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Re: [Qemu-devel] [PATCH 2/4] tcg/aarch64: implement new TCG target for a

From: Claudio Fontana
Subject: Re: [Qemu-devel] [PATCH 2/4] tcg/aarch64: implement new TCG target for aarch64
Date: Mon, 27 May 2013 13:43:40 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20130509 Thunderbird/17.0.6

On 24.05.2013 19:02, Richard Henderson wrote:
> On 05/24/2013 01:53 AM, Claudio Fontana wrote:
>>> No real need to special case zero; it's just an extra test slowing down the
>>> compiler.
>> Yes, we need to handle the special case zero.
>> Otherwise no instruction at all would be emitted for value 0.
> Hmm, true.  Although I'd been thinking more along the lines of
> arranging the code such that we'd use movz to set the zero.

I think we need to keep treating zero specially if we want to keep the 
optimization where we don't emit needless MOVK instructions for half-words of 
value 0000h.

I can however make one single function out of movi32 and movi64, it could look 
like this:

if (!value) {
    tcg_out_movr(s, 0, rd, TCG_REG_ZXR);

base = (value > 0xffffffff) ? 0xd2800000 : 0x52800000;

while (value) {
    /* etc etc */

>> I actually don't know whether to prefer ext=0 or ext=1,
>> in the sense that it would be useful to know whether using the extended 
>> registers
>> with a small constant is performance-wise preferable to using the 32bit 
>> operation,
>> and relying on 0-extension. See also the rotation comment below.
>>From the armv8 isa overview:
> # Rationale: [...] By maintaining this semantic information in the instruction
> # set, implementations can exploit this information to avoid expending energy
> # or cycles to compute, forward and store the unused upper 32 bits of such
> # data types. Implementations are free to exploit this freedom in whatever way
> # they choose to save energy.

I did not notice that, that solves the issue.

>>> addr_reg almost certainly needs to be zero-extended for 32-bit guests, 
>>> easily
>>> done by setting ext = 0 here.
>> I can easily put an #ifdef just to be sure.
> No ifdef, just the TARGET_LONG_BITS == 64 comparison works.
>>> You initialize FP, but you don't reserve the register, so it's going to get
>>> clobbered.  We don't actually use the frame pointer in the translated code, 
>>> so
>>> I don't think there's any call to actually initialize it either.
>> The FP is not going to be clobbered, not by code here and not by called code.
>> It is not going to be clobbered between our use before the jump and after the
>> jump, because all the called functions need to preserve FP as mandated by the
>> calling conventions.
>> It is not going to be clobbered from the point of view of our caller,
>> because we save (FP, LR) along with (X19, X20) .. (X27, X28) and restore them
>> before returning.
> Ah, well, I didn't see it mentioned here,
>> +    tcg_regset_clear(s->reserved_regs);
>> +    tcg_regset_set_reg(s->reserved_regs, TCG_REG_SP);
>> +    tcg_regset_set_reg(s->reserved_regs, TCG_REG_TMP);
>> +    tcg_regset_set_reg(s->reserved_regs, TCG_REG_X18); /* platform register 
>> */
> but hadn't noticed that it's not listed in the reg_alloc_order.
>> We use FP to point to the callee_saved registers, and to move to/from them
>> in the tcg_out_store_pair and tcg_out_load_pair functions.
> I hadn't noticed you'd hard-coded FP into the load/store_pair functions.
> Let's *really* not do that.  Even if we decide to continue using it, let's
> pass it in explicitly.
> But I don't see that you're really gaining anything in the prologue from
> using FP instead of SP.  It seems like a waste of a register to me.
> r~

Claudio Fontana
Server OS Architect
Huawei Technologies Duesseldorf GmbH
Riesstra├če 25 - 80992 M├╝nchen

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