Re: [IRA] Skip empty register classes in setup_reg_class_relations

[Vladimir Makarov <vmakarov@redhat.com>]

On 7/12/23 07:05, SenthilKumar.Selvaraj@microchip.com wrote:
> Hi,
>
>    I've been spending some (spare) time trying to get LRA working
>    for the avr target.

Thank you for addressing this problem.

The code you changing is very sensitive and was a source of multiple PRs 
in the past.  But I found the change your propose logical and I think it 
will not create problems.  Still please be alert and revert the patch if 
people reports the problem with this change.

>   After making a couple of changes to get
>    libgcc going, I'm now hitting an assert at
>    lra-constraints.cc:4423 for a subarch (avrtiny) that has a
>    couple of regclasses with no available registers.
>
>    The assert fires because in_class_p (correctly) returns
>    false for get_reg_class (regno) = ALL_REGS, and new_class =
>    NO_LD_REGS. For avrtiny, NO_LD_REGS is an empty regset, and
>    therefore hard_reg_set_subset_p (NO_LD_REGS, lra_no_alloc_regs)
>    is always true, making in_class_p return false.
>
>    in_class_p picks NO_LD_REGS as new_class because common_class =
>    ira_reg_class_subset[ALL_REGS][NO_REGS] evaluates as
>    NO_LD_REGS. This appears wrong to me - it should be NO_REGS
>    instead (lra-constraints.cc:4421 checks for NO_REGS).
>
>    ira.cc:setup_reg_class_relations sets up
>    ira_reg_class_subset (among other things), and the problem
>    appears to be a missing continue statement if
>    reg_class_contents[cl3] (in the innermost loop) is empty.
>
>    In this case, for cl1 = ALL_REGS and cl2 = NO_REGS, cl3 =
>    NO_LD_REGS, temp_hard_regset and temp_set2 are both empty, and
>    hard_reg_subset_p (<emptyset>, <anyval>) is always true, so
>    ira_reg_class_subset[ALL_REGS][NO_REGS] ends up being set to
>    cl3 = NO_LD_REGS. Adding a continue if hard_reg_set_empty_p (temp_hard_regset)
>    fixes the problem for me.
>
>    Does the below patch look ok? Bootstrapping and regression
>    testing passed on x86_64.
OK.