This is Info file gcc.info, produced by Makeinfo version 1.67 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  File: gcc.info, Node: Peephole Definitions, Next: Expander Definitions, Prev: Insn Canonicalizations, Up: Machine Desc Machine-Specific Peephole Optimizers ==================================== In addition to instruction patterns the `md' file may contain definitions of machine-specific peephole optimizations. The combiner does not notice certain peephole optimizations when the data flow in the program does not suggest that it should try them. For example, sometimes two consecutive insns related in purpose can be combined even though the second one does not appear to use a register computed in the first one. A machine-specific peephole optimizer can detect such opportunities. A definition looks like this: (define_peephole [INSN-PATTERN-1 INSN-PATTERN-2 ...] "CONDITION" "TEMPLATE" "OPTIONAL INSN-ATTRIBUTES") The last string operand may be omitted if you are not using any machine-specific information in this machine description. If present, it must obey the same rules as in a `define_insn'. In this skeleton, INSN-PATTERN-1 and so on are patterns to match consecutive insns. The optimization applies to a sequence of insns when INSN-PATTERN-1 matches the first one, INSN-PATTERN-2 matches the next, and so on. Each of the insns matched by a peephole must also match a `define_insn'. Peepholes are checked only at the last stage just before code generation, and only optionally. Therefore, any insn which would match a peephole but no `define_insn' will cause a crash in code generation in an unoptimized compilation, or at various optimization stages. The operands of the insns are matched with `match_operands', `match_operator', and `match_dup', as usual. What is not usual is that the operand numbers apply to all the insn patterns in the definition. So, you can check for identical operands in two insns by using `match_operand' in one insn and `match_dup' in the other. The operand constraints used in `match_operand' patterns do not have any direct effect on the applicability of the peephole, but they will be validated afterward, so make sure your constraints are general enough to apply whenever the peephole matches. If the peephole matches but the constraints are not satisfied, the compiler will crash. It is safe to omit constraints in all the operands of the peephole; or you can write constraints which serve as a double-check on the criteria previously tested. Once a sequence of insns matches the patterns, the CONDITION is checked. This is a C expression which makes the final decision whether to perform the optimization (we do so if the expression is nonzero). If CONDITION is omitted (in other words, the string is empty) then the optimization is applied to every sequence of insns that matches the patterns. The defined peephole optimizations are applied after register allocation is complete. Therefore, the peephole definition can check which operands have ended up in which kinds of registers, just by looking at the operands. The way to refer to the operands in CONDITION is to write `operands[I]' for operand number I (as matched by `(match_operand I ...)'). Use the variable `insn' to refer to the last of the insns being matched; use `prev_active_insn' to find the preceding insns. When optimizing computations with intermediate results, you can use CONDITION to match only when the intermediate results are not used elsewhere. Use the C expression `dead_or_set_p (INSN, OP)', where INSN is the insn in which you expect the value to be used for the last time (from the value of `insn', together with use of `prev_nonnote_insn'), and OP is the intermediate value (from `operands[I]'). Applying the optimization means replacing the sequence of insns with one new insn. The TEMPLATE controls ultimate output of assembler code for this combined insn. It works exactly like the template of a `define_insn'. Operand numbers in this template are the same ones used in matching the original sequence of insns. The result of a defined peephole optimizer does not need to match any of the insn patterns in the machine description; it does not even have an opportunity to match them. The peephole optimizer definition itself serves as the insn pattern to control how the insn is output. Defined peephole optimizers are run as assembler code is being output, so the insns they produce are never combined or rearranged in any way. Here is an example, taken from the 68000 machine description: (define_peephole [(set (reg:SI 15) (plus:SI (reg:SI 15) (const_int 4))) (set (match_operand:DF 0 "register_operand" "=f") (match_operand:DF 1 "register_operand" "ad"))] "FP_REG_P (operands[0]) && ! FP_REG_P (operands[1])" "* { rtx xoperands[2]; xoperands[1] = gen_rtx (REG, SImode, REGNO (operands[1]) + 1); #ifdef MOTOROLA output_asm_insn (\"move.l %1,(sp)\", xoperands); output_asm_insn (\"move.l %1,-(sp)\", operands); return \"fmove.d (sp)+,%0\"; #else output_asm_insn (\"movel %1,sp@\", xoperands); output_asm_insn (\"movel %1,sp@-\", operands); return \"fmoved sp@+,%0\"; #endif } ") The effect of this optimization is to change jbsr _foobar addql #4,sp movel d1,sp@- movel d0,sp@- fmoved sp@+,fp0 into jbsr _foobar movel d1,sp@ movel d0,sp@- fmoved sp@+,fp0 INSN-PATTERN-1 and so on look *almost* like the second operand of `define_insn'. There is one important difference: the second operand of `define_insn' consists of one or more RTX's enclosed in square brackets. Usually, there is only one: then the same action can be written as an element of a `define_peephole'. But when there are multiple actions in a `define_insn', they are implicitly enclosed in a `parallel'. Then you must explicitly write the `parallel', and the square brackets within it, in the `define_peephole'. Thus, if an insn pattern looks like this, (define_insn "divmodsi4" [(set (match_operand:SI 0 "general_operand" "=d") (div:SI (match_operand:SI 1 "general_operand" "0") (match_operand:SI 2 "general_operand" "dmsK"))) (set (match_operand:SI 3 "general_operand" "=d") (mod:SI (match_dup 1) (match_dup 2)))] "TARGET_68020" "divsl%.l %2,%3:%0") then the way to mention this insn in a peephole is as follows: (define_peephole [... (parallel [(set (match_operand:SI 0 "general_operand" "=d") (div:SI (match_operand:SI 1 "general_operand" "0") (match_operand:SI 2 "general_operand" "dmsK"))) (set (match_operand:SI 3 "general_operand" "=d") (mod:SI (match_dup 1) (match_dup 2)))]) ...] ...)  File: gcc.info, Node: Expander Definitions, Next: Insn Splitting, Prev: Peephole Definitions, Up: Machine Desc Defining RTL Sequences for Code Generation ========================================== On some target machines, some standard pattern names for RTL generation cannot be handled with single insn, but a sequence of RTL insns can represent them. For these target machines, you can write a `define_expand' to specify how to generate the sequence of RTL. A `define_expand' is an RTL expression that looks almost like a `define_insn'; but, unlike the latter, a `define_expand' is used only for RTL generation and it can produce more than one RTL insn. A `define_expand' RTX has four operands: * The name. Each `define_expand' must have a name, since the only use for it is to refer to it by name. * The RTL template. This is just like the RTL template for a `define_peephole' in that it is a vector of RTL expressions each being one insn. * The condition, a string containing a C expression. This expression is used to express how the availability of this pattern depends on subclasses of target machine, selected by command-line options when GNU CC is run. This is just like the condition of a `define_insn' that has a standard name. Therefore, the condition (if present) may not depend on the data in the insn being matched, but only the target-machine-type flags. The compiler needs to test these conditions during initialization in order to learn exactly which named instructions are available in a particular run. * The preparation statements, a string containing zero or more C statements which are to be executed before RTL code is generated from the RTL template. Usually these statements prepare temporary registers for use as internal operands in the RTL template, but they can also generate RTL insns directly by calling routines such as `emit_insn', etc. Any such insns precede the ones that come from the RTL template. Every RTL insn emitted by a `define_expand' must match some `define_insn' in the machine description. Otherwise, the compiler will crash when trying to generate code for the insn or trying to optimize it. The RTL template, in addition to controlling generation of RTL insns, also describes the operands that need to be specified when this pattern is used. In particular, it gives a predicate for each operand. A true operand, which needs to be specified in order to generate RTL from the pattern, should be described with a `match_operand' in its first occurrence in the RTL template. This enters information on the operand's predicate into the tables that record such things. GNU CC uses the information to preload the operand into a register if that is required for valid RTL code. If the operand is referred to more than once, subsequent references should use `match_dup'. The RTL template may also refer to internal "operands" which are temporary registers or labels used only within the sequence made by the `define_expand'. Internal operands are substituted into the RTL template with `match_dup', never with `match_operand'. The values of the internal operands are not passed in as arguments by the compiler when it requests use of this pattern. Instead, they are computed within the pattern, in the preparation statements. These statements compute the values and store them into the appropriate elements of `operands' so that `match_dup' can find them. There are two special macros defined for use in the preparation statements: `DONE' and `FAIL'. Use them with a following semicolon, as a statement. `DONE' Use the `DONE' macro to end RTL generation for the pattern. The only RTL insns resulting from the pattern on this occasion will be those already emitted by explicit calls to `emit_insn' within the preparation statements; the RTL template will not be generated. `FAIL' Make the pattern fail on this occasion. When a pattern