GNU Compiler Collection (GCC) Internals: Insns

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### 14.19 Insns

The RTL representation of the code for a function is a doubly-linked chain of objects called insns. Insns are expressions with special codes that are used for no other purpose. Some insns are actual instructions; others represent dispatch tables for `switch` statements; others represent labels to jump to or various sorts of declarative information.

In addition to its own specific data, each insn must have a unique id-number that distinguishes it from all other insns in the current function (after delayed branch scheduling, copies of an insn with the same id-number may be present in multiple places in a function, but these copies will always be identical and will only appear inside a `sequence`), and chain pointers to the preceding and following insns. These three fields occupy the same position in every insn, independent of the expression code of the insn. They could be accessed with `XEXP` and `XINT`, but instead three special macros are always used:

`INSN_UID (i)`

  Accesses the unique id of insn i.

`PREV_INSN (i)`

  Accesses the chain pointer to the insn preceding i. If i is the first insn, this is a null pointer.

`NEXT_INSN (i)`

  Accesses the chain pointer to the insn following i. If i is the last insn, this is a null pointer.

The first insn in the chain is obtained by calling `get_insns`; the last insn is the result of calling `get_last_insn`. Within the chain delimited by these insns, the `NEXT_INSN` and `PREV_INSN` pointers must always correspond: if insn is not the first insn,

    NEXT_INSN (PREV_INSN (insn)) == insn

is always true and if insn is not the last insn,

    PREV_INSN (NEXT_INSN (insn)) == insn

is always true.

After delay slot scheduling, some of the insns in the chain might be `sequence` expressions, which contain a vector of insns. The value of `NEXT_INSN` in all but the last of these insns is the next insn in the vector; the value of `NEXT_INSN` of the last insn in the vector is the same as the value of `NEXT_INSN` for the `sequence` in which it is contained. Similar rules apply for `PREV_INSN`.

This means that the above invariants are not necessarily true for insns inside `sequence` expressions. Specifically, if insn is the first insn in a `sequence`, `NEXT_INSN (PREV_INSN (insn))` is the insn containing the `sequence` expression, as is the value of `PREV_INSN (NEXT_INSN (insn))` if insn is the last insn in the `sequence` expression. You can use these expressions to find the containing `sequence` expression.

Every insn has one of the following expression codes:

`insn`

  The expression code `insn` is used for instructions that do not jump and do not do function calls. `sequence` expressions are always contained in insns with code `insn` even if one of those insns should jump or do function calls.

  Insns with code `insn` have four additional fields beyond the three mandatory ones listed above. These four are described in a table below.

`jump_insn`

  The expression code `jump_insn` is used for instructions that may jump (or, more generally, may contain `label_ref` expressions to which `pc` can be set in that instruction). If there is an instruction to return from the current function, it is recorded as a `jump_insn`.

  `jump_insn` insns have the same extra fields as `insn` insns, accessed in the same way and in addition contain a field `JUMP_LABEL` which is defined once jump optimization has completed.

  For simple conditional and unconditional jumps, this field contains the `code_label` to which this insn will (possibly conditionally) branch. In a more complex jump, `JUMP_LABEL` records one of the labels that the insn refers to; other jump target labels are recorded as `REG_LABEL_TARGET` notes. The exception is `addr_vec` and `addr_diff_vec`, where `JUMP_LABEL` is `NULL_RTX` and the only way to find the labels is to scan the entire body of the insn.

  Return insns count as jumps, but their `JUMP_LABEL` is `RETURN` or `SIMPLE_RETURN`.

`call_insn`

  The expression code `call_insn` is used for instructions that may do function calls. It is important to distinguish these instructions because they imply that certain registers and memory locations may be altered unpredictably.

  `call_insn` insns have the same extra fields as `insn` insns, accessed in the same way and in addition contain a field `CALL_INSN_FUNCTION_USAGE`, which contains a list (chain of `expr_list` expressions) containing `use`, `clobber` and sometimes `set` expressions that denote hard registers and `mem`s used or clobbered by the called function.

  A `mem` generally points to a stack slot in which arguments passed to the libcall by reference (see TARGET_PASS_BY_REFERENCE) are stored. If the argument is caller-copied (see TARGET_CALLEE_COPIES), the stack slot will be mentioned in `clobber` and `use` entries; if it’s callee-copied, only a `use` will appear, and the `mem` may point to addresses that are not stack slots.

