GNU Compiler Collection (GCC) Internals: Flags

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13.5 Flags in an RTL Expression

RTL expressions contain several flags (one-bit bit-fields) that are used in certain types of expression. Most often they are accessed with the following macros, which expand into lvalues.

CONSTANT_POOL_ADDRESS_P (x)

Nonzero in a symbol_ref if it refers to part of the current function’s constant pool. For most targets these addresses are in a .rodata section entirely separate from the function, but for some targets the addresses are close to the beginning of the function. In either case GCC assumes these addresses can be addressed directly, perhaps with the help of base registers. Stored in the unchanging field and printed as ‘/u’.

RTL_CONST_CALL_P (x)

In a call_insn indicates that the insn represents a call to a const function. Stored in the unchanging field and printed as ‘/u’.

RTL_PURE_CALL_P (x)

In a call_insn indicates that the insn represents a call to a pure function. Stored in the return_val field and printed as ‘/i’.

RTL_CONST_OR_PURE_CALL_P (x)

In a call_insn, true if RTL_CONST_CALL_P or RTL_PURE_CALL_P is true.

RTL_LOOPING_CONST_OR_PURE_CALL_P (x)

In a call_insn indicates that the insn represents a possibly infinite looping call to a const or pure function. Stored in the call field and printed as ‘/c’. Only true if one of RTL_CONST_CALL_P or RTL_PURE_CALL_P is true.

INSN_ANNULLED_BRANCH_P (x)

In a jump_insn, call_insn, or insn indicates that the branch is an annulling one. See the discussion under sequence below. Stored in the unchanging field and printed as ‘/u’.

INSN_DELETED_P (x)

In an insn, call_insn, jump_insn, code_label, jump_table_data, barrier, or note, nonzero if the insn has been deleted. Stored in the volatil field and printed as ‘/v’.

INSN_FROM_TARGET_P (x)

In an insn or jump_insn or call_insn in a delay slot of a branch, indicates that the insn is from the target of the branch. If the branch insn has INSN_ANNULLED_BRANCH_P set, this insn will only be executed if the branch is taken. For annulled branches with INSN_FROM_TARGET_P clear, the insn will be executed only if the branch is not taken. When INSN_ANNULLED_BRANCH_P is not set, this insn will always be executed. Stored in the in_struct field and printed as ‘/s’.

LABEL_PRESERVE_P (x)

In a code_label or note, indicates that the label is referenced by code or data not visible to the RTL of a given function. Labels referenced by a non-local goto will have this bit set. Stored in the in_struct field and printed as ‘/s’.

LABEL_REF_NONLOCAL_P (x)

In label_ref and reg_label expressions, nonzero if this is a reference to a non-local label. Stored in the volatil field and printed as ‘/v’.

MEM_KEEP_ALIAS_SET_P (x)

In mem expressions, 1 if we should keep the alias set for this mem unchanged when we access a component. Set to 1, for example, when we are already in a non-addressable component of an aggregate. Stored in the jump field and printed as ‘/j’.

MEM_VOLATILE_P (x)

In mem, asm_operands, and asm_input expressions, nonzero for volatile memory references. Stored in the volatil field and printed as ‘/v’.

MEM_NOTRAP_P (x)

In mem, nonzero for memory references that will not trap. Stored in the call field and printed as ‘/c’.

MEM_POINTER (x)

Nonzero in a mem if the memory reference holds a pointer. Stored in the frame_related field and printed as ‘/f’.

REG_FUNCTION_VALUE_P (x)

Nonzero in a reg if it is the place in which this function’s value is going to be returned. (This happens only in a hard register.) Stored in the return_val field and printed as ‘/i’.

REG_POINTER (x)

Nonzero in a reg if the register holds a pointer. Stored in the frame_related field and printed as ‘/f’.

REG_USERVAR_P (x)

In a reg, nonzero if it corresponds to a variable present in the user’s source code. Zero for temporaries generated internally by the compiler. Stored in the volatil field and printed as ‘/v’.

The same hard register may be used also for collecting the values of functions called by this one, but REG_FUNCTION_VALUE_P is zero in this kind of use.