fails, it means that the pattern was not truly available. The calling routines in the compiler will try other strategies for code generation using other patterns. Failure is currently supported only for binary (addition, multiplication, shifting, etc.) and bitfield (`extv', `extzv', and `insv') operations. Here is an example, the definition of left-shift for the SPUR chip: (define_expand "ashlsi3" [(set (match_operand:SI 0 "register_operand" "") (ashift:SI (match_operand:SI 1 "register_operand" "") (match_operand:SI 2 "nonmemory_operand" "")))] "" " { if (GET_CODE (operands[2]) != CONST_INT || (unsigned) INTVAL (operands[2]) > 3) FAIL; }") This example uses `define_expand' so that it can generate an RTL insn for shifting when the shift-count is in the supported range of 0 to 3 but fail in other cases where machine insns aren't available. When it fails, the compiler tries another strategy using different patterns (such as, a library call). If the compiler were able to handle nontrivial condition-strings in patterns with names, then it would be possible to use a `define_insn' in that case. Here is another case (zero-extension on the 68000) which makes more use of the power of `define_expand': (define_expand "zero_extendhisi2" [(set (match_operand:SI 0 "general_operand" "") (const_int 0)) (set (strict_low_part (subreg:HI (match_dup 0) 0)) (match_operand:HI 1 "general_operand" ""))] "" "operands[1] = make_safe_from (operands[1], operands[0]);") Here two RTL insns are generated, one to clear the entire output operand and the other to copy the input operand into its low half. This sequence is incorrect if the input operand refers to [the old value of] the output operand, so the preparation statement makes sure this isn't so. The function `make_safe_from' copies the `operands[1]' into a temporary register if it refers to `operands[0]'. It does this by emitting another RTL insn. Finally, a third example shows the use of an internal operand. Zero-extension on the SPUR chip is done by `and'-ing the result against a halfword mask. But this mask cannot be represented by a `const_int' because the constant value is too large to be legitimate on this machine. So it must be copied into a register with `force_reg' and then the register used in the `and'. (define_expand "zero_extendhisi2" [(set (match_operand:SI 0 "register_operand" "") (and:SI (subreg:SI (match_operand:HI 1 "register_operand" "") 0) (match_dup 2)))] "" "operands[2] = force_reg (SImode, gen_rtx (CONST_INT, VOIDmode, 65535)); ") *Note:* If the `define_expand' is used to serve a standard binary or unary arithmetic operation or a bitfield operation, then the last insn it generates must not be a `code_label', `barrier' or `note'. It must be an `insn', `jump_insn' or `call_insn'. If you don't need a real insn at the end, emit an insn to copy the result of the operation into itself. Such an insn will generate no code, but it can avoid problems in the compiler.  File: gcc.info, Node: Insn Splitting, Next: Insn Attributes, Prev: Expander Definitions, Up: Machine Desc Defining How to Split Instructions ================================== There are two cases where you should specify how to split a pattern into multiple insns. On machines that have instructions requiring delay slots (*note Delay Slots::.) or that have instructions whose output is not available for multiple cycles (*note Function Units::.), the compiler phases that optimize these cases need to be able to move insns into one-instruction delay slots. However, some insns may generate more than one machine instruction. These insns cannot be placed into a delay slot. Often you can rewrite the single insn as a list of individual insns, each corresponding to one machine instruction. The disadvantage of doing so is that it will cause the compilation to be slower and require more space. If the resulting insns are too complex, it may also suppress some optimizations. The compiler splits the insn if there is a reason to believe that it might improve instruction or delay slot scheduling. The insn combiner phase also splits putative insns. If three insns are merged into one insn with a complex expression that cannot be matched by some `define_insn' pattern, the combiner phase attempts to split the complex pattern into two insns that are recognized. Usually it can break the complex pattern into two patterns by splitting out some subexpression. However, in some other cases, such as performing an addition of a large constant in two insns on a RISC machine, the way to split the addition into two insns is machine-dependent. The `define_split' definition tells the compiler how to split a complex insn into several simpler insns. It looks like this: (define_split [INSN-PATTERN] "CONDITION" [NEW-INSN-PATTERN-1 NEW-INSN-PATTERN-2 ...] "PREPARATION STATEMENTS") INSN-PATTERN is a pattern that needs to be split and CONDITION is the final condition to be tested, as in a `define_insn'. When an insn matching INSN-PATTERN and satisfying CONDITION is found, it is replaced in the insn list with the insns given by NEW-INSN-PATTERN-1, NEW-INSN-PATTERN-2, etc. The PREPARATION STATEMENTS are similar to those statements that