. It is necessary because
machines can generally refer to each register in more than one mode.
For example, a register may contain a full word but there may be
instructions to refer to it as a half word or as a single byte, as
well as instructions to refer to it as a floating point number of
various precisions.
Even for a register that the machine can access in only one mode, the
mode must always be specified.
A hard register may be accessed in various modes throughout one
function, but each pseudo register is given a natural mode and is
accessed only in that mode. When it is necessary to describe an
access to a pseudo register using a nonnatural mode, a `subreg'
expression is used.
A `reg' expression with a machine mode that specifies more than one
word of data may actually stand for several consecutive registers. If
in addition the register number specifies a hardware register, then it
actually represents several consecutive hardware registers starting
with the specified one.
Such multi-word hardware register `reg' expressions may not be live
across the boundary of a basic block. The lifetime analysis pass does
not know how to record properly that several consecutive registers are
actually live there, and therefore register allocation would be
confused. The CSE pass must go out of its way to make sure the
situation does not arise.
`(subreg:M REG WORDNUM)'
`subreg' expressions are used to refer to a register in a machine mode
other than its natural one, or to refer to one register of a
multi-word `reg' that actually refers to several registers.
Each pseudo-register has a natural mode. If it is necessary to
operate on it in a different mode---for example, to perform a fullword
move instruction on a pseudo-register that contains a single byte---
the pseudo-register must be enclosed in a `subreg'. In such a case,
WORDNUM is zero.
The other use of `subreg' is to extract the individual registers of a
multi-register value. Machine modes such as `DImode' and `EPmode'
indicate values longer than a word, values which usually require two
consecutive registers. To access one of the registers, use a `subreg'
with mode `SImode' and a WORDNUM that says which register.
The compilation parameter `WORDS_BIG_ENDIAN', if defined, says that
word number zero is the most significant part; otherwise, it is the
least significant part.
Note that it is not valid to access a `DFmode' value in `SFmode' using
a `subreg'. On some machines the most significant part of a `DFmode'
value does not have the same format as a single-precision floating
value.
`(cc0)'
This refers to the machine's condition code register. It has no
operands and may not have a machine mode. It may be validly used in
only two contexts: as the destination of an assignment (in test and
compare instructions) and in comparison operators comparing against
zero (`const_int' with value zero; that is to say, `const0_rtx').
There is only one expression object of code `cc0'; it is the value of
the variable `cc0_rtx'. Any attempt to create an expression of code
`cc0' will return `cc0_rtx'.
One special thing about the condition code register is that
instructions can set it implicitly. On many machines, nearly all
instructions set the condition code based on the value that they
compute or store. It is not necessary to record these actions
explicitly in the RTL because the machine description includes a
prescription for recognizing the instructions that do so (by means of
the macro `NOTICE_UPDATE_CC'). Only instructions whose sole purpose
is to set the condition code, and instructions that use the condition
code, need mention `(cc0)'.
`(pc)'
This represents the machine's program counter. It has no operands and
may not have a machine mode. `(pc)' may be validly used only in
certain specific contexts in jump instructions.
There is only one expression object of code `pc'; it is the value of
the variable `pc_rtx'. Any attempt to create an expression of code
`pc' will return `pc_rtx'.
All instructions that do not jump alter the program counter implicitly
by incrementing it, but there is no need to mention this in the RTL.
`(mem:M ADDR)'
This RTX represents a reference to main memory at an address
represented by the expression ADDR. M specifies how large a unit of
memory is accessed.
File: internals, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL
RTL Expressions for Arithmetic
==============================
`(plus:M X Y)'
Represents the sum of the values represented by X and Y carried out in
machine mode M. This is valid only if X and Y both are valid for mode
M.
`(minus:M X Y)'
Like `plus' but represents subtraction.
`(minus X Y)'
Represents the result of subtracting Y from X for purposes of
comparison. The absence of a machine mode in the `minus' expression
indicates that the result is computed without overflow, as if with
infinite precision.
Of course, machines can't really subtract with infinite precision.
However, they can pretend to do so when only the sign of the result
will be used, which is the case when the result is stored in `(cc0)'.
And that is the only way this kind of expression may validly be used:
as a value to be stored in the condition codes.
`(neg:M X)'
Represents the negation (subtraction from zero) of the value
represented by X, carried out in mode M. X must be valid for mode M.