RTX_FRAME_RELATED_P (x)

Nonzero in an insn, call_insn, jump_insn, barrier, or set which is part of a function prologue and sets the stack pointer, sets the frame pointer, or saves a register. This flag should also be set on an instruction that sets up a temporary register to use in place of the frame pointer. Stored in the frame_related field and printed as ‘/f’.

In particular, on RISC targets where there are limits on the sizes of immediate constants, it is sometimes impossible to reach the register save area directly from the stack pointer. In that case, a temporary register is used that is near enough to the register save area, and the Canonical Frame Address, i.e., DWARF2’s logical frame pointer, register must (temporarily) be changed to be this temporary register. So, the instruction that sets this temporary register must be marked as RTX_FRAME_RELATED_P.

If the marked instruction is overly complex (defined in terms of what dwarf2out_frame_debug_expr can handle), you will also have to create a REG_FRAME_RELATED_EXPR note and attach it to the instruction. This note should contain a simple expression of the computation performed by this instruction, i.e., one that dwarf2out_frame_debug_expr can handle.

This flag is required for exception handling support on targets with RTL prologues.

MEM_READONLY_P (x)

Nonzero in a mem, if the memory is statically allocated and read-only.

Read-only in this context means never modified during the lifetime of the program, not necessarily in ROM or in write-disabled pages. A common example of the later is a shared library’s global offset table. This table is initialized by the runtime loader, so the memory is technically writable, but after control is transferred from the runtime loader to the application, this memory will never be subsequently modified.

Stored in the unchanging field and printed as ‘/u’.

SCHED_GROUP_P (x)

During instruction scheduling, in an insn, call_insn, jump_insn or jump_table_data, indicates that the previous insn must be scheduled together with this insn. This is used to ensure that certain groups of instructions will not be split up by the instruction scheduling pass, for example, use insns before a call_insn may not be separated from the call_insn. Stored in the in_struct field and printed as ‘/s’.

SET_IS_RETURN_P (x)

For a set, nonzero if it is for a return. Stored in the jump field and printed as ‘/j’.

SIBLING_CALL_P (x)

For a call_insn, nonzero if the insn is a sibling call. Stored in the jump field and printed as ‘/j’.

STRING_POOL_ADDRESS_P (x)

For a symbol_ref expression, nonzero if it addresses this function’s string constant pool. Stored in the frame_related field and printed as ‘/f’.

SUBREG_PROMOTED_UNSIGNED_P (x)

Returns a value greater then zero for a subreg that has SUBREG_PROMOTED_VAR_P nonzero if the object being referenced is kept zero-extended, zero if it is kept sign-extended, and less then zero if it is extended some other way via the ptr_extend instruction. Stored in the unchanging field and volatil field, printed as ‘/u’ and ‘/v’. This macro may only be used to get the value it may not be used to change the value. Use SUBREG_PROMOTED_UNSIGNED_SET to change the value.

SUBREG_PROMOTED_UNSIGNED_SET (x)

Set the unchanging and volatil fields in a subreg to reflect zero, sign, or other extension. If volatil is zero, then unchanging as nonzero means zero extension and as zero means sign extension. If volatil is nonzero then some other type of extension was done via the ptr_extend instruction.

SUBREG_PROMOTED_VAR_P (x)

Nonzero in a subreg if it was made when accessing an object that was promoted to a wider mode in accord with the PROMOTED_MODE machine description macro (see Storage Layout). In this case, the mode of the subreg is the declared mode of the object and the mode of SUBREG_REG is the mode of the register that holds the object. Promoted variables are always either sign- or zero-extended to the wider mode on every assignment. Stored in the in_struct field and printed as ‘/s’.

SYMBOL_REF_USED (x)

In a symbol_ref, indicates that x has been used. This is normally only used to ensure that x is only declared external once. Stored in the used field.

SYMBOL_REF_WEAK (x)

In a symbol_ref, indicates that x has been declared weak. Stored in the return_val field and printed as ‘/i’.