are specified for `define_expand' (*note Expander Definitions::.) and are executed before the new RTL is generated to prepare for the generated code or emit some insns whose pattern is not fixed. Unlike those in `define_expand', however, these statements must not generate any new pseudo-registers. Once reload has completed, they also must not allocate any space in the stack frame. Patterns are matched against INSN-PATTERN in two different circumstances. If an insn needs to be split for delay slot scheduling or insn scheduling, the insn is already known to be valid, which means that it must have been matched by some `define_insn' and, if `reload_completed' is non-zero, is known to satisfy the constraints of that `define_insn'. In that case, the new insn patterns must also be insns that are matched by some `define_insn' and, if `reload_completed' is non-zero, must also satisfy the constraints of those definitions. As an example of this usage of `define_split', consider the following example from `a29k.md', which splits a `sign_extend' from `HImode' to `SImode' into a pair of shift insns: (define_split [(set (match_operand:SI 0 "gen_reg_operand" "") (sign_extend:SI (match_operand:HI 1 "gen_reg_operand" "")))] "" [(set (match_dup 0) (ashift:SI (match_dup 1) (const_int 16))) (set (match_dup 0) (ashiftrt:SI (match_dup 0) (const_int 16)))] " { operands[1] = gen_lowpart (SImode, operands[1]); }") When the combiner phase tries to split an insn pattern, it is always the case that the pattern is *not* matched by any `define_insn'. The combiner pass first tries to split a single `set' expression and then the same `set' expression inside a `parallel', but followed by a `clobber' of a pseudo-reg to use as a scratch register. In these cases, the combiner expects exactly two new insn patterns to be generated. It will verify that these patterns match some `define_insn' definitions, so you need not do this test in the `define_split' (of course, there is no point in writing a `define_split' that will never produce insns that match). Here is an example of this use of `define_split', taken from `rs6000.md': (define_split [(set (match_operand:SI 0 "gen_reg_operand" "") (plus:SI (match_operand:SI 1 "gen_reg_operand" "") (match_operand:SI 2 "non_add_cint_operand" "")))] "" [(set (match_dup 0) (plus:SI (match_dup 1) (match_dup 3))) (set (match_dup 0) (plus:SI (match_dup 0) (match_dup 4)))] " { int low = INTVAL (operands[2]) & 0xffff; int high = (unsigned) INTVAL (operands[2]) >> 16; if (low & 0x8000) high++, low |= 0xffff0000; operands[3] = gen_rtx (CONST_INT, VOIDmode, high << 16); operands[4] = gen_rtx (CONST_INT, VOIDmode, low); }") Here the predicate `non_add_cint_operand' matches any `const_int' that is *not* a valid operand of a single add insn. The add with the smaller displacement is written so that it can be substituted into the address of a subsequent operation. An example that uses a scratch register, from the same file, generates an equality comparison of a register and a large constant: (define_split [(set (match_operand:CC 0 "cc_reg_operand" "") (compare:CC (match_operand:SI 1 "gen_reg_operand" "") (match_operand:SI 2 "non_short_cint_operand" ""))) (clobber (match_operand:SI 3 "gen_reg_operand" ""))] "find_single_use (operands[0], insn, 0) && (GET_CODE (*find_single_use (operands[0], insn, 0)) == EQ || GET_CODE (*find_single_use (operands[0], insn, 0)) == NE)" [(set (match_dup 3) (xor:SI (match_dup 1) (match_dup 4))) (set (match_dup 0) (compare:CC (match_dup 3) (match_dup 5)))] " { /* Get the constant we are comparing against, C, and see what it looks like sign-extended to 16 bits. Then see what constant could be XOR'ed with C to get the sign-extended value. */ int c = INTVAL (operands[2]); int sextc = (c << 16) >> 16; int xorv = c ^ sextc; operands[4] = gen_rtx (CONST_INT, VOIDmode, xorv); operands[5] = gen_rtx (CONST_INT, VOIDmode, sextc); }") To avoid confusion, don't write a single `define_split' that accepts some insns that match some `define_insn' as well as some insns that don't. Instead, write two separate `define_split' definitions, one for the insns that are valid and one for the insns that are not valid.  File: gcc.info, Node: Insn Attributes, Prev: Insn Splitting, Up: Machine Desc Instruction Attributes ====================== In addition to describing the instruction supported by the target machine, the `md' file also defines a group of "attributes" and a set of values for each. Every generated insn is assigned a value for each attribute. One possible attribute would be the effect that the insn has on the machine's condition code. This attribute can then be used by `NOTICE_UPDATE_CC' to track the condition codes. * Menu: * Defining Attributes:: Specifying attributes and their values. * Expressions:: Valid expressions for attribute values. * Tagging Insns:: Assigning attribute values to insns. * Attr Example:: An example of assigning attributes. * Insn Lengths:: Computing the length of insns. * Constant Attributes:: Defining attributes that are constant. * Delay Slots:: Defining delay slots required for a machine. * Function Units:: Specifying information for insn scheduling.  