The various expression codes are divided into several classes,
which are represented by single characters. You can determine the class
of an RTX code with the macro
rtx.def defines these classes:
REG) or a memory location (
SYMBOL_REF). Constants and basic transforms on objects (
LO_SUM) are also included. Note that
STRICT_LOW_PARTare not in this class, but in class
ABS. This category also includes value extension (sign or zero) and conversions between integer and floating point.
EQare comparisons, so they have class
SIGN_EXTRACT. These have three inputs and are lvalues (so they can be used for insertion as well). See Bit-Fields.
CALL_INSN. See Insns.
MATCH_DUP. These only occur in machine descriptions.
DEFINE_*, etc.). It also includes all the codes describing side effects (
CLOBBER, etc.) and the non-insns that may appear on an insn chain, such as
For each expression code,
rtl.def specifies the number of
contained objects and their kinds using a sequence of characters
called the format of the expression code. For example,
the format of
These are the most commonly used format characters:
A few other format characters are used occasionally:
uis equivalent to
eexcept that it is printed differently in debugging dumps. It is used for pointers to insns.
nis equivalent to
iexcept that it is printed differently in debugging dumps. It is used for the line number or code number of a
Sindicates a string which is optional. In the RTL objects in core,
Sis equivalent to
s, but when the object is read, from an
mdfile, the string value of this operand may be omitted. An omitted string is taken to be the null string.
Vindicates a vector which is optional. In the RTL objects in core,
Vis equivalent to
E, but when the object is read from an
mdfile, the vector value of this operand may be omitted. An omitted vector is effectively the same as a vector of no elements.
0means a slot whose contents do not fit any normal category.
0slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler.
There are macros to get the number of operands and the format of an expression code:
Some classes of RTX codes always have the same format. For example, it
is safe to assume that all comparison operations have format
iuueiee. See Insns. Note that not all RTL objects linked onto an insn chain are of class