`(mult:M X Y)'
Represents the signed product of the values represented by X and Y
carried out in machine mode M. If X and Y are both valid for mode M,
this is ordinary size-preserving multiplication. Alternatively, both
X and Y may be valid for a different, narrower mode. This represents
the kind of multiplication that generates a product wider than the
operands. Widening multiplication and same-size multiplication are
completely distinct and supported by different machine instructions;
machines may support one but not the other.
`mult' may be used for floating point division as well. Then M is a
floating point machine mode.
`(umult:M X Y)'
Like `mult' but represents unsigned multiplication. It may be used in
both same-size and widening forms, like `mult'. `umult' is used only
for fixed-point multiplication.
`(div:M X Y)'
Represents the quotient in signed division of X by Y, carried out in
machine mode M. If M is a floating-point mode, it represents the
exact quotient; otherwise, the integerized quotient. If X and Y are
both valid for mode M, this is ordinary size-preserving division.
Some machines have division instructions in which the operands and
quotient widths are not all the same; such instructions are
represented by `div' expressions in which the machine modes are not
all the same.
`(udiv:M X Y)'
Like `div' but represents unsigned division.
`(mod:M X Y)'
`(umod:M X Y)'
Like `div' and `udiv' but represent the remainder instead of the
quotient.
`(not:M X)'
Represents the bitwise complement of the value represented by X,
carried out in mode M, which must be a fixed-point machine mode. X
must be valid for mode M, which must be a fixed-point mode.
`(and:M X Y)'
Represents the bitwise logical-and of the values represented by X and
Y, carried out in machine mode M. This is valid only if X and Y both
are valid for mode M, which must be a fixed-point mode.
`(ior:M X Y)'
Represents the bitwise inclusive-or of the values represented by X and
Y, carried out in machine mode M. This is valid only if X and Y both
are valid for mode M, which must be a fixed-point mode.
`(xor:M X Y)'
Represents the bitwise exclusive-or of the values represented by X and
Y, carried out in machine mode M. This is valid only if X and Y both
are valid for mode M, which must be a fixed-point mode.
`(lshift:M X C)'
Represents the result of logically shifting X left by C places. X
must be valid for the mode M, a fixed-point machine mode. C must be
valid for a fixed-point mode; which mode is determined by the mode
called for in the machine description entry for the left-shift
instruction. For example, on the Vax, the mode of C is `QImode'
regardless of M.
On some machines, negative values of C may be meaningful; this is why
logical left shift and arithmetic left shift are distinguished. For
example, Vaxes have no right-shift instructions, and right shifts are
represented as left-shift instructions whose counts happen to be
negative constants or else computed (in a previous instruction) by
negation.
`(ashift:M X C)'
Like `lshift' but for arithmetic left shift.
`(lshiftrt:M X C)'
`(ashiftrt:M X C)'
Like `lshift' and `ashift' but for right shift.
`(rotate:M X C)'
`(rotatert:M X C)'
Similar but represent left and right rotate.
`(abs:M X)'
Represents the absolute value of X, computed in mode M. X must be
valid for M.
`(sqrt:M X)'
Represents the square root of X, computed in mode M. X must be valid
for M. Most often M will be a floating point mode.
`(ffs:M X)'
Represents the one plus the index of the least significant 1-bit in X,
represented as an integer of mode M. (The value is zero if X is
zero.) The mode of X need not be M; depending on the target machine,
various mode combinations may be valid.
File: internals, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL
Comparison Operations
=====================
Comparison operators test a relation on two operands and are considered to
represent the value 1 if the relation holds, or zero if it does not. The
mode of the comparison is determined by the operands; they must both be
valid for a common machine mode. A comparison with both operands constant
would be invalid as the machine mode could not be deduced from it, but such
a comparison should never exist in RTL due to constant folding.
Inequality comparisons come in two flavors, signed and unsigned. Thus,
there are distinct expression codes `gt' and `gtu' for signed and unsigned
greater-than. These can produce different results for the same pair of
integer values: for example, 1 is signed greater-than -1 but not unsigned
greater-than, because -1 when regarded as unsigned is actually `0xffffffff'
which is greater than 1.
The signed comparisons are also used for floating point values. Floating
point comparisons are distinguished by the machine modes of the operands.
The comparison operators may be used to compare the condition codes `(cc0)'
against zero, as in `(eq (cc0) (const_int 0))'. Such a construct actually
refers to the result of the preceding instruction in which the condition
codes were set. The above example stands for 1 if the condition codes were
set to say ``zero'' or ``equal'', 0 otherwise. Although the same
comparison operators are used for this as may be used in other contexts on
actual data, no confusion can result since the machine description would
never allow both kinds of uses in the same context.