File: gcc.info, Node: Defining Attributes, Next: Expressions, Up: Insn Attributes Defining Attributes and their Values ------------------------------------ The `define_attr' expression is used to define each attribute required by the target machine. It looks like: (define_attr NAME LIST-OF-VALUES DEFAULT) NAME is a string specifying the name of the attribute being defined. LIST-OF-VALUES is either a string that specifies a comma-separated list of values that can be assigned to the attribute, or a null string to indicate that the attribute takes numeric values. DEFAULT is an attribute expression that gives the value of this attribute for insns that match patterns whose definition does not include an explicit value for this attribute. *Note Attr Example::, for more information on the handling of defaults. *Note Constant Attributes::, for information on attributes that do not depend on any particular insn. For each defined attribute, a number of definitions are written to the `insn-attr.h' file. For cases where an explicit set of values is specified for an attribute, the following are defined: * A `#define' is written for the symbol `HAVE_ATTR_NAME'. * An enumeral class is defined for `attr_NAME' with elements of the form `UPPER-NAME_UPPER-VALUE' where the attribute name and value are first converted to upper case. * A function `get_attr_NAME' is defined that is passed an insn and returns the attribute value for that insn. For example, if the following is present in the `md' file: (define_attr "type" "branch,fp,load,store,arith" ...) the following lines will be written to the file `insn-attr.h'. #define HAVE_ATTR_type enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD, TYPE_STORE, TYPE_ARITH}; extern enum attr_type get_attr_type (); If the attribute takes numeric values, no `enum' type will be defined and the function to obtain the attribute's value will return `int'.  File: gcc.info, Node: Expressions, Next: Tagging Insns, Prev: Defining Attributes, Up: Insn Attributes Attribute Expressions --------------------- RTL expressions used to define attributes use the codes described above plus a few specific to attribute definitions, to be discussed below. Attribute value expressions must have one of the following forms: `(const_int I)' The integer I specifies the value of a numeric attribute. I must be non-negative. The value of a numeric attribute can be specified either with a `const_int' or as an integer represented as a string in `const_string', `eq_attr' (see below), and `set_attr' (*note Tagging Insns::.) expressions. `(const_string VALUE)' The string VALUE specifies a constant attribute value. If VALUE is specified as `"*"', it means that the default value of the attribute is to be used for the insn containing this expression. `"*"' obviously cannot be used in the DEFAULT expression of a `define_attr'. If the attribute whose value is being specified is numeric, VALUE must be a string containing a non-negative integer (normally `const_int' would be used in this case). Otherwise, it must contain one of the valid values for the attribute. `(if_then_else TEST TRUE-VALUE FALSE-VALUE)' TEST specifies an attribute test, whose format is defined below. The value of this expression is TRUE-VALUE if TEST is true, otherwise it is FALSE-VALUE. `(cond [TEST1 VALUE1 ...] DEFAULT)' The first operand of this expression is a vector containing an even number of expressions and consisting of pairs of TEST and VALUE expressions. The value of the `cond' expression is that of the VALUE corresponding to the first true TEST expression. If none of the TEST expressions are true, the value of the `cond' expression is that of the DEFAULT expression. TEST expressions can have one of the following forms: `(const_int I)' This test is true if I is non-zero and false otherwise. `(not TEST)' `(ior TEST1 TEST2)' `(and TEST1 TEST2)' These tests are true if the indicated logical function is true. `(match_operand:M N PRED CONSTRAINTS)' This test is true if operand N of the insn whose attribute value is being determined has mode M (this part of the test is ignored if M is `VOIDmode') and the function specified by the string PRED returns a non-zero value when passed operand N and mode M (this part of the test is ignored if PRED is the null string). The CONSTRAINTS operand is ignored and should be the null string. `(le ARITH1 ARITH2)' `(leu ARITH1 ARITH2)' `(lt ARITH1 ARITH2)' `(ltu ARITH1 ARITH2)' `(gt ARITH1 ARITH2)' `(gtu ARITH1 ARITH2)' `(ge ARITH1 ARITH2)' `(geu ARITH1 ARITH2)' `(ne ARITH1 ARITH2)' `(eq ARITH1 ARITH2)' These tests are true if the indicated comparison of the two arithmetic expressions is true. Arithmetic expressions are formed with `plus', `minus', `mult', `div', `mod', `abs', `neg', `and', `ior', `xor', `not', `ashift', `lshiftrt', and `ashiftrt' expressions. `const_int' and `symbol_ref' are always valid terms (*note Insn Lengths::.,for additional forms). `symbol_ref' is a string denoting a C expression that yields an `int' when evaluated by the `get_attr_...' routine. It should normally be a global variable. `(eq_attr NAME VALUE)' NAME is a string specifying the name of an attribute. VALUE is a string that is either a valid