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### 15.29 Miscellaneous Parameters

Here are several miscellaneous parameters.

— Macro: HAS_LONG_COND_BRANCH

> Define this boolean macro to indicate whether or not your architecture has conditional branches that can span all of memory. It is used in conjunction with an optimization that partitions hot and cold basic blocks into separate sections of the executable. If this macro is set to false, gcc will convert any conditional branches that attempt to cross between sections into unconditional branches or indirect jumps.

— Macro: HAS_LONG_UNCOND_BRANCH

> Define this boolean macro to indicate whether or not your architecture has unconditional branches that can span all of memory. It is used in conjunction with an optimization that partitions hot and cold basic blocks into separate sections of the executable. If this macro is set to false, gcc will convert any unconditional branches that attempt to cross between sections into indirect jumps.

— Macro: CASE_VECTOR_MODE

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

— Macro: 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.

— Macro: CASE_VECTOR_PC_RELATIVE

> Define this macro to be a C expression to indicate when jump-tables should contain relative addresses. You need not define this macro if jump-tables never contain relative addresses, or jump-tables should contain relative addresses only when -fPIC or -fPIC is in effect.

— Macro: 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.

— Macro: CASE_USE_BIT_TESTS

> Define this macro to be a C expression to indicate whether C switch statements may be implemented by a sequence of bit tests. This is advantageous on processors that can efficiently implement left shift of 1 by the number of bits held in a register, but inappropriate on targets that would require a loop. By default, this macro returns `true` if the target defines an `ashlsi3` pattern, and `false` otherwise.

— Macro: 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.

— Macro: LOAD_EXTEND_OP (mem_mode)

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

> This macro is not called with mem_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 `UNKNOWN`. On machines where this macro is defined, you will normally define it as the constant `SIGN_EXTEND` or `ZERO_EXTEND`.

> You may return a non-`UNKNOWN` value even if for some hard registers the sign extension is not performed, if for the `REGNO_REG_CLASS` of these hard registers `CANNOT_CHANGE_MODE_CLASS` returns nonzero when the from mode is mem_mode and the to mode is any integral mode larger than this but not larger than `word_mode`.

> You must return `UNKNOWN` if for some hard registers that allow this mode, `CANNOT_CHANGE_MODE_CLASS` says that they cannot change to `word_mode`, but that they can change to another integral mode that is larger then mem_mode but still smaller than `word_mode`.

— Macro: SHORT_IMMEDIATES_SIGN_EXTEND

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

— Macro: 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.

— Target Hook: int TARGET_MIN_DIVISIONS_FOR_RECIP_MUL (enum machine_mode mode)

> When -ffast-math is in effect, GCC tries to optimize divisions by the same divisor, by turning them into multiplications by the reciprocal. This target hook specifies the minimum number of divisions that should be there for GCC to perform the optimization for a variable of mode mode. The default implementation returns 3 if the machine has an instruction for the division, and 2 if it does not.

— Macro: MOVE_MAX

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

— Macro: 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.

— Macro: 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 nonzero, 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.

— Target Hook: int TARGET_SHIFT_TRUNCATION_MASK (enum machine_mode mode)

> This function describes how the standard shift patterns for mode deal with shifts by negative amounts or by more than the width of the mode. See shift patterns.

> On many machines, the shift patterns will apply a mask m to the shift count, meaning that a fixed-width shift of x by y is equivalent to an arbitrary-width shift of x by y & m. If this is true for mode mode, the function should return m, otherwise it should return 0. A return value of 0 indicates that no particular behavior is guaranteed.

> Note that, unlike `SHIFT_COUNT_TRUNCATED`, this function does not apply to general shift rtxes; it applies only to instructions that are generated by the named shift patterns.

> The default implementation of this function returns `GET_MODE_BITSIZE (`mode`) - 1` if `SHIFT_COUNT_TRUNCATED` and 0 otherwise. This definition is always safe, but if `SHIFT_COUNT_TRUNCATED` is false, and some shift patterns nevertheless truncate the shift count, you may get better code by overriding it.

— Macro: 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.

