https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116550
--- Comment #4 from denisc at gcc dot gnu.org ---
Explanation.
Test file: udivmoddi.c
problem insn: 484
Before LRA pass we have:
(insn 484 483 485 72 (parallel [
(set (reg/v:SI 143 [ __q1 ])
(plus:SI (reg/v:SI 143 [ __q1 ])
(const_int -2 [0xfffffffffffffffe])))
(clobber (scratch:QI))
]) "udivmoddi.c":163:405 discrim 5 186 {addsi3}
(nil))
LRA substitute all scratches with new pseudos, so we have:
(insn 484 483 485 72 (parallel [
(set (reg/v:SI 143 [ __q1 ])
(plus:SI (reg/v:SI 143 [ __q1 ])
(const_int -2 [0xfffffffffffffffe])))
(clobber (reg:QI 619))
]) "/mnt/d/avr-lra/udivmoddi.c":163:405 discrim 5 186 {addsi3}
(expr_list:REG_UNUSED (reg:QI 619)
(nil)))
Pseudo 619 is a special scratch register generated by LRA which is marked in
`scratch_bitmap' and can be tested by call `ira_former_scratch_p(regno)'.
In dump file (udivmoddi.c.317r.reload) we have:
Creating newreg=619
Removing SCRATCH to p619 in insn #484 (nop 3)
rescanning insn with uid = 484.
After that LRA trying to spill (reg:QI 619)
It's a bug because (reg:QI 619) is an output scratch register which is already
something like spill register.
Fragment from udivmoddi.c.317r.reload:
Choosing alt 2 in insn 484: (0) r (1) 0 (2) nYnn (3) &d {addsi3}
Creating newreg=728 from oldreg=619, assigning class LD_REGS to r728
IMHO: the bug is in lra-constraints.cc in function `get_reload_reg'
fragment of `get_reload_reg':
if (type == OP_OUT)
{
/* Output reload registers tend to start out with a conservative
choice of register class. Usually this is ALL_REGS, although
a target might narrow it (for performance reasons) through
targetm.preferred_reload_class. It's therefore quite common
for a reload instruction to require a more restrictive class
than the class that was originally assigned to the reload register.
In these situations, it's more efficient to refine the choice
of register class rather than create a second reload register.
This also helps to avoid cycling for registers that are only
used by reload instructions. */
if (REG_P (original)
&& (int) REGNO (original) >= new_regno_start
&& INSN_UID (curr_insn) >= new_insn_uid_start
__________________________________^^
&& in_class_p (original, rclass, &new_class, true))
{
unsigned int regno = REGNO (original);
if (lra_dump_file != NULL)
{
fprintf (lra_dump_file, " Reuse r%d for output ", regno);
dump_value_slim (lra_dump_file, original, 1);
}
This condition incorrectly limits register reuse to ONLY newly generated
instructions.
i.e. LRA can reuse register only from insns generated by himself.
IMHO:It's wrong.
Scratch registers generated by LRA also have to be reused.
The patch is very simple.
