Re: Using secondary reload to reload CONST_INT?

[Richard Sandiford <richard dot sandiford at linaro dot org>]

Georg-Johann Lay <avr@gjlay.de> writes:
> 1) The internals just mention TARGET_SECONDARY_RELOAD for REG-MEM and
>     for REG-REG moves, no word about REG-CONST moves. So is using
>     secondary reloads for CONST_INT (or other consts) like outlined
>     above a defined use case I can rely on?

Yeah, it should be.  Other targets rely on this too.  E.g. MIPS allows
integers to be stored in FPRs as well as GPRs, but you can't load a
symbolic constant directly into an FPR; it has to go through a GPR.

> 2) The secondary reload hook is always called with
>     regclass = GENERAL_REGS, even in cases where the class
>     is known to be NO_LD_REGS like, e.g. when preparing arguments
>     for a function call. Why this? I would expect to get the smallest
>     available regclass. If a reg lies in LD_REGS, a secondary reload
>     is not needed, but how can I know if class is always GENERAL_REGS?
>     Is it ensured that secondary reload hook gets never called when
>     a constraint alternative matches, like "d,i"?

As far as the last bit goes: once reload has decided that it needs to
reload A into B, it's uses the secondary reload hook (and only that hook)
to decide whether a secondary reload is needed.  The movsi constraints
only matter when reloading a pre-reload movsi instruction.

I think the reason you only ever see GENERAL_REGS being passed in is
because (from a quick look at avr.md) very few non-move patterns use
the "d" and "l" constraints.  They all seem to use the "r" constraint.
Thus reloads from those instructions will use GENERAL_REGS rather than
NO_LD_REGS.

If you want to make reload use NO_LD_REGS for these GENERAL_REGS reloads,
at least when the reloaded value is a constant, then it might be worth
defining TARGET_PREFERRED_RELOAD_CLASS.

> 3) What is the "unit" of sri->extra_cost? Compared to COST_N_INSNS?
>     Or compared to "?" constraint cost?

I think it's measured in the same units as REGISTER_MOVE_COST.
(2 == a simple register move).

Richard