  Registers occurring inside a `clobber` in this list augment registers specified in `CALL_USED_REGISTERS` (see Register Basics).

  If the list contains a `set` involving two registers, it indicates that the function returns one of its arguments. Such a `set` may look like a no-op if the same register holds the argument and the return value.

`code_label`

  A `code_label` insn represents a label that a jump insn can jump to. It contains two special fields of data in addition to the three standard ones. `CODE_LABEL_NUMBER` is used to hold the label number, a number that identifies this label uniquely among all the labels in the compilation (not just in the current function). Ultimately, the label is represented in the assembler output as an assembler label, usually of the form ‘`Ln`’ where n is the label number.

  When a `code_label` appears in an RTL expression, it normally appears within a `label_ref` which represents the address of the label, as a number.

  Besides as a `code_label`, a label can also be represented as a `note` of type `NOTE_INSN_DELETED_LABEL`.

  The field `LABEL_NUSES` is only defined once the jump optimization phase is completed. It contains the number of times this label is referenced in the current function.

  The field `LABEL_KIND` differentiates four different types of labels: `LABEL_NORMAL`, `LABEL_STATIC_ENTRY`, `LABEL_GLOBAL_ENTRY`, and `LABEL_WEAK_ENTRY`. The only labels that do not have type `LABEL_NORMAL` are alternate entry points to the current function. These may be static (visible only in the containing translation unit), global (exposed to all translation units), or weak (global, but can be overridden by another symbol with the same name).

  Much of the compiler treats all four kinds of label identically. Some of it needs to know whether or not a label is an alternate entry point; for this purpose, the macro `LABEL_ALT_ENTRY_P` is provided. It is equivalent to testing whether ‘`LABEL_KIND (label) == LABEL_NORMAL`’. The only place that cares about the distinction between static, global, and weak alternate entry points, besides the front-end code that creates them, is the function `output_alternate_entry_point`, in `final.cc`.

  To set the kind of a label, use the `SET_LABEL_KIND` macro.

`jump_table_data`

  A `jump_table_data` insn is a placeholder for the jump-table data of a `casesi` or `tablejump` insn. They are placed after a `tablejump_p` insn. A `jump_table_data` insn is not part o a basic blockm but it is associated with the basic block that ends with the `tablejump_p` insn. The `PATTERN` of a `jump_table_data` is always either an `addr_vec` or an `addr_diff_vec`, and a `jump_table_data` insn is always preceded by a `code_label`. The `tablejump_p` insn refers to that `code_label` via its `JUMP_LABEL`.

`barrier`

  Barriers are placed in the instruction stream when control cannot flow past them. They are placed after unconditional jump instructions to indicate that the jumps are unconditional and after calls to `volatile` functions, which do not return (e.g., `exit`). They contain no information beyond the three standard fields.

`note`

  `note` insns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro `NOTE_LINE_NUMBER` and a string accessed with `NOTE_SOURCE_FILE`.

  If `NOTE_LINE_NUMBER` is positive, the note represents the position of a source line and `NOTE_SOURCE_FILE` is the source file name that the line came from. These notes control generation of line number data in the assembler output.

  Otherwise, `NOTE_LINE_NUMBER` is not really a line number but a code with one of the following values (and `NOTE_SOURCE_FILE` must contain a null pointer):

  `NOTE_INSN_DELETED`

    Such a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind.

  `NOTE_INSN_DELETED_LABEL`

    This marks what used to be a `code_label`, but was not used for other purposes than taking its address and was transformed to mark that no code jumps to it.

  `NOTE_INSN_BLOCK_BEG`
  `NOTE_INSN_BLOCK_END`

    These types of notes indicate the position of the beginning and end of a level of scoping of variable names. They control the output of debugging information.

  `NOTE_INSN_EH_REGION_BEG`
  `NOTE_INSN_EH_REGION_END`

    These types of notes indicate the position of the beginning and end of a level of scoping for exception handling. `NOTE_EH_HANDLER` identifies which region is associated with these notes.

  `NOTE_INSN_FUNCTION_BEG`

    Appears at the start of the function body, after the function prologue.