value for attribute NAME, a comma-separated list of values, or `!' followed by a value or list. If VALUE does not begin with a `!', this test is true if the value of the NAME attribute of the current insn is in the list specified by VALUE. If VALUE begins with a `!', this test is true if the attribute's value is *not* in the specified list. For example, (eq_attr "type" "load,store") is equivalent to (ior (eq_attr "type" "load") (eq_attr "type" "store")) If NAME specifies an attribute of `alternative', it refers to the value of the compiler variable `which_alternative' (*note Output Statement::.) and the values must be small integers. For example, (eq_attr "alternative" "2,3") is equivalent to (ior (eq (symbol_ref "which_alternative") (const_int 2)) (eq (symbol_ref "which_alternative") (const_int 3))) Note that, for most attributes, an `eq_attr' test is simplified in cases where the value of the attribute being tested is known for all insns matching a particular pattern. This is by far the most common case. `(attr_flag NAME)' The value of an `attr_flag' expression is true if the flag specified by NAME is true for the `insn' currently being scheduled. NAME is a string specifying one of a fixed set of flags to test. Test the flags `forward' and `backward' to determine the direction of a conditional branch. Test the flags `very_likely', `likely', `very_unlikely', and `unlikely' to determine if a conditional branch is expected to be taken. If the `very_likely' flag is true, then the `likely' flag is also true. Likewise for the `very_unlikely' and `unlikely' flags. This example describes a conditional branch delay slot which can be nullified for forward branches that are taken (annul-true) or for backward branches which are not taken (annul-false). (define_delay (eq_attr "type" "cbranch") [(eq_attr "in_branch_delay" "true") (and (eq_attr "in_branch_delay" "true") (attr_flag "forward")) (and (eq_attr "in_branch_delay" "true") (attr_flag "backward"))]) The `forward' and `backward' flags are false if the current `insn' being scheduled is not a conditional branch. The `very_likely' and `likely' flags are true if the `insn' being scheduled is not a conditional branch. The `very_unlikely' and `unlikely' flags are false if the `insn' being scheduled is not a conditional branch. `attr_flag' is only used during delay slot scheduling and has no meaning to other passes of the compiler.  File: gcc.info, Node: Tagging Insns, Next: Attr Example, Prev: Expressions, Up: Insn Attributes Assigning Attribute Values to Insns ----------------------------------- The value assigned to an attribute of an insn is primarily determined by which pattern is matched by that insn (or which `define_peephole' generated it). Every `define_insn' and `define_peephole' can have an optional last argument to specify the values of attributes for matching insns. The value of any attribute not specified in a particular insn is set to the default value for that attribute, as specified in its `define_attr'. Extensive use of default values for attributes permits the specification of the values for only one or two attributes in the definition of most insn patterns, as seen in the example in the next section. The optional last argument of `define_insn' and `define_peephole' is a vector of expressions, each of which defines the value for a single attribute. The most general way of assigning an attribute's value is to use a `set' expression whose first operand is an `attr' expression giving the name of the attribute being set. The second operand of the `set' is an attribute expression (*note Expressions::.) giving the value of the attribute. When the attribute value depends on the `alternative' attribute (i.e., which is the applicable alternative in the constraint of the insn), the `set_attr_alternative' expression can be used. It allows the specification of a vector of attribute expressions, one for each alternative. When the generality of arbitrary attribute expressions is not required, the simpler `set_attr' expression can be used, which allows specifying a string giving either a single attribute value or a list of attribute values, one for each alternative. The form of each of the above specifications is shown below. In each case, NAME is a string specifying the attribute to be set. `(set_attr NAME VALUE-STRING)' VALUE-STRING is either a string giving the desired attribute value, or a string containing a comma-separated list giving the values for succeeding alternatives. The number of elements must match the number of alternatives in the constraint of the insn pattern. Note that it may be useful to specify `*' for some alternative, in which case the attribute will assume its default value for insns matching that alternative. `(set_attr_alternative NAME [VALUE1 VALUE2 ...])' Depending on the alternative of the insn, the value will be one of the specified values. This is a shorthand for using a `cond' with tests on the `alternative' attribute. `(set (attr NAME) VALUE)' The first operand of this `set' must be the special RTL expression `attr', whose sole operand is a string giving the name of the attribute being set. VALUE is the value of the