21.21 Miscellaneous Parameters

Here are several miscellaneous parameters.

PREDICATE_CODES

Define this if you have defined special-purpose predicates in the file `machine.c'. This macro is called within an initializer of an array of structures. The first field in the structure is the name of a predicate and the second field is an array of rtl codes. For each predicate, list all rtl codes that can be in expressions matched by the predicate. The list should have a trailing comma. Here is an example of two entries in the list for a typical RISC machine:

#define PREDICATE_CODES \ {"gen_reg_rtx_operand", {SUBREG, REG}}, \ {"reg_or_short_cint_operand", {SUBREG, REG, CONST_INT}},

Defining this macro does not affect the generated code (however, incorrect definitions that omit an rtl code that may be matched by the predicate can cause the compiler to malfunction). Instead, it allows the table built by `genrecog' to be more compact and efficient, thus speeding up the compiler. The most important predicates to include in the list specified by this macro are those used in the most insn patterns.

For each predicate function named in PREDICATE_CODES, a declaration will be generated in `insn-codes.h'.

SPECIAL_MODE_PREDICATES

Define this if you have special predicates that know special things about modes. Genrecog will warn about certain forms of match_operand without a mode; if the operand predicate is listed in SPECIAL_MODE_PREDICATES, the warning will be suppressed.

Here is an example from the IA-32 port (ext_register_operand specially checks for HImode or SImode in preparation for a byte extraction from %ah etc.).

#define SPECIAL_MODE_PREDICATES \ "ext_register_operand",

CASE_VECTOR_MODE

An alias for a machine mode name. This is the machine mode that elements of a jump-table should have.

CASE_VECTOR_SHORTEN_MODE (min_offset, max_offset, body)

Optional: return the preferred mode for an addr_diff_vec when the minimum and maximum offset are known. If you define this, it enables extra code in branch shortening to deal with addr_diff_vec. To make this work, you also have to define INSN_ALIGN and make the alignment for addr_diff_vec explicit. The body argument is provided so that the offset_unsigned and scale flags can be updated.

CASE_VECTOR_PC_RELATIVE

Define this macro to be a C expression to indicate when jump-tables should contain relative addresses. If jump-tables never contain relative addresses, then you need not define this macro.

CASE_DROPS_THROUGH

Define this if control falls through a case insn when the index value is out of range. This means the specified default-label is actually ignored by the case insn proper.

CASE_VALUES_THRESHOLD

Define this to be the smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. The default is four for machines with a casesi instruction and five otherwise. This is best for most machines.

WORD_REGISTER_OPERATIONS

Define this macro if operations between registers with integral mode smaller than a word are always performed on the entire register. Most RISC machines have this property and most CISC machines do not.

LOAD_EXTEND_OP (mode)

Define this macro to be a C expression indicating when insns that read memory in mode, an integral mode narrower than a word, set the bits outside of mode to be either the sign-extension or the zero-extension of the data read. Return SIGN_EXTEND for values of mode for which the insn sign-extends, ZERO_EXTEND for which it zero-extends, and NIL for other modes.

This macro is not called with mode non-integral or with a width greater than or equal to BITS_PER_WORD, so you may return any value in this case. Do not define this macro if it would always return NIL. On machines where this macro is defined, you will normally define it as the constant SIGN_EXTEND or ZERO_EXTEND.

SHORT_IMMEDIATES_SIGN_EXTEND

Define this macro if loading short immediate values into registers sign extends.

IMPLICIT_FIX_EXPR

An alias for a tree code that should be used by default for conversion of floating point values to fixed point. Normally, FIX_ROUND_EXPR is used.

FIXUNS_TRUNC_LIKE_FIX_TRUNC

Define this macro if the same instructions that convert a floating point number to a signed fixed point number also convert validly to an unsigned one.

EASY_DIV_EXPR

An alias for a tree code that is the easiest kind of division to compile code for in the general case. It may be TRUNC_DIV_EXPR, FLOOR_DIV_EXPR, CEIL_DIV_EXPR or ROUND_DIV_EXPR. These four division operators differ in how they round the result to an integer. EASY_DIV_EXPR is used when it is permissible to use any of those kinds of division and the choice should be made on the basis of efficiency.