— Target Hook: int TARGET_MODE_REP_EXTENDED (enum machine_mode mode, enum machine_mode rep_mode)

> The representation of an integral mode can be such that the values are always extended to a wider integral mode. Return `SIGN_EXTEND` if values of mode are represented in sign-extended form to rep_mode. Return `UNKNOWN` otherwise. (Currently, none of the targets use zero-extended representation this way so unlike `LOAD_EXTEND_OP`, `TARGET_MODE_REP_EXTENDED` is expected to return either `SIGN_EXTEND` or `UNKNOWN`. Also no target extends mode to mode_rep so that mode_rep is not the next widest integral mode and currently we take advantage of this fact.)

> Similarly to `LOAD_EXTEND_OP` you may return a non-`UNKNOWN` value even if the extension is not performed on certain hard registers as long as for the `REGNO_REG_CLASS` of these hard registers `CANNOT_CHANGE_MODE_CLASS` returns nonzero.

> Note that `TARGET_MODE_REP_EXTENDED` and `LOAD_EXTEND_OP` describe two related properties. If you define `TARGET_MODE_REP_EXTENDED (mode, word_mode)` you probably also want to define `LOAD_EXTEND_OP (mode)` to return the same type of extension.

> In order to enforce the representation of `mode`, `TRULY_NOOP_TRUNCATION` should return false when truncating to `mode`.

— Macro: 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 (s cond) when the condition is true. This description must apply to all the s cond 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 s cond 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 at gcc@gcc.gnu.org.

> 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 s cond 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.

> If this macro is not defined, the default value, 1, is used. You need not define `STORE_FLAG_VALUE` if the machine has no store-flag instructions, or if the value generated by these instructions is 1.

— Macro: FLOAT_STORE_FLAG_VALUE (mode)

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

— Macro: VECTOR_STORE_FLAG_VALUE (mode)

> A C expression that gives a rtx representing the nonzero true element for vector comparisons. The returned rtx should be valid for the inner mode of mode which is guaranteed to be a vector mode. Define this macro on machines that have vector comparison operations that return a vector result. If there are no such operations, do not define this macro. Typically, this macro is defined as `const1_rtx` or `constm1_rtx`. This macro may return `NULL_RTX` to prevent the compiler optimizing such vector comparison operations for the given mode.

— Macro: CLZ_DEFINED_VALUE_AT_ZERO (mode, value)
— Macro: CTZ_DEFINED_VALUE_AT_ZERO (mode, value)

> A C expression that indicates whether the architecture defines a value for `clz` or `ctz` with a zero operand. A result of `0` indicates the value is undefined. If the value is defined for only the RTL expression, the macro should evaluate to `1`; if the value applies also to the corresponding optab entry (which is normally the case if it expands directly into the corresponding RTL), then the macro should evaluate to `2`. In the cases where the value is defined, value should be set to this value.

> If this macro is not defined, the value of `clz` or `ctz` at zero is assumed to be undefined.

> This macro must be defined if the target's expansion for `ffs` relies on a particular value to get correct results. Otherwise it is not necessary, though it may be used to optimize some corner cases, and to provide a default expansion for the `ffs` optab.

> Note that regardless of this macro the “definedness” of `clz` and `ctz` at zero do not extend to the builtin functions visible to the user. Thus one may be free to adjust the value at will to match the target expansion of these operations without fear of breaking the API.

— 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`.

— Macro: FUNCTION_MODE

> An alias for the machine mode used for memory references to functions being called, in `call` RTL expressions. On most CISC machines, where an instruction can begin at any byte address, this should be `QImode`. On most RISC machines, where all instructions have fixed size and alignment, this should be a mode with the same size and alignment as the machine instruction words - typically `SImode` or `HImode`.

— Macro: STDC_0_IN_SYSTEM_HEADERS

> In normal operation, the preprocessor expands `__STDC__` to the constant 1, to signify that GCC conforms to ISO Standard C. On some hosts, like Solaris, the system compiler uses a different convention, where `__STDC__` is normally 0, but is 1 if the user specifies strict conformance to the C Standard.

> Defining `STDC_0_IN_SYSTEM_HEADERS` makes GNU CPP follows the host convention when processing system header files, but when processing user files `__STDC__` will always expand to 1.

— Macro: 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" {...}.

— Macro: REGISTER_TARGET_PRAGMAS ()

> 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 `c_register_pragma` or `c_register_pragma_with_expansion` for each pragma. The macro may also do any 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 you should consider defining the target hook TARGET_INSERT_ATTRIBUTES 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 c_register_pragma (const char *space, const char *name, void (*callback) (struct cpp_reader *))
— Function: void c_register_pragma_with_expansion (const char *space, const char *name, void (*callback) (struct cpp_reader *))

> Each call to `c_register_pragma` or `c_register_pragma_with_expansion` establishes one pragma. The callback routine will be called when the preprocessor encounters a pragma of the form

>               #pragma [space] name ...

