Re: Reload pass ignores constraints. Why?

[Ian Lance Taylor <iant at google dot com>]

Georg-Johann Lay <avr@gjlay.de> writes:

> iii) Expand into MOV+LSHIFTRT and allow the resulting const in
>      *movsi_insn:
>      CSE et. al. will reconstruct the original constant and
>      replace MOV+LSHIFTRT with a single SET:
>      Bad: expanding was in vain and we Goto i)

I think this is the option you should use.  You should use
TARGET_RTX_COSTS to make the constants which may not be immediate
operands expensive.  Then CSE will not reconstruct them.


>> I don't know exactly what is going on.  But it is most likely just a
>> coincidence that it is failing when using SHIFTRT.  There is probably
>> some way to make it fail in other ways as well.
>
> Would state it like this: If the movMM expander expands the move into
> several insns, each insn must be able to handle an alternative (which
> reload might select) without needing a reload.

Yes, but also when can_create_pseudo_p returns false the move expander
must not call force_reg or gen_reg_reg or anything else which creates a
new pseudo register.  When can_create_pseudo_p returns true, it is often
preferable to generate a new pseudo-reg rather than reuse an existing
one.

Ian