MOVE_MAX

The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations.

MAX_MOVE_MAX

The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations. If this is undefined, the default is MOVE_MAX. Otherwise, it is the constant value that is the largest value that MOVE_MAX can have at run-time.

SHIFT_COUNT_TRUNCATED

A C expression that is nonzero if on this machine the number of bits actually used for the count of a shift operation is equal to the number of bits needed to represent the size of the object being shifted. When this macro is non-zero, the compiler will assume that it is safe to omit a sign-extend, zero-extend, and certain bitwise `and' instructions that truncates the count of a shift operation. On machines that have instructions that act on bit-fields at variable positions, which may include `bit test' instructions, a nonzero SHIFT_COUNT_TRUNCATED also enables deletion of truncations of the values that serve as arguments to bit-field instructions.

If both types of instructions truncate the count (for shifts) and position (for bit-field operations), or if no variable-position bit-field instructions exist, you should define this macro.

However, on some machines, such as the 80386 and the 680x0, truncation only applies to shift operations and not the (real or pretended) bit-field operations. Define SHIFT_COUNT_TRUNCATED to be zero on such machines. Instead, add patterns to the `md' file that include the implied truncation of the shift instructions.

You need not define this macro if it would always have the value of zero.

TRULY_NOOP_TRUNCATION (outprec, inprec)

A C expression which is nonzero if on this machine it is safe to "convert" an integer of inprec bits to one of outprec bits (where outprec is smaller than inprec) by merely operating on it as if it had only outprec bits.

On many machines, this expression can be 1.

When TRULY_NOOP_TRUNCATION returns 1 for a pair of sizes for modes for which MODES_TIEABLE_P is 0, suboptimal code can result. If this is the case, making TRULY_NOOP_TRUNCATION return 0 in such cases may improve things.

STORE_FLAG_VALUE

A C expression describing the value returned by a comparison operator with an integral mode and stored by a store-flag instruction (`scond') when the condition is true. This description must apply to all the `scond' patterns and all the comparison operators whose results have a MODE_INT mode.

A value of 1 or -1 means that the instruction implementing the comparison operator returns exactly 1 or -1 when the comparison is true and 0 when the comparison is false. Otherwise, the value indicates which bits of the result are guaranteed to be 1 when the comparison is true. This value is interpreted in the mode of the comparison operation, which is given by the mode of the first operand in the `scond' pattern. Either the low bit or the sign bit of STORE_FLAG_VALUE be on. Presently, only those bits are used by the compiler.

If STORE_FLAG_VALUE is neither 1 or -1, the compiler will generate code that depends only on the specified bits. It can also replace comparison operators with equivalent operations if they cause the required bits to be set, even if the remaining bits are undefined. For example, on a machine whose comparison operators return an SImode value and where STORE_FLAG_VALUE is defined as `0x80000000', saying that just the sign bit is relevant, the expression

(ne:SI (and:SI x (const_int power-of-2)) (const_int 0))

can be converted to

(ashift:SI x (const_int n))

where n is the appropriate shift count to move the bit being tested into the sign bit.

There is no way to describe a machine that always sets the low-order bit for a true value, but does not guarantee the value of any other bits, but we do not know of any machine that has such an instruction. If you are trying to port GCC to such a machine, include an instruction to perform a logical-and of the result with 1 in the pattern for the comparison operators and let us know (see section How to Report Bugs).

Often, a machine will have multiple instructions that obtain a value from a comparison (or the condition codes). Here are rules to guide the choice of value for STORE_FLAG_VALUE, and hence the instructions to be used:

• Use the shortest sequence that yields a valid definition for STORE_FLAG_VALUE. It is more efficient for the compiler to "normalize" the value (convert it to, e.g., 1 or 0) than for the comparison operators to do so because there may be opportunities to combine the normalization with other operations.

• For equal-length sequences, use a value of 1 or -1, with -1 being slightly preferred on machines with expensive jumps and 1 preferred on other machines.

• As a second choice, choose a value of `0x80000001' if instructions exist that set both the sign and low-order bits but do not define the others.