> space is the case-sensitive namespace of the pragma, or `NULL` to put the pragma in the global namespace. The callback routine receives pfile as its first argument, which can be passed on to cpplib's functions if necessary. You can lex tokens after the name by calling `pragma_lex`. Tokens that are not read by the callback will be silently ignored. The end of the line is indicated by a token of type `CPP_EOF`. Macro expansion occurs on the arguments of pragmas registered with `c_register_pragma_with_expansion` but not on the arguments of pragmas registered with `c_register_pragma`.

> Note that the use of `pragma_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 `pragma_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 `pragma_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.

— Macro: 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 behavior to the default.

> A subtlety for Microsoft Visual C/C++ style bit-field packing (e.g. -mms-bitfields) for targets that support it: When a bit-field is inserted into a packed record, the whole size of the underlying type is used by one or more same-size adjacent bit-fields (that is, if its long:3, 32 bits is used in the record, and any additional adjacent long bit-fields are packed into the same chunk of 32 bits. However, if the size changes, a new field of that size is allocated).

> If both MS bit-fields and __attribute__((packed)) are used, the latter will take precedence. If __attribute__((packed)) is used on a single field when MS bit-fields are in use, it will take precedence for that field, but the alignment of the rest of the structure may affect its placement.

> 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.

— Macro: 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 behavior 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.

— Macro: HANDLE_PRAGMA_PACK_WITH_EXPANSION

> Define this macro, as well as `HANDLE_SYSV_PRAGMA`, if macros should be expanded in the arguments of #pragma pack.

— Macro: TARGET_DEFAULT_PACK_STRUCT

> If your target requires a structure packing default other than 0 (meaning the machine default), define this macro to the necessary value (in bytes). This must be a value that would also be valid to use with #pragma pack() (that is, a small power of two).

— Macro: HANDLE_PRAGMA_PUSH_POP_MACRO

> Define this macro if you want to support the Win32 style pragmas #pragma push_macro(macro-name-as-string) and #pragma pop_macro(macro-name-as-string). The #pragma push_macro( macro-name-as-string) pragma saves the named macro and via #pragma pop_macro(macro-name-as-string) it will return to the previous value.

— Macro: DOLLARS_IN_IDENTIFIERS

> Define this macro to control use of the character $ in identifier names for the C family of languages. 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.

— Macro: 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.

— Macro: 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 ..

— Macro: 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.

— Macro: 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.

— Macro: MULTIPLE_SYMBOL_SPACES

> Define this macro as a C expression that is nonzero 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).

> You need not define this macro if it would always evaluate to zero.

— Target Hook: tree TARGET_MD_ASM_CLOBBERS (tree outputs, tree inputs, tree clobbers)

> This target hook should add to clobbers `STRING_CST` trees for any hard regs the port wishes to automatically clobber for an asm. It should return the result of the last `tree_cons` used to add a clobber. The outputs, inputs and clobber lists are the corresponding parameters to the asm and may be inspected to avoid clobbering a register that is an input or output of the asm. You can use `tree_overlaps_hard_reg_set`, declared in tree.h, to test for overlap with regards to asm-declared registers.

— 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.

— Macro: 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.

— Macro: TARGET_POSIX_IO

> Define this macro if the target supports the following POSIX file functions, access, mkdir and file locking with fcntl / F_SETLKW. Defining `TARGET_POSIX_IO` will enable the test coverage code to use file locking when exiting a program, which avoids race conditions if the program has forked. It will also create directories at run-time for cross-profiling.

— Macro: 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.

— Macro: IFCVT_MODIFY_TESTS (ce_info, true_expr, false_expr)

> Used if the target needs to perform machine-dependent modifications on the conditionals used for turning basic blocks into conditionally executed code. ce_info points to a data structure, `struct ce_if_block`, which contains information about the currently processed blocks. true_expr and false_expr are the tests that are used for converting the then-block and the else-block, respectively. Set either true_expr or false_expr to a null pointer if the tests cannot be converted.