  `NOTE_INSN_VAR_LOCATION`

    This note is used to generate variable location debugging information. It indicates that the user variable in its `VAR_LOCATION` operand is at the location given in the RTL expression, or holds a value that can be computed by evaluating the RTL expression from that static point in the program up to the next such note for the same user variable.

  `NOTE_INSN_BEGIN_STMT`

    This note is used to generate `is_stmt` markers in line number debugging information. It indicates the beginning of a user statement.

  `NOTE_INSN_INLINE_ENTRY`

    This note is used to generate `entry_pc` for inlined subroutines in debugging information. It indicates an inspection point at which all arguments for the inlined function have been bound, and before its first statement.

  These codes are printed symbolically when they appear in debugging dumps.

`debug_insn`

  The expression code `debug_insn` is used for pseudo-instructions that hold debugging information for variable tracking at assignments (see `-fvar-tracking-assignments` option). They are the RTL representation of `GIMPLE_DEBUG` statements (`GIMPLE_DEBUG`), with a `VAR_LOCATION` operand that binds a user variable tree to an RTL representation of the `value` in the corresponding statement. A `DEBUG_EXPR` in it stands for the value bound to the corresponding `DEBUG_EXPR_DECL`.

  `GIMPLE_DEBUG_BEGIN_STMT` and `GIMPLE_DEBUG_INLINE_ENTRY` are expanded to RTL as a `DEBUG_INSN` with a `DEBUG_MARKER` `PATTERN`; the difference is the RTL mode: the former’s `DEBUG_MARKER` is `VOIDmode`, whereas the latter is `BLKmode`; information about the inlined function can be taken from the lexical block encoded in the `INSN_LOCATION`. These `DEBUG_INSN`s, that do not carry `VAR_LOCATION` information, just `DEBUG_MARKER`s, can be detected by testing `DEBUG_MARKER_INSN_P`, whereas those that do can be recognized as `DEBUG_BIND_INSN_P`.

  Throughout optimization passes, `DEBUG_INSN`s are not reordered with respect to each other, particularly during scheduling. Binding information is kept in pseudo-instruction form, so that, unlike notes, it gets the same treatment and adjustments that regular instructions would. It is the variable tracking pass that turns these pseudo-instructions into `NOTE_INSN_VAR_LOCATION`, `NOTE_INSN_BEGIN_STMT` and `NOTE_INSN_INLINE_ENTRY` notes, analyzing control flow, value equivalences and changes to registers and memory referenced in value expressions, propagating the values of debug temporaries and determining expressions that can be used to compute the value of each user variable at as many points (ranges, actually) in the program as possible.

  Unlike `NOTE_INSN_VAR_LOCATION`, the value expression in an `INSN_VAR_LOCATION` denotes a value at that specific point in the program, rather than an expression that can be evaluated at any later point before an overriding `VAR_LOCATION` is encountered. E.g., if a user variable is bound to a `REG` and then a subsequent insn modifies the `REG`, the note location would keep mapping the user variable to the register across the insn, whereas the insn location would keep the variable bound to the value, so that the variable tracking pass would emit another location note for the variable at the point in which the register is modified.

The machine mode of an insn is normally `VOIDmode`, but some phases use the mode for various purposes.

The common subexpression elimination pass sets the mode of an insn to `QImode` when it is the first insn in a block that has already been processed.

The second Haifa scheduling pass, for targets that can multiple issue, sets the mode of an insn to `TImode` when it is believed that the instruction begins an issue group. That is, when the instruction cannot issue simultaneously with the previous. This may be relied on by later passes, in particular machine-dependent reorg.

Here is a table of the extra fields of `insn`, `jump_insn` and `call_insn` insns:

`PATTERN (i)`

  An expression for the side effect performed by this insn. This must be one of the following codes: `set`, `call`, `use`, `clobber`, `return`, `simple_return`, `asm_input`, `asm_output`, `addr_vec`, `addr_diff_vec`, `trap_if`, `unspec`, `unspec_volatile`, `parallel`, `cond_exec`, or `sequence`. If it is a `parallel`, each element of the `parallel` must be one these codes, except that `parallel` expressions cannot be nested and `addr_vec` and `addr_diff_vec` are not permitted inside a `parallel` expression.