• Otherwise, use a value of `0x80000000'.

Many machines can produce both the value chosen for STORE_FLAG_VALUE and its negation in the same number of instructions. On those machines, you should also define a pattern for those cases, e.g., one matching

(set A (neg:m (ne:m B C)))

Some machines can also perform and or plus operations on condition code values with less instructions than the corresponding `scond' insn followed by and or plus. On those machines, define the appropriate patterns. Use the names incscc and decscc, respectively, for the patterns which perform plus or minus operations on condition code values. See `rs6000.md' for some examples. The GNU Superoptizer can be used to find such instruction sequences on other machines.

You need not define STORE_FLAG_VALUE if the machine has no store-flag instructions.

FLOAT_STORE_FLAG_VALUE (mode)

A C expression that gives a non-zero REAL_VALUE_TYPE value that is returned when comparison operators with floating-point results are true. Define this macro on machine that have comparison operations that return floating-point values. If there are no such operations, do not define this macro.

Pmode

An alias for the machine mode for pointers. On most machines, define this to be the integer mode corresponding to the width of a hardware pointer; SImode on 32-bit machine or DImode on 64-bit machines. On some machines you must define this to be one of the partial integer modes, such as PSImode.

The width of Pmode must be at least as large as the value of POINTER_SIZE. If it is not equal, you must define the macro POINTERS_EXTEND_UNSIGNED to specify how pointers are extended to Pmode.

FUNCTION_MODE

An alias for the machine mode used for memory references to functions being called, in call RTL expressions. On most machines this should be QImode.

INTEGRATE_THRESHOLD (decl)

A C expression for the maximum number of instructions above which the function decl should not be inlined. decl is a FUNCTION_DECL node.

The default definition of this macro is 64 plus 8 times the number of arguments that the function accepts. Some people think a larger threshold should be used on RISC machines.

SCCS_DIRECTIVE

Define this if the preprocessor should ignore #sccs directives and print no error message.

NO_IMPLICIT_EXTERN_C

Define this macro if the system header files support C++ as well as C. This macro inhibits the usual method of using system header files in C++, which is to pretend that the file's contents are enclosed in `extern "C" {...}'.

HANDLE_PRAGMA (getc, ungetc, name)

This macro is no longer supported. You must use REGISTER_TARGET_PRAGMAS instead.

REGISTER_TARGET_PRAGMAS (pfile)

Define this macro if you want to implement any target-specific pragmas. If defined, it is a C expression which makes a series of calls to the cpp_register_pragma and/or cpp_register_pragma_space functions. The pfile argument is the first argument to supply to these functions. The macro may also do setup required for the pragmas.

The primary reason to define this macro is to provide compatibility with other compilers for the same target. In general, we discourage definition of target-specific pragmas for GCC.

If the pragma can be implemented by attributes then the macro `INSERT_ATTRIBUTES' might be a useful one to define as well.

Preprocessor macros that appear on pragma lines are not expanded. All `#pragma' directives that do not match any registered pragma are silently ignored, unless the user specifies `-Wunknown-pragmas'.

Function: void cpp_register_pragma (cpp_reader *pfile, const char *space, const char *name, void (*callback) (cpp_reader *))

Each call to cpp_register_pragma establishes one pragma. The callback routine will be called when the preprocessor encounters a pragma of the form

#pragma [space] name ...

space must have been the subject of a previous call to cpp_register_pragma_space, or else be a null pointer. The callback routine receives pfile as its first argument, but must not use it for anything (this may change in the future). It may read any text after the name by making calls to c_lex. Text which is not read by the callback will be silently ignored.

Note that both space and name are case sensitive.

For an example use of this routine, see `c4x.h' and the callback routines defined in `c4x.c'.

Note that the use of c_lex is specific to the C and C++ compilers. It will not work in the Java or Fortran compilers, or any other language compilers for that matter. Thus if c_lex is going to be called from target-specific code, it must only be done so when building the C and C++ compilers. This can be done by defining the variables c_target_objs and cxx_target_objs in the target entry in the config.gcc file. These variables should name the target-specific, language-specific object file which contains the code that uses c_lex. Note it will also be necessary to add a rule to the makefile fragment pointed to by tmake_file that shows how to build this object file.

