linux-kernel - Re: [PATCH 5/9][v5] powerpc: implement is_instr_load

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Date:	Thu, 3 Oct 2013 15:35:19 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Michael Ellerman <michaele@....ibm.com>,
	linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>, linuxppc-dev@...abs.org,
	Paul Mackerras <paulus@...ba.org>,
	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Subject: Re: [PATCH 5/9][v5] powerpc: implement is_instr_load_store().

On Tue, Oct 01, 2013 at 05:15:06PM -0700, Sukadev Bhattiprolu wrote:
> Implement is_instr_load_store() to detect whether a given instruction
> is one of the fixed-point or floating-point load/store instructions.
> This function will be used in a follow-on patch to save memory hierarchy
> information of the load/store.

The search over the array is a bit of a pity, especially as the worst
case penalises you when you haven't hit a load/store.

I think we can do better. If you look at the opcode maps, and in
particular the extended table for opcode 31, you'll see there's a
reasonable amount of structure.

The following is only valid for arch 2.06, ie. it will classify reserved
opcodes as being load/store, but I think that's fine for the moment. If
we need to use it somewhere in future we can update it. But we should