GNU Info

(gcc-295.info)Insn Canonicalizations

---

Next: Peephole Definitions Prev: Jump Patterns Up: Machine Desc

---

Enter node , (file) or (file)node

Canonicalization of Instructions
================================

   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:

   * For commutative and comparison operators, a constant is always
     made the second operand.  If a machine only supports a constant as
     the second operand, only patterns that match a constant in the
     second operand need be supplied.

     For these operators, if only one operand is a `neg', `not',
     `mult', `plus', or `minus' expression, it will be the first
     operand.

   * For the `compare' operator, a constant is always the second operand
     on machines where `cc0' is used (Note: Jump Patterns.).  On other
     machines, there are rare cases where the compiler might want to
     construct a `compare' with 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 `neg', `not', `mult', `plus', or `minus' is made the
     first operand under the same conditions as above.

   * `(minus X (const_int N))' is converted to `(plus X (const_int
     -N))'.

   * Within address computations (i.e., inside `mem'), a left shift is
     converted into the appropriate multiplication by a power of two.

   * De`Morgan's Law is used to move bitwise negation inside a bitwise
     logical-and or logical-or operation.  If this results in only one
     operand being a `not' expression, 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.

   * The only possible RTL expressions involving both bitwise
     exclusive-or and bitwise negation are `(xor:M X Y)' and `(not:M
     (xor:M X Y))'.

   * The sum of three items, one of which is a constant, will only
     appear in the form

          (plus:M (plus:M X Y) CONSTANT)

   * On machines that do not use `cc0', `(compare X (const_int 0))'
     will be converted to X.

   * Equality comparisons of a group of bits (usually a single bit)
     with zero will be written using `zero_extract' rather than the
     equivalent `and' or `sign_extract' operations.