Function: void cpp_register_pragma_space (cpp_reader *pfile, const char *space)

This routine establishes a namespace for pragmas, which will be registered by subsequent calls to cpp_register_pragma. For example, pragmas defined by the C standard are in the `STDC' namespace, and pragmas specific to GCC are in the `GCC' namespace.

For an example use of this routine in a target header, see `v850.h'.

HANDLE_SYSV_PRAGMA

Define this macro (to a value of 1) if you want the System V style pragmas `#pragma pack(<n>)' and `#pragma weak <name> [=<value>]' to be supported by gcc.

The pack pragma specifies the maximum alignment (in bytes) of fields within a structure, in much the same way as the `__aligned__' and `__packed__' __attribute__s do. A pack value of zero resets the behaviour to the default.

The weak pragma only works if SUPPORTS_WEAK and ASM_WEAKEN_LABEL are defined. If enabled it allows the creation of specifically named weak labels, optionally with a value.

HANDLE_PRAGMA_PACK_PUSH_POP

Define this macro (to a value of 1) if you want to support the Win32 style pragmas `#pragma pack(push,<n>)' and `#pragma pack(pop)'. The pack(push,<n>) pragma specifies the maximum alignment (in bytes) of fields within a structure, in much the same way as the `__aligned__' and `__packed__' __attribute__s do. A pack value of zero resets the behaviour to the default. Successive invocations of this pragma cause the previous values to be stacked, so that invocations of `#pragma pack(pop)' will return to the previous value.

VALID_MACHINE_DECL_ATTRIBUTE (decl, attributes, identifier, args)

If defined, a C expression whose value is nonzero if identifier with arguments args is a valid machine specific attribute for decl. The attributes in attributes have previously been assigned to decl.

VALID_MACHINE_TYPE_ATTRIBUTE (type, attributes, identifier, args)

If defined, a C expression whose value is nonzero if identifier with arguments args is a valid machine specific attribute for type. The attributes in attributes have previously been assigned to type.

COMP_TYPE_ATTRIBUTES (type1, type2)

If defined, a C expression whose value is zero if the attributes on type1 and type2 are incompatible, one if they are compatible, and two if they are nearly compatible (which causes a warning to be generated).

SET_DEFAULT_TYPE_ATTRIBUTES (type)

If defined, a C statement that assigns default attributes to newly defined type.

MERGE_MACHINE_TYPE_ATTRIBUTES (type1, type2)

Define this macro if the merging of type attributes needs special handling. If defined, the result is a list of the combined TYPE_ATTRIBUTES of type1 and type2. It is assumed that comptypes has already been called and returned 1.

MERGE_MACHINE_DECL_ATTRIBUTES (olddecl, newdecl)

Define this macro if the merging of decl attributes needs special handling. If defined, the result is a list of the combined DECL_MACHINE_ATTRIBUTES of olddecl and newdecl. newdecl is a duplicate declaration of olddecl. Examples of when this is needed are when one attribute overrides another, or when an attribute is nullified by a subsequent definition.

INSERT_ATTRIBUTES (node, attr_ptr, prefix_ptr)

Define this macro if you want to be able to add attributes to a decl when it is being created. This is normally useful for back ends which wish to implement a pragma by using the attributes which correspond to the pragma's effect. The node argument is the decl which is being created. The attr_ptr argument is a pointer to the attribute list for this decl. The prefix_ptr is a pointer to the list of attributes that have appeared after the specifiers and modifiers of the declaration, but before the declaration proper.

SET_DEFAULT_DECL_ATTRIBUTES (decl, attributes)

If defined, a C statement that assigns default attributes to newly defined decl.

DOLLARS_IN_IDENTIFIERS

Define this macro to control use of the character `$' in identifier names. 0 means `$' is not allowed by default; 1 means it is allowed. 1 is the default; there is no need to define this macro in that case. This macro controls the compiler proper; it does not affect the preprocessor.