SYMBOL_REF_FLAG (x)

In a symbol_ref, this is used as a flag for machine-specific purposes. Stored in the volatil field and printed as ‘/v’.

Most uses of SYMBOL_REF_FLAG are historic and may be subsumed by SYMBOL_REF_FLAGS. Certainly use of SYMBOL_REF_FLAGS is mandatory if the target requires more than one bit of storage.

PREFETCH_SCHEDULE_BARRIER_P (x)

In a prefetch, indicates that the prefetch is a scheduling barrier. No other INSNs will be moved over it. Stored in the volatil field and printed as ‘/v’.

These are the fields to which the above macros refer:

call

In a mem, 1 means that the memory reference will not trap.

In a call, 1 means that this pure or const call may possibly infinite loop.

In an RTL dump, this flag is represented as ‘/c’.

frame_related

In an insn or set expression, 1 means that it is part of a function prologue and sets the stack pointer, sets the frame pointer, saves a register, or sets up a temporary register to use in place of the frame pointer.

In reg expressions, 1 means that the register holds a pointer.

In mem expressions, 1 means that the memory reference holds a pointer.

In symbol_ref expressions, 1 means that the reference addresses this function’s string constant pool.

In an RTL dump, this flag is represented as ‘/f’.

in_struct

In reg expressions, it is 1 if the register has its entire life contained within the test expression of some loop.

In subreg expressions, 1 means that the subreg is accessing an object that has had its mode promoted from a wider mode.

In label_ref expressions, 1 means that the referenced label is outside the innermost loop containing the insn in which the label_ref was found.

In code_label expressions, it is 1 if the label may never be deleted. This is used for labels which are the target of non-local gotos. Such a label that would have been deleted is replaced with a note of type NOTE_INSN_DELETED_LABEL.

In an insn during dead-code elimination, 1 means that the insn is dead code.

In an insn or jump_insn during reorg for an insn in the delay slot of a branch, 1 means that this insn is from the target of the branch.

In an insn during instruction scheduling, 1 means that this insn must be scheduled as part of a group together with the previous insn.

In an RTL dump, this flag is represented as ‘/s’.

return_val

In reg expressions, 1 means the register contains the value to be returned by the current function. On machines that pass parameters in registers, the same register number may be used for parameters as well, but this flag is not set on such uses.

In symbol_ref expressions, 1 means the referenced symbol is weak.

In call expressions, 1 means the call is pure.

In an RTL dump, this flag is represented as ‘/i’.

jump

In a mem expression, 1 means we should keep the alias set for this mem unchanged when we access a component.

In a set, 1 means it is for a return.

In a call_insn, 1 means it is a sibling call.

In an RTL dump, this flag is represented as ‘/j’.

unchanging

In reg and mem expressions, 1 means that the value of the expression never changes.

In subreg expressions, it is 1 if the subreg references an unsigned object whose mode has been promoted to a wider mode.

In an insn or jump_insn in the delay slot of a branch instruction, 1 means an annulling branch should be used.

In a symbol_ref expression, 1 means that this symbol addresses something in the per-function constant pool.

In a call_insn 1 means that this instruction is a call to a const function.

In an RTL dump, this flag is represented as ‘/u’.

used

This flag is used directly (without an access macro) at the end of RTL generation for a function, to count the number of times an expression appears in insns. Expressions that appear more than once are copied, according to the rules for shared structure (see Sharing).

For a reg, it is used directly (without an access macro) by the leaf register renumbering code to ensure that each register is only renumbered once.

In a symbol_ref, it indicates that an external declaration for the symbol has already been written.

volatil

In a mem, asm_operands, or asm_input expression, it is 1 if the memory reference is volatile. Volatile memory references may not be deleted, reordered or combined.

In a symbol_ref expression, it is used for machine-specific purposes.

In a reg expression, it is 1 if the value is a user-level variable. 0 indicates an internal compiler temporary.

In an insn, 1 means the insn has been deleted.

In label_ref and reg_label expressions, 1 means a reference to a non-local label.

In prefetch expressions, 1 means that the containing insn is a scheduling barrier.

In an RTL dump, this flag is represented as ‘/v’.

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