Hi, I've been spending some (spare) time trying to get LRA working for the avr target. 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. Regards Senthil gcc/ChangeLog: * ira.cc (setup_reg_class_relations): Skip if cl3 is an empty register class. --- gcc/ira.cc +++ gcc/ira.cc @@ -1259,6 +1259,9 @@ setup_reg_class_relations (void) for (cl3 = 0; cl3 < N_REG_CLASSES; cl3++) { temp_hard_regset = reg_class_contents[cl3] & ~no_unit_alloc_regs; + if (hard_reg_set_empty_p (temp_hard_regset)) + continue; + if (hard_reg_set_subset_p (temp_hard_regset, intersection_set)) { /* CL3 allocatable hard register set is inside of