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From: | Richard Henderson |
Subject: | Re: [Qemu-devel] [PATCH] tcg/mips: Bugfix for crash when runningprogram with qemu-i386. |
Date: | Sun, 9 Jul 2017 16:15:57 -1000 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 |
On 07/09/2017 04:04 PM, address@hidden wrote:
>> if (TCG_TARGET_REG_BITS > TARGET_LONG_BITS) { >> tcg_out_ext32u(s, base, addr_regl); >> - addr_regl = base; >> + tcg_out_mov(s, TCG_TYPE_PTR, addr_regl, base); >> } >> if (guest_base == 0 && data_regl != addr_regl) { >> base = addr_regl; > > This is wrong, because you're not allowed to modify the input operands. > > Try this, just a few lines lower in the function: > - tcg_out_movi(s, TCG_TYPE_PTR, base, guest_base); > - tcg_out_opc_reg(s, ALIAS_PADD, base, base, addr_regl); > + tcg_out_movi(s, TCG_TYPE_PTR, TCG_TMP0, guest_base); > + tcg_out_opc_reg(s, ALIAS_PADD, base, TCG_TMP0, addr_regl); > Got it, but the real problem is for addr_regl instead of guest_base.
Guest base is a problem simply because we require a temporary for it, and we were trying to put two temporaries into the same register.
If we retain guest_base in a register all of the time, then (1) we do not have to recompute it for every memory load and (2) we do not need a temporary for it.
> Better would be to reserve a register for the guest_base, like we do for ppc. > See all of the uses of TCG_GUEST_BASE_REG in tcg/ppc/tcg-target.inc.c. It uses base(TCG_REG_A0) for temperary use for guest_base in this case.
No it doesn't. It computes guest_base into a register in the prologue:
#ifndef CONFIG_SOFTMMU if (guest_base) { tcg_out_movi(s, TCG_TYPE_PTR, TCG_GUEST_BASE_REG, guest_base); tcg_regset_set_reg(s->reserved_regs, TCG_GUEST_BASE_REG); } #endif
and then it uses a reg+reg addressing mode during qemu_ld/st:
rbase = guest_base ? TCG_GUEST_BASE_REG : 0;
...
insn = qemu_ldx_opc[opc & (MO_SIZE | MO_BSWAP)]; tcg_out32(s, insn | TAB(datalo, rbase, addrlo));
Obviously mips doesn't have a reg+reg addressing mode, so a PADD instruction is required, but otherwise you can use the same scheme. Using TCG_REG_S1 on mips for TCG_GUEST_BASE_REG would be fine.
r~
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