NO_DOLLAR_IN_LABEL

Define this macro if the assembler does not accept the character `$' in label names. By default constructors and destructors in G++ have `$' in the identifiers. If this macro is defined, `.' is used instead.

NO_DOT_IN_LABEL

Define this macro if the assembler does not accept the character `.' in label names. By default constructors and destructors in G++ have names that use `.'. If this macro is defined, these names are rewritten to avoid `.'.

DEFAULT_MAIN_RETURN

Define this macro if the target system expects every program's main function to return a standard "success" value by default (if no other value is explicitly returned).

The definition should be a C statement (sans semicolon) to generate the appropriate rtl instructions. It is used only when compiling the end of main.

NEED_ATEXIT

Define this if the target system lacks the function atexit from the ISO C standard. If this macro is defined, a default definition will be provided to support C++. If ON_EXIT is not defined, a default exit function will also be provided.

ON_EXIT

Define this macro if the target has another way to implement atexit functionality without replacing exit. For instance, SunOS 4 has a similar on_exit library function.

The definition should be a functional macro which can be used just like the atexit function.

EXIT_BODY

Define this if your exit function needs to do something besides calling an external function _cleanup before terminating with _exit. The EXIT_BODY macro is only needed if NEED_ATEXIT is defined and ON_EXIT is not defined.

INSN_SETS_ARE_DELAYED (insn)

Define this macro as a C expression that is nonzero if it is safe for the delay slot scheduler to place instructions in the delay slot of insn, even if they appear to use a resource set or clobbered in insn. insn is always a jump_insn or an insn; GCC knows that every call_insn has this behavior. On machines where some insn or jump_insn is really a function call and hence has this behavior, you should define this macro.

You need not define this macro if it would always return zero.

INSN_REFERENCES_ARE_DELAYED (insn)

Define this macro as a C expression that is nonzero if it is safe for the delay slot scheduler to place instructions in the delay slot of insn, even if they appear to set or clobber a resource referenced in insn. insn is always a jump_insn or an insn. On machines where some insn or jump_insn is really a function call and its operands are registers whose use is actually in the subroutine it calls, you should define this macro. Doing so allows the delay slot scheduler to move instructions which copy arguments into the argument registers into the delay slot of insn.

You need not define this macro if it would always return zero.

MACHINE_DEPENDENT_REORG (insn)

In rare cases, correct code generation requires extra machine dependent processing between the second jump optimization pass and delayed branch scheduling. On those machines, define this macro as a C statement to act on the code starting at insn.

MULTIPLE_SYMBOL_SPACES

Define this macro if in some cases global symbols from one translation unit may not be bound to undefined symbols in another translation unit without user intervention. For instance, under Microsoft Windows symbols must be explicitly imported from shared libraries (DLLs).

MD_ASM_CLOBBERS

A C statement that adds to CLOBBERS STRING_CST trees for any hard regs the port wishes to automatically clobber for all asms.

ISSUE_RATE

A C expression that returns how many instructions can be issued at the same time if the machine is a superscalar machine.

MD_SCHED_INIT (file, verbose, max_ready)

A C statement which is executed by the scheduler at the beginning of each block of instructions that are to be scheduled. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by `-fsched-verbose-n'. max_ready is the maximum number of insns in the current scheduling region that can be live at the same time. This can be used to allocate scratch space if it is needed.

MD_SCHED_FINISH (file, verbose)

A C statement which is executed by the scheduler at the end of each block of instructions that are to be scheduled. It can be used to perform cleanup of any actions done by the other scheduling macros. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by `-fsched-verbose-n'.

MD_SCHED_REORDER (file, verbose, ready, n_ready, clock, can_issue_more)

A C statement which is executed by the scheduler after it has scheduled the ready list to allow the machine description to reorder it (for example to combine two small instructions together on `VLIW' machines). file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by `-fsched-verbose-n'. ready is a pointer to the ready list of instructions that are ready to be scheduled. n_ready is the number of elements in the ready list. The scheduler reads the ready list in reverse order, starting with ready[n_ready-1] and going to ready[0]. clock is the timer tick of the scheduler. can_issue_more is an output parameter that is set to the number of insns that can issue this clock; normally this is just issue_rate. See also `MD_SCHED_REORDER2'.