`INSN_CODE (i)`

  An integer that says which pattern in the machine description matches this insn, or -1 if the matching has not yet been attempted.

  Such matching is never attempted and this field remains -1 on an insn whose pattern consists of a single `use`, `clobber`, `asm_input`, `addr_vec` or `addr_diff_vec` expression.

  Matching is also never attempted on insns that result from an `asm` statement. These contain at least one `asm_operands` expression. The function `asm_noperands` returns a non-negative value for such insns.

  In the debugging output, this field is printed as a number followed by a symbolic representation that locates the pattern in the `md` file as some small positive or negative offset from a named pattern.

`REG_NOTES (i)`

  A list (chain of `expr_list`, `insn_list` and `int_list` expressions) giving miscellaneous information about the insn. It is often information pertaining to the registers used in this insn.

The `REG_NOTES` field of an insn is a chain that includes `expr_list` and `int_list` expressions as well as `insn_list` expressions. There are several kinds of register notes, which are distinguished by the machine mode, which in a register note is really understood as being an `enum reg_note`. The first operand op of the note is data whose meaning depends on the kind of note.

The macro `REG_NOTE_KIND (x)` returns the kind of register note. Its counterpart, the macro `PUT_REG_NOTE_KIND (x, newkind)` sets the register note type of x to be newkind.

Register notes are of three classes: They may say something about an input to an insn, they may say something about an output of an insn, or they may create a linkage between two insns.

These register notes annotate inputs to an insn:

`REG_DEAD`

  The value in op dies in this insn; that is to say, altering the value immediately after this insn would not affect the future behavior of the program.

  It does not follow that the register op has no useful value after this insn since op is not necessarily modified by this insn. Rather, no subsequent instruction uses the contents of op.

`REG_UNUSED`

  The register op being set by this insn will not be used in a subsequent insn. This differs from a `REG_DEAD` note, which indicates that the value in an input will not be used subsequently. These two notes are independent; both may be present for the same register.

`REG_INC`

  The register op is incremented (or decremented; at this level there is no distinction) by an embedded side effect inside this insn. This means it appears in a `post_inc`, `pre_inc`, `post_dec` or `pre_dec` expression.

`REG_NONNEG`

  The register op is known to have a nonnegative value when this insn is reached. This is used by special looping instructions that terminate when the register goes negative.

  The `REG_NONNEG` note is added only to ‘`doloop_end`’ insns, if its pattern uses a `ge` condition.

`REG_LABEL_OPERAND`

  This insn uses op, a `code_label` or a `note` of type `NOTE_INSN_DELETED_LABEL`, but is not a `jump_insn`, or it is a `jump_insn` that refers to the operand as an ordinary operand. The label may still eventually be a jump target, but if so in an indirect jump in a subsequent insn. The presence of this note allows jump optimization to be aware that op is, in fact, being used, and flow optimization to build an accurate flow graph.

`REG_LABEL_TARGET`

  This insn is a `jump_insn` but not an `addr_vec` or `addr_diff_vec`. It uses op, a `code_label` as a direct or indirect jump target. Its purpose is similar to that of `REG_LABEL_OPERAND`. This note is only present if the insn has multiple targets; the last label in the insn (in the highest numbered insn-field) goes into the `JUMP_LABEL` field and does not have a `REG_LABEL_TARGET` note. See JUMP_LABEL.

`REG_SETJMP`

  Appears attached to each `CALL_INSN` to `setjmp` or a related function.

The following notes describe attributes of outputs of an insn:

`REG_EQUIV`
`REG_EQUAL`

  This note is only valid on an insn that sets only one register and indicates that that register will be equal to op at run time; the scope of this equivalence differs between the two types of notes. The value which the insn explicitly copies into the register may look different from op, but they will be equal at run time. If the output of the single `set` is a `strict_low_part` or `zero_extract` expression, the note refers to the register that is contained in its first operand.

  For `REG_EQUIV`, the register is equivalent to op throughout the entire function, and could validly be replaced in all its occurrences by op. (“Validly” here refers to the data flow of the program; simple replacement may make some insns invalid.) For example, when a constant is loaded into a register that is never assigned any other value, this kind of note is used.

