Using and Porting GNU CC

14.7: RTL Expressions for Arithmetic

Unless otherwise specified, all the operands of arithmetic expressions must be valid for mode m. An operand is valid for mode m if it has mode m, or if it is a const_int or const_double and m is a mode of class MODE_INT.

For commutative binary operations, constants should be placed in the second operand.

(plus:m x y)

Represents the sum of the values represented by x and y carried out in machine mode m.

(lo_sum:m x y)

Like plus, except that it represents that sum of x and the low-order bits of y. The number of low order bits is machine-dependent but is normally the number of bits in a Pmode item minus the number of bits set by the high code (see Constants).

m should be Pmode.

(minus:m x y)

Like plus but represents subtraction.

(compare:m x y)

Represents the result of subtracting y from x for purposes of comparison. The result is computed without overflow, as if with infinite precision.

Of course, machines can't really subtract with infinite precision. However, they can pretend to do so when only the sign of the result will be used, which is the case when the result is stored in the condition code. And that is the only way this kind of expression may validly be used: as a value to be stored in the condition codes.

The mode m is not related to the modes of x and y, but instead is the mode of the condition code value. If (cc0) is used, it is VOIDmode. Otherwise it is some mode in class MODE_CC, often CCmode. See Condition Code.

Normally, x and y must have the same mode. Otherwise, compare is valid only if the mode of x is in class MODE_INT and y is a const_int or const_double with mode VOIDmode. The mode of x determines what mode the comparison is to be done in; thus it must not be VOIDmode.

If one of the operands is a constant, it should be placed in the second operand and the comparison code adjusted as appropriate.

A compare specifying two VOIDmode constants is not valid since there is no way to know in what mode the comparison is to be performed; the comparison must either be folded during the compilation or the first operand must be loaded into a register while its mode is still known.

(neg:m x)

Represents the negation (subtraction from zero) of the value represented by x, carried out in mode m.

(mult:m x y)

Represents the signed product of the values represented by x and y carried out in machine mode m.

Some machines support a multiplication that generates a product wider than the operands. Write the pattern for this as

(mult:m (sign_extend:m x) (sign_extend:m y))

where m is wider than the modes of x and y, which need not be the same.

Write patterns for unsigned widening multiplication similarly using zero_extend.

(div:m x y)

Represents the quotient in signed division of x by y, carried out in machine mode m. If m is a floating point mode, it represents the exact quotient; otherwise, the integerized quotient.

Some machines have division instructions in which the operands and quotient widths are not all the same; you should represent such instructions using truncate and sign_extend as in,

(truncate:m1 (div:m2 x (sign_extend:m2 y)))

(udiv:m x y)

Like div but represents unsigned division.

(mod:m x y)

(umod:m x y)

Like div and udiv but represent the remainder instead of the quotient.

(smin:m x y)

(smax:m x y)

Represents the smaller (for smin) or larger (for smax) of x and y, interpreted as signed integers in mode m.

(umin:m x y)

(umax:m x y)

Like smin and smax, but the values are interpreted as unsigned integers.

(not:m x)

Represents the bitwise complement of the value represented by x, carried out in mode m, which must be a fixed-point machine mode.

(and:m x y)

Represents the bitwise logical-and of the values represented by x and y, carried out in machine mode m, which must be a fixed-point machine mode.

(ior:m x y)

Represents the bitwise inclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode.

(xor:m x y)

Represents the bitwise exclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode.

(ashift:m x c)

Represents the result of arithmetically shifting x left by c places. x have mode m, a fixed-point machine mode. c be a fixed-point mode or be a constant with mode VOIDmode; which mode is determined by the mode called for in the machine description entry for the left-shift instruction. For example, on the Vax, the mode of c is QImode regardless of m.

(lshiftrt:m x c)

(ashiftrt:m x c)

Like ashift but for right shift. Unlike the case for left shift, these two operations are distinct.

(rotate:m x c)

(rotatert:m x c)

Similar but represent left and right rotate. If c is a constant, use rotate.

(abs:m x)

Represents the absolute value of x, computed in mode m.

(sqrt:m x)

Represents the square root of x, computed in mode m. Most often m will be a floating point mode.

(ffs:m x)

Represents one plus the index of the least significant 1-bit in x, represented as an integer of mode m. (The value is zero if x is zero.) The mode of x need not be m; depending on the target machine, various mode combinations may be valid.