`(eq X Y)'
1 if the values represented by X and Y are equal, otherwise 0.
`(ne X Y)'
1 if the values represented by X and Y are not equal, otherwise 0.
`(gt X Y)'
1 if the X is greater than Y. If they are fixed-point, the comparison
is done in a signed sense.
`(gtu X Y)'
Like `gt' but does unsigned comparison, on fixed-point numbers only.
`(lt X Y)'
`(ltu X Y)'
Like `gt' and `gtu' but test for ``less than''.
`(ge X Y)'
`(geu X Y)'
Like `gt' and `gtu' but test for ``greater than or equal''.
`(le X Y)'
`(leu X Y)'
Like `gt' and `gtu' but test for ``less than or equal''.
`(if_then_else COND THEN ELSE)'
This is not a comparison operation but is listed here because it is
always used in conjunction with a comparison operation. To be
precISE, COND is a comparison expression. This expression represents
a choice, according to COND, between the value represented by THEN and
the one represented by ELSE.
On most machines, `if_then_else' expressions are valid only to express
conditional jumps.
File: internals, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL
Bit-fields
==========
Special expression codes exist to represent bit-field instructions. These
types of expressions are lvalues in RTL; they may appear on the left side
of a assignment, indicating insertion of a value into the specified bit
field.
`(sign_extract:SI LOC SIZE POS)'
This represents a reference to a sign-extended bit-field contained or
starting in LOC (a memory or register reference). The bit field is
SIZE bits wide and starts at bit POS. The compilation option
`BITS_BIG_ENDIAN' says which end of the memory unit POS counts from.
Which machine modes are valid for LOC depends on the machine, but
typically LOC should be a single byte when in memory or a full word in
a register.
`(zero_extract:SI LOC SIZE POS)'
Like `sign_extract' but refers to an unsigned or zero-extended bit
field. The same sequence of bits are extracted, but they are filled
to an entire word with zeros instead of by sign-extension.
File: internals, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL
Conversions
===========
All conversions between machine modes must be represented by explicit
conversion operations. For example, an expression which is the sum of a
byte and a full word cannot be written as `(plus:SI (reg:QI 34) (reg:SI
80))' because the `plus' operation requires two operands of the same
machine mode. Therefore, the byte-sized operand is enclosed in a
conversion operation, as in
(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
The conversion operation is not a mere placeholder, because there may be
more than one way of converting from a given starting mode to the desired
final mode. The conversion operation code says how to do it.
`(sign_extend:M X)'
Represents the result of sign-extending the value X to machine mode M.
M must be a fixed-point mode and X a fixed-point value of a mode
narrower than M.
`(zero_extend:M X)'
Represents the result of zero-extending the value X to machine mode M.
M must be a fixed-point mode and X a fixed-point value of a mode
narrower than M.
`(float_extend:M X)'
Represents the result of extending the value X to machine mode M. M
must be a floating point mode and X a floating point value of a mode
narrower than M.
`(truncate:M X)'
Represents the result of truncating the value X to machine mode M. M
must be a fixed-point mode and X a fixed-point value of a mode wider
than M.
`(float_truncate:M X)'
Represents the result of truncating the value X to machine mode M. M
must be a floating point mode and X a floating point value of a mode
wider than M.
`(float:M X)'
Represents the result of converting fixed point value X, regarded as
signed, to floating point mode M.
`(unsigned_float:M X)'
Represents the result of converting fixed point value X, regarded as
unsigned, to floating point mode M.
`(fix:M X)'
When M is a fixed point mode, represents the result of converting
floating point value X to mode M, regarded as signed. How rounding is
done is not specified, so this operation may be used validly in
compiling C code only for integer-valued operands.
`(unsigned_fix:M X)'
Represents the result of converting floating point value X to fixed
point mode M, regarded as unsigned. How rounding is done is not
specified.
`(fix:M X)'
When M is a floating point mode, represents the result of converting
floating point value X (valid for mode M) to an integer, still
represented in floating point mode M, by rounding towards zero.
File: internals, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL
Declarations
============
Declaration expression codes do not represent arithmetic operations but
rather state assertions about their operands.
`(strict_low_part (subreg:M (reg:N R) 0))'
This expression code is used in only one context: operand 0 of a `set'
expression. In addition, the operand of this expression must be a
`subreg' expression.
The presence of `strict_low_part' says that the part of the register
which is meaningful in mode N, but is not part of mode M, is not to be
altered. Normally, an assignment to such a subreg is allowed to have
undefined effects on the rest of the register when M is less than a
word.