— Macro: IFCVT_MODIFY_MULTIPLE_TESTS (ce_info, bb, true_expr, false_expr)

> Like `IFCVT_MODIFY_TESTS`, but used when converting more complicated if-statements into conditions combined by `and` and `or` operations. bb contains the basic block that contains the test that is currently being processed and about to be turned into a condition.

— Macro: IFCVT_MODIFY_INSN (ce_info, pattern, insn)

> A C expression to modify the PATTERN of an INSN that is to be converted to conditional execution format. ce_info points to a data structure, `struct ce_if_block`, which contains information about the currently processed blocks.

— Macro: IFCVT_MODIFY_FINAL (ce_info)

> A C expression to perform any final machine dependent modifications in converting code to conditional execution. The involved basic blocks can be found in the `struct ce_if_block` structure that is pointed to by ce_info.

— Macro: IFCVT_MODIFY_CANCEL (ce_info)

> A C expression to cancel any machine dependent modifications in converting code to conditional execution. The involved basic blocks can be found in the `struct ce_if_block` structure that is pointed to by ce_info.

— Macro: IFCVT_INIT_EXTRA_FIELDS (ce_info)

> A C expression to initialize any extra fields in a `struct ce_if_block` structure, which are defined by the `IFCVT_EXTRA_FIELDS` macro.

— Macro: IFCVT_EXTRA_FIELDS

> If defined, it should expand to a set of field declarations that will be added to the `struct ce_if_block` structure. These should be initialized by the `IFCVT_INIT_EXTRA_FIELDS` macro.

— Target Hook: void TARGET_MACHINE_DEPENDENT_REORG ()

> If non-null, this hook performs a target-specific pass over the instruction stream. The compiler will run it at all optimization levels, just before the point at which it normally does delayed-branch scheduling.

> The exact purpose of the hook varies from target to target. Some use it to do transformations that are necessary for correctness, such as laying out in-function constant pools or avoiding hardware hazards. Others use it as an opportunity to do some machine-dependent optimizations.

> You need not implement the hook if it has nothing to do. The default definition is null.

— Target Hook: void TARGET_INIT_BUILTINS ()

> Define this hook if you have any machine-specific built-in functions that need to be defined. It should be a function 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 `lang_hooks.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 those two functions will call TARGET_INIT_BUILTINS.

— Target Hook: rtx TARGET_EXPAND_BUILTIN (tree exp, rtx target, rtx subtarget, enum machine_mode mode, int ignore)

> Expand a call to a machine specific built-in function that was set up by TARGET_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 function should return the result of the call to the built-in function.

— Target Hook: tree TARGET_RESOLVE_OVERLOADED_BUILTIN (tree fndecl, tree arglist)

> Select a replacement for a machine specific built-in function that was set up by TARGET_INIT_BUILTINS. This is done before regular type checking, and so allows the target to implement a crude form of function overloading. fndecl is the declaration of the built-in function. arglist is the list of arguments passed to the built-in function. The result is a complete expression that implements the operation, usually another `CALL_EXPR`.

— Target Hook: tree TARGET_FOLD_BUILTIN (tree fndecl, tree arglist, bool ignore)

> Fold a call to a machine specific built-in function that was set up by TARGET_INIT_BUILTINS. fndecl is the declaration of the built-in function. arglist is the list of arguments passed to the built-in function. The result is another tree containing a simplified expression for the call's result. If ignore is true the value will be ignored.

— Target Hook: const char * TARGET_INVALID_WITHIN_DOLOOP (rtx insn)

> Take an instruction in insn and return NULL if it is valid within a low-overhead loop, otherwise return a string why doloop could not be applied.

> Many targets use special registers for low-overhead looping. For any instruction that clobbers these this function should return a string indicating the reason why the doloop could not be applied. By default, the RTL loop optimizer does not use a present doloop pattern for loops containing function calls or branch on table instructions.

— Macro: MD_CAN_REDIRECT_BRANCH (branch1, branch2)

> Take a branch insn in branch1 and another in branch2. Return true if redirecting branch1 to the destination of branch2 is possible.

> On some targets, branches may have a limited range. Optimizing the filling of delay slots can result in branches being redirected, and this may in turn cause a branch offset to overflow.

— Target Hook: bool TARGET_COMMUTATIVE_P (rtx x, outer_code)

> This target hook returns `true` if x is considered to be commutative. Usually, this is just COMMUTATIVE_P (x), but the HP PA doesn't consider PLUS to be commutative inside a MEM. outer_code is the rtx code of the enclosing rtl, if known, otherwise it is UNKNOWN.

