Sunday, March 20, 2011

OK, back to 8-bit registers. If you remember, the last problem I hit here was that I found an instruction which was failing to allocate a register. I mentioned that the problem was in the not_usable allocation mask.

To make things easier to explain, I've copied the instruction below.

2k_chess.c:123: error: unable to find a register to spill in class ‘REAL_REGS’
this is the insn:
(insn 740 739 742 126 2k_chess.c:113 (set (reg/v:QI 69 [ x ])
(subreg:QI (reg:HI 6 r3 [322]) 1)) 70 {movqi} (expr_list:REG_DEAD(reg:HI 6 r3 [322])

This instruction was MOVQI, which should take any register as an operand. For some reason, GCC is convinced it needs to use the REAL_REGS class for pseudoreg r322. The problem is that we're trying to allocate a two-byte value, but we are limited by the class to only using one register.

reload failure for reload 1

Reloads for insn # 740
Reload 0: REAL_REGS, RELOAD_FOR_OUTPUT (opnum = 0)
reload_out_reg: (reg/v:QI 69 [ x ])
Reload 1: reload_in (HI) = (reg:HI 6 r3 [322])
reload_out (QI) = (reg/v:QI 69 [ x ])
reload_in_reg: (reg:HI 6 r3 [322])
reload_out_reg: (reg/v:QI 69 [ x ])

I suspect the problem is in allocating a register for pseudoregister 322. The subreg is causing problems. I wish GCC would let me make a generic method to handle subregs. it would make my life so much easier.

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