Using and Porting GNU CC
Four special side-effect expression codes appear as memory addresses.
mem, but most machines allow only a
reg. m must be the machine mode for pointers on the machine in use. The amount x is decremented by is the length in bytes of the machine mode of the containing memory reference of which this expression serves as the address. Here is an example of its use:
(mem:DF (pre_dec:SI (reg:SI 39)))
This says to decrement pseudo register 39 by the length of a
value and use the result to address a
pre_decbut a different value. The value represented here is the value x has before being decremented.
These embedded side effect expressions must be used with care. Instruction
patterns may not use them. Until the `
flow' pass of the compiler,
they may occur only to represent pushes onto the stack. The `
pass finds cases where registers are incremented or decremented in one
instruction and used as an address shortly before or after; these cases are
then transformed to use pre- or post-increment or -decrement.
If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave differently on different machines and hence must be treated as ambiguous and disallowed.
An instruction that can be represented with an embedded side effect
could also be represented using
parallel containing an additional
set to describe how the address register is altered. This is not
done because machines that allow these operations at all typically
allow them wherever a memory address is called for. Describing them as
additional parallel stores would require doubling the number of entries
in the machine description.