attribute. The following shows three different ways of representing the same attribute value specification: (set_attr "type" "load,store,arith") (set_attr_alternative "type" [(const_string "load") (const_string "store") (const_string "arith")]) (set (attr "type") (cond [(eq_attr "alternative" "1") (const_string "load") (eq_attr "alternative" "2") (const_string "store")] (const_string "arith"))) The `define_asm_attributes' expression provides a mechanism to specify the attributes assigned to insns produced from an `asm' statement. It has the form: (define_asm_attributes [ATTR-SETS]) where ATTR-SETS is specified the same as for both the `define_insn' and the `define_peephole' expressions. These values will typically be the "worst case" attribute values. For example, they might indicate that the condition code will be clobbered. A specification for a `length' attribute is handled specially. The way to compute the length of an `asm' insn is to multiply the length specified in the expression `define_asm_attributes' by the number of machine instructions specified in the `asm' statement, determined by counting the number of semicolons and newlines in the string. Therefore, the value of the `length' attribute specified in a `define_asm_attributes' should be the maximum possible length of a single machine instruction.  File: gcc.info, Node: Attr Example, Next: Insn Lengths, Prev: Tagging Insns, Up: Insn Attributes Example of Attribute Specifications ----------------------------------- The judicious use of defaulting is important in the efficient use of insn attributes. Typically, insns are divided into "types" and an attribute, customarily called `type', is used to represent this value. This attribute is normally used only to define the default value for other attributes. An example will clarify this usage. Assume we have a RISC machine with a condition code and in which only full-word operations are performed in registers. Let us assume that we can divide all insns into loads, stores, (integer) arithmetic operations, floating point operations, and branches. Here we will concern ourselves with determining the effect of an insn on the condition code and will limit ourselves to the following possible effects: The condition code can be set unpredictably (clobbered), not be changed, be set to agree with the results of the operation, or only changed if the item previously set into the condition code has been modified. Here is part of a sample `md' file for such a machine: (define_attr "type" "load,store,arith,fp,branch" (const_string "arith")) (define_attr "cc" "clobber,unchanged,set,change0" (cond [(eq_attr "type" "load") (const_string "change0") (eq_attr "type" "store,branch") (const_string "unchanged") (eq_attr "type" "arith") (if_then_else (match_operand:SI 0 "" "") (const_string "set") (const_string "clobber"))] (const_string "clobber"))) (define_insn "" [(set (match_operand:SI 0 "general_operand" "=r,r,m") (match_operand:SI 1 "general_operand" "r,m,r"))] "" "@ move %0,%1 load %0,%1 store %0,%1" [(set_attr "type" "arith,load,store")]) Note that we assume in the above example that arithmetic operations performed on quantities smaller than a machine word clobber the condition code since they will set the condition code to a value corresponding to the full-word result.  File: gcc.info, Node: Insn Lengths, Next: Constant Attributes, Prev: Attr Example, Up: Insn Attributes Computing the Length of an Insn ------------------------------- For many machines, multiple types of branch instructions are provided, each for different length branch displacements. In most cases, the assembler will choose the correct instruction to use. However, when the assembler cannot do so, GCC can when a special attribute, the `length' attribute, is defined. This attribute must be defined to have numeric values by specifying a null string in its `define_attr'. In the case of the `length' attribute, two additional forms of arithmetic terms are allowed in test expressions: `(match_dup N)' This refers to the address of operand N of the current insn, which must be a `label_ref'. `(pc)' This refers to the address of the *current* insn. It might have been more consistent with other usage to make this the address of the *next* insn but this would be confusing because the length of the current insn is to be computed. For normal insns, the length will be determined by value of the `length' attribute. In the case of `addr_vec' and `addr_diff_vec' insn patterns, the length is computed as the number of vectors multiplied by the size of each vector. Lengths are measured in addressable storage units (bytes). The following macros can be used to refine the length computation: `FIRST_INSN_ADDRESS' When the `length' insn attribute is used, this macro specifies the value to be assigned to the address of the first insn in a function. If not specified, 0 is used. `ADJUST_INSN_LENGTH (INSN, LENGTH)' If defined, modifies the length assigned to instruction INSN as a function of the context in which it is used. LENGTH is an lvalue that contains the initially computed length of the insn and should be updated with the correct length of the insn. If updating is