There are often cases where multiple RTL expressions could represent an operation performed by a single machine instruction. This situation is most commonly encountered with logical, branch, and multiply-accumulate instructions. In such cases, the compiler attempts to convert these multiple RTL expressions into a single canonical form to reduce the number of insn patterns required.
In addition to algebraic simplifications, following canonicalizations are performed:
minusexpression, it will be the first operand.
compareoperator, a constant is always the second operand on machines where
cc0is used (see section Defining Jump Instruction Patterns). On other machines, there are rare cases where the compiler might want to construct a
comparewith a constant as the first operand. However, these cases are not common enough for it to be worthwhile to provide a pattern matching a constant as the first operand unless the machine actually has such an instruction. An operand of
minusis made the first operand under the same conditions as above.
(minus x (const_int n))is converted to
(plus x (const_int -n)).
mem), a left shift is converted into the appropriate multiplication by a power of two.
notexpression, it will be the first one. A machine that has an instruction that performs a bitwise logical-and of one operand with the bitwise negation of the other should specify the pattern for that instruction as
(define_insn "" [(set (match_operand:m 0 ...) (and:m (not:m (match_operand:m 1 ...)) (match_operand:m 2 ...)))] "..." "...")Similarly, a pattern for a "NAND" instruction should be written
(define_insn "" [(set (match_operand:m 0 ...) (ior:m (not:m (match_operand:m 1 ...)) (not:m (match_operand:m 2 ...))))] "..." "...")In both cases, it is not necessary to include patterns for the many logically equivalent RTL expressions.
(xor:m x y)and
(not:m (xor:m x y)).
(plus:m (plus:m x y) constant)
(compare x (const_int 0))will be converted to x.
zero_extractrather than the equivalent