MD_SCHED_REORDER2 (file, verbose, ready, n_ready, clock, can_issue_more)

Like `MD_SCHED_REORDER', but called at a different time. While the `MD_SCHED_REORDER' macro is called whenever the scheduler starts a new cycle, this macro is used immediately after `MD_SCHED_VARIABLE_ISSUE' is called; it can reorder the ready list and set can_issue_more to determine whether there are more insns to be scheduled in the same cycle. Defining this macro can be useful if there are frequent situations where scheduling one insn causes other insns to become ready in the same cycle, these other insns can then be taken into account properly.

MD_SCHED_VARIABLE_ISSUE (file, verbose, insn, more)

A C statement which is executed by the scheduler after it has scheduled an insn from the ready list. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by `-fsched-verbose-n'. insn is the instruction that was scheduled. more is the number of instructions that can be issued in the current cycle. The `MD_SCHED_VARIABLE_ISSUE' macro is responsible for updating the value of more (typically by `more--').

MAX_INTEGER_COMPUTATION_MODE

Define this to the largest integer machine mode which can be used for operations other than load, store and copy operations.

You need only define this macro if the target holds values larger than word_mode in general purpose registers. Most targets should not define this macro.

MATH_LIBRARY

Define this macro as a C string constant for the linker argument to link in the system math library, or `""' if the target does not have a separate math library.

You need only define this macro if the default of `"-lm"' is wrong.

LIBRARY_PATH_ENV

Define this macro as a C string constant for the environment variable that specifies where the linker should look for libraries.

You need only define this macro if the default of `"LIBRARY_PATH"' is wrong.

TARGET_HAS_F_SETLKW

Define this macro if the target supports file locking with fcntl / F_SETLKW. Note that this functionality is part of POSIX. Defining TARGET_HAS_F_SETLKW will enable the test coverage code to use file locking when exiting a program, which avoids race conditions if the program has forked.

MAX_CONDITIONAL_EXECUTE

A C expression for the maximum number of instructions to execute via conditional execution instructions instead of a branch. A value of BRANCH_COST+1 is the default if the machine does not use cc0, and 1 if it does use cc0.

IFCVT_MODIFY_TESTS

A C expression to modify the tests in TRUE_EXPR, and FALSE_EXPR for use in converting insns in TEST_BB, THEN_BB, ELSE_BB, and JOIN_BB basic blocks to conditional execution. Set either TRUE_EXPR or FALSE_EXPR to a null pointer if the tests cannot be converted.

IFCVT_MODIFY_INSN

A C expression to modify the PATTERN of an INSN that is to be converted to conditional execution format.

IFCVT_MODIFY_FINAL

A C expression to perform any final machine dependent modifications in converting code to conditional execution in the basic blocks TEST_BB, THEN_BB, ELSE_BB, and JOIN_BB.

IFCVT_MODIFY_CANCEL

A C expression to cancel any machine dependent modifications in converting code to conditional execution in the basic blocks TEST_BB, THEN_BB, ELSE_BB, and JOIN_BB.

MD_INIT_BUILTINS

Define this macro if you have any machine-specific built-in functions that need to be defined. It should be a C expression that performs the necessary setup.

Machine specific built-in functions can be useful to expand special machine instructions that would otherwise not normally be generated because they have no equivalent in the source language (for example, SIMD vector instructions or prefetch instructions).

To create a built-in function, call the function builtin_function which is defined by the language front end. You can use any type nodes set up by build_common_tree_nodes and build_common_tree_nodes_2; only language front ends that use these two functions will use `MD_INIT_BUILTINS'.

MD_EXPAND_BUILTIN(exp, target, subtarget, mode, ignore)

Expand a call to a machine specific built-in function that was set up by `MD_INIT_BUILTINS'. exp is the expression for the function call; the result should go to target if that is convenient, and have mode mode if that is convenient. subtarget may be used as the target for computing one of exp's operands. ignore is nonzero if the value is to be ignored. This macro should return the result of the call to the built-in function.