required, INSN must not be a varying-length insn. This macro will normally not be required. A case in which it is required is the ROMP. On this machine, the size of an `addr_vec' insn must be increased by two to compensate for the fact that alignment may be required. The routine that returns `get_attr_length' (the value of the `length' attribute) can be used by the output routine to determine the form of the branch instruction to be written, as the example below illustrates. As an example of the specification of variable-length branches, consider the IBM 360. If we adopt the convention that a register will be set to the starting address of a function, we can jump to labels within 4k of the start using a four-byte instruction. Otherwise, we need a six-byte sequence to load the address from memory and then branch to it. On such a machine, a pattern for a branch instruction might be specified as follows: (define_insn "jump" [(set (pc) (label_ref (match_operand 0 "" "")))] "" "* { return (get_attr_length (insn) == 4 ? \"b %l0\" : \"l r15,=a(%l0); br r15\"); }" [(set (attr "length") (if_then_else (lt (match_dup 0) (const_int 4096)) (const_int 4) (const_int 6)))])  File: gcc.info, Node: Constant Attributes, Next: Delay Slots, Prev: Insn Lengths, Up: Insn Attributes Constant Attributes ------------------- A special form of `define_attr', where the expression for the default value is a `const' expression, indicates an attribute that is constant for a given run of the compiler. Constant attributes may be used to specify which variety of processor is used. For example, (define_attr "cpu" "m88100,m88110,m88000" (const (cond [(symbol_ref "TARGET_88100") (const_string "m88100") (symbol_ref "TARGET_88110") (const_string "m88110")] (const_string "m88000")))) (define_attr "memory" "fast,slow" (const (if_then_else (symbol_ref "TARGET_FAST_MEM") (const_string "fast") (const_string "slow")))) The routine generated for constant attributes has no parameters as it does not depend on any particular insn. RTL expressions used to define the value of a constant attribute may use the `symbol_ref' form, but may not use either the `match_operand' form or `eq_attr' forms involving insn attributes.  File: gcc.info, Node: Delay Slots, Next: Function Units, Prev: Constant Attributes, Up: Insn Attributes Delay Slot Scheduling --------------------- The insn attribute mechanism can be used to specify the requirements for delay slots, if any, on a target machine. An instruction is said to require a "delay slot" if some instructions that are physically after the instruction are executed as if they were located before it. Classic examples are branch and call instructions, which often execute the following instruction before the branch or call is performed. On some machines, conditional branch instructions can optionally "annul" instructions in the delay slot. This means that the instruction will not be executed for certain branch outcomes. Both instructions that annul if the branch is true and instructions that annul if the branch is false are supported. Delay slot scheduling differs from instruction scheduling in that determining whether an instruction needs a delay slot is dependent only on the type of instruction being generated, not on data flow between the instructions. See the next section for a discussion of data-dependent instruction scheduling. The requirement of an insn needing one or more delay slots is indicated via the `define_delay' expression. It has the following form: (define_delay TEST [DELAY-1 ANNUL-TRUE-1 ANNUL-FALSE-1 DELAY-2 ANNUL-TRUE-2 ANNUL-FALSE-2 ...]) TEST is an attribute test that indicates whether this `define_delay' applies to a particular insn. If so, the number of required delay slots is determined by the length of the vector specified as the second argument. An insn placed in delay slot N must satisfy attribute test DELAY-N. ANNUL-TRUE-N is an attribute test that specifies which insns may be annulled if the branch is true. Similarly, ANNUL-FALSE-N specifies which insns in the delay slot may be annulled if the branch is false. If annulling is not supported for that delay slot, `(nil)' should be coded. For example, in the common case where branch and call insns require a single delay slot, which may contain any insn other than a branch or call, the following would be placed in the `md' file: (define_delay (eq_attr "type" "branch,call") [(eq_attr "type" "!branch,call") (nil) (nil)]) Multiple `define_delay' expressions may be specified. In this case, each such expression specifies different delay slot requirements and there must be no insn for which tests in two `define_delay' expressions are both true. For example, if we have a machine that requires one delay slot for branches but two for calls, no delay slot can contain a branch or call insn, and any valid insn in the delay slot for the branch can be annulled if the branch is true, we might represent this as follows: (define_delay (eq_attr "type" "branch") [(eq_attr "type" "!branch,call") (eq_attr "type" "!branch,call") (nil)]) (define_delay (eq_attr "type" "call") [(eq_attr "type" "!branch,call") (nil) (nil) (eq_attr "type" "!branch,call") (nil) (nil)])