— Target Hook: rtx TARGET_ALLOCATE_INITIAL_VALUE (rtx hard_reg)

> When the initial value of a hard register has been copied in a pseudo register, it is often not necessary to actually allocate another register to this pseudo register, because the original hard register or a stack slot it has been saved into can be used. `TARGET_ALLOCATE_INITIAL_VALUE` is called at the start of register allocation once for each hard register that had its initial value copied by using `get_func_hard_reg_initial_val` or `get_hard_reg_initial_val`. Possible values are `NULL_RTX`, if you don't want to do any special allocation, a `REG` rtx—that would typically be the hard register itself, if it is known not to be clobbered—or a `MEM`. If you are returning a `MEM`, this is only a hint for the allocator; it might decide to use another register anyways. You may use `current_function_leaf_function` in the hook, functions that use `REG_N_SETS`, to determine if the hard register in question will not be clobbered. The default value of this hook is `NULL`, which disables any special allocation.

— Target Hook: int TARGET_UNSPEC_MAY_TRAP_P (const_rtx x, unsigned flags)

> This target hook returns nonzero if x, an `unspec` or `unspec_volatile` operation, might cause a trap. Targets can use this hook to enhance precision of analysis for `unspec` and `unspec_volatile` operations. You may call `may_trap_p_1` to analyze inner elements of x in which case flags should be passed along.

— Target Hook: void TARGET_SET_CURRENT_FUNCTION (tree decl)

> The compiler invokes this hook whenever it changes its current function context (`cfun`). You can define this function if the back end needs to perform any initialization or reset actions on a per-function basis. For example, it may be used to implement function attributes that affect register usage or code generation patterns. The argument decl is the declaration for the new function context, and may be null to indicate that the compiler has left a function context and is returning to processing at the top level. The default hook function does nothing.

> GCC sets `cfun` to a dummy function context during initialization of some parts of the back end. The hook function is not invoked in this situation; you need not worry about the hook being invoked recursively, or when the back end is in a partially-initialized state.

— Macro: TARGET_OBJECT_SUFFIX

> Define this macro to be a C string representing the suffix for object files on your target machine. If you do not define this macro, GCC will use .o as the suffix for object files.

— Macro: TARGET_EXECUTABLE_SUFFIX

> Define this macro to be a C string representing the suffix to be automatically added to executable files on your target machine. If you do not define this macro, GCC will use the null string as the suffix for executable files.

— Macro: COLLECT_EXPORT_LIST

> If defined, `collect2` will scan the individual object files specified on its command line and create an export list for the linker. Define this macro for systems like AIX, where the linker discards object files that are not referenced from `main` and uses export lists.

— Macro: MODIFY_JNI_METHOD_CALL (mdecl)

> Define this macro to a C expression representing a variant of the method call mdecl, if Java Native Interface (JNI) methods must be invoked differently from other methods on your target. For example, on 32-bit Microsoft Windows, JNI methods must be invoked using the `stdcall` calling convention and this macro is then defined as this expression:

>               build_type_attribute_variant (mdecl,
>                                             build_tree_list
>                                             (get_identifier ("stdcall"),
>                                              NULL))

— Target Hook: bool TARGET_CANNOT_MODIFY_JUMPS_P (void)

> This target hook returns `true` past the point in which new jump instructions could be created. On machines that require a register for every jump such as the SHmedia ISA of SH5, this point would typically be reload, so this target hook should be defined to a function such as:

>               static bool
>               cannot_modify_jumps_past_reload_p ()
>               {
>                 return (reload_completed || reload_in_progress);
>               }

— Target Hook: int TARGET_BRANCH_TARGET_REGISTER_CLASS (void)

> This target hook returns a register class for which branch target register optimizations should be applied. All registers in this class should be usable interchangeably. After reload, registers in this class will be re-allocated and loads will be hoisted out of loops and be subjected to inter-block scheduling.

— Target Hook: bool TARGET_BRANCH_TARGET_REGISTER_CALLEE_SAVED (bool after_prologue_epilogue_gen)

> Branch target register optimization will by default exclude callee-saved registers that are not already live during the current function; if this target hook returns true, they will be included. The target code must than make sure that all target registers in the class returned by TARGET_BRANCH_TARGET_REGISTER_CLASS that might need saving are saved. after_prologue_epilogue_gen indicates if prologues and epilogues have already been generated. Note, even if you only return true when after_prologue_epilogue_gen is false, you still are likely to have to make special provisions in `INITIAL_ELIMINATION_OFFSET` to reserve space for caller-saved target registers.