  When a parameter is copied into a pseudo-register at entry to a function, a note of this kind records that the register is equivalent to the stack slot where the parameter was passed. Although in this case the register may be set by other insns, it is still valid to replace the register by the stack slot throughout the function.

  A `REG_EQUIV` note is also used on an instruction which copies a register parameter into a pseudo-register at entry to a function, if there is a stack slot where that parameter could be stored. Although other insns may set the pseudo-register, it is valid for the compiler to replace the pseudo-register by stack slot throughout the function, provided the compiler ensures that the stack slot is properly initialized by making the replacement in the initial copy instruction as well. This is used on machines for which the calling convention allocates stack space for register parameters. See `REG_PARM_STACK_SPACE` in Stack Arguments.

  In the case of `REG_EQUAL`, the register that is set by this insn will be equal to op at run time at the end of this insn but not necessarily elsewhere in the function. In this case, op is typically an arithmetic expression. For example, when a sequence of insns such as a library call is used to perform an arithmetic operation, this kind of note is attached to the insn that produces or copies the final value.

  These two notes are used in different ways by the compiler passes. `REG_EQUAL` is used by passes prior to register allocation (such as common subexpression elimination and loop optimization) to tell them how to think of that value. `REG_EQUIV` notes are used by register allocation to indicate that there is an available substitute expression (either a constant or a `mem` expression for the location of a parameter on the stack) that may be used in place of a register if insufficient registers are available.

  Except for stack homes for parameters, which are indicated by a `REG_EQUIV` note and are not useful to the early optimization passes and pseudo registers that are equivalent to a memory location throughout their entire life, which is not detected until later in the compilation, all equivalences are initially indicated by an attached `REG_EQUAL` note. In the early stages of register allocation, a `REG_EQUAL` note is changed into a `REG_EQUIV` note if op is a constant and the insn represents the only set of its destination register.

  Thus, compiler passes prior to register allocation need only check for `REG_EQUAL` notes and passes subsequent to register allocation need only check for `REG_EQUIV` notes.

These notes describe linkages between insns. They occur in pairs: one insn has one of a pair of notes that points to a second insn, which has the inverse note pointing back to the first insn.

`REG_DEP_TRUE`

  This indicates a true dependence (a read after write dependence).

`REG_DEP_OUTPUT`

  This indicates an output dependence (a write after write dependence).

`REG_DEP_ANTI`

  This indicates an anti dependence (a write after read dependence).

These notes describe information gathered from gcov profile data. They are stored in the `REG_NOTES` field of an insn.

`REG_BR_PROB`

  This is used to specify the ratio of branches to non-branches of a branch insn according to the profile data. The note is represented as an `int_list` expression whose integer value is an encoding of `profile_probability` type. `profile_probability` provide member function `from_reg_br_prob_note` and `to_reg_br_prob_note` to extract and store the probability into the RTL encoding.

`REG_BR_PRED`

  These notes are found in JUMP insns after delayed branch scheduling has taken place. They indicate both the direction and the likelihood of the JUMP. The format is a bitmask of ATTR_FLAG_* values.

`REG_FRAME_RELATED_EXPR`

  This is used on an RTX_FRAME_RELATED_P insn wherein the attached expression is used in place of the actual insn pattern. This is done in cases where the pattern is either complex or misleading.

The note `REG_CALL_NOCF_CHECK` is used in conjunction with the `-fcf-protection=branch` option. The note is set if a `nocf_check` attribute is specified for a function type or a pointer to function type. The note is stored in the `REG_NOTES` field of an insn.

`REG_CALL_NOCF_CHECK`

  Users have control through the `nocf_check` attribute to identify which calls to a function should be skipped from control-flow instrumentation when the option `-fcf-protection=branch` is specified. The compiler puts a `REG_CALL_NOCF_CHECK` note on each `CALL_INSN` instruction that has a function type marked with a `nocf_check` attribute.

For convenience, the machine mode in an `insn_list` or `expr_list` is printed using these symbolic codes in debugging dumps.

The only difference between the expression codes `insn_list` and `expr_list` is that the first operand of an `insn_list` is assumed to be an insn and is printed in debugging dumps as the insn’s unique id; the first operand of an `expr_list` is printed in the ordinary way as an expression.

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