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Re: [Qemu-devel] [PATCH v4 04/33] tcg-aarch64: Hoist common argument loa

From: Claudio Fontana
Subject: Re: [Qemu-devel] [PATCH v4 04/33] tcg-aarch64: Hoist common argument loads in tcg_out_op
Date: Tue, 17 Sep 2013 10:01:23 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20130801 Thunderbird/17.0.8

On 16.09.2013 18:20, Richard Henderson wrote:
> On 09/16/2013 12:42 AM, Claudio Fontana wrote:
>>> +    /* Hoist the loads of the most common arguments.  */
>>>> +    TCGArg a0 = args[0];
>>>> +    TCGArg a1 = args[1];
>>>> +    TCGArg a2 = args[2];
>>>> +    int c2 = const_args[2];
>>>> +
>> Either all or none (add c0, c1), I would expect the compiler not to
>> generate code for the paths that don't use C[n].
> I chose the most common.  Those used in 90% of all of the cases.

What you did is clear, does not change the fact that the mixing of use of the 
variables and args[] is confusing.
So either we add int c0 and int c1, and use the hoisted variables exclusively 
later, or we don't do it.

>> Btw, if the compiler generates bloated code without this, we should notify
>> the projects working on gcc for aarch64.
> It's not the compiler's fault.  After parameter decomposition, the arrays
> become pointers, and the compiler can't tell that it's always safe to perform
> the loads.  So in general it can't hoist the loads higher than the first
> explicit reference that proves the pointers must be non-null.
> Now that I think about it, we might actually do better, generically, to 
> package
> all of these arguments up into a struct.  The compiler can more easily reason
> about the collective safety of structure access...

I don't have anything against it in principle, but just adding c0 and c1, which 
iirc should cover all uses, would be fine by me.


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