— Macro: POWI_MAX_MULTS

> If defined, this macro is interpreted as a signed integer C expression that specifies the maximum number of floating point multiplications that should be emitted when expanding exponentiation by an integer constant inline. When this value is defined, exponentiation requiring more than this number of multiplications is implemented by calling the system library's `pow`, `powf` or `powl` routines. The default value places no upper bound on the multiplication count.

— Macro: void TARGET_EXTRA_INCLUDES (const char *sysroot, const char *iprefix, int stdinc)

> This target hook should register any extra include files for the target. The parameter stdinc indicates if normal include files are present. The parameter sysroot is the system root directory. The parameter iprefix is the prefix for the gcc directory.

— Macro: void TARGET_EXTRA_PRE_INCLUDES (const char *sysroot, const char *iprefix, int stdinc)

> This target hook should register any extra include files for the target before any standard headers. The parameter stdinc indicates if normal include files are present. The parameter sysroot is the system root directory. The parameter iprefix is the prefix for the gcc directory.

— Macro: void TARGET_OPTF (char *path)

> This target hook should register special include paths for the target. The parameter path is the include to register. On Darwin systems, this is used for Framework includes, which have semantics that are different from -I.

— Target Hook: bool TARGET_USE_LOCAL_THUNK_ALIAS_P (tree fndecl)

> This target hook returns `true` if it is safe to use a local alias for a virtual function fndecl when constructing thunks, `false` otherwise. By default, the hook returns `true` for all functions, if a target supports aliases (i.e. defines `ASM_OUTPUT_DEF`), `false` otherwise,

— Macro: TARGET_FORMAT_TYPES

> If defined, this macro is the name of a global variable containing target-specific format checking information for the -Wformat option. The default is to have no target-specific format checks.

— Macro: TARGET_N_FORMAT_TYPES

> If defined, this macro is the number of entries in `TARGET_FORMAT_TYPES`.

— Target Hook: bool TARGET_RELAXED_ORDERING

> If set to `true`, means that the target's memory model does not guarantee that loads which do not depend on one another will access main memory in the order of the instruction stream; if ordering is important, an explicit memory barrier must be used. This is true of many recent processors which implement a policy of “relaxed,” “weak,” or “release” memory consistency, such as Alpha, PowerPC, and ia64. The default is `false`.

— Target Hook: const char *TARGET_INVALID_ARG_FOR_UNPROTOTYPED_FN (tree typelist, tree funcdecl, tree val)

> If defined, this macro returns the diagnostic message when it is illegal to pass argument val to function funcdecl with prototype typelist.

— Target Hook: const char * TARGET_INVALID_CONVERSION (tree fromtype, tree totype)

> If defined, this macro returns the diagnostic message when it is invalid to convert from fromtype to totype, or `NULL` if validity should be determined by the front end.

— Target Hook: const char * TARGET_INVALID_UNARY_OP (int op, tree type)

> If defined, this macro returns the diagnostic message when it is invalid to apply operation op (where unary plus is denoted by `CONVERT_EXPR`) to an operand of type type, or `NULL` if validity should be determined by the front end.

— Target Hook: const char * TARGET_INVALID_BINARY_OP (int op, tree type1, tree type2)

> If defined, this macro returns the diagnostic message when it is invalid to apply operation op to operands of types type1 and type2, or `NULL` if validity should be determined by the front end.

— Macro: TARGET_USE_JCR_SECTION

> This macro determines whether to use the JCR section to register Java classes. By default, TARGET_USE_JCR_SECTION is defined to 1 if both SUPPORTS_WEAK and TARGET_HAVE_NAMED_SECTIONS are true, else 0.

— Macro: OBJC_JBLEN

> This macro determines the size of the objective C jump buffer for the NeXT runtime. By default, OBJC_JBLEN is defined to an innocuous value.

— Macro: LIBGCC2_UNWIND_ATTRIBUTE

> Define this macro if any target-specific attributes need to be attached to the functions in libgcc that provide low-level support for call stack unwinding. It is used in declarations in unwind-generic.h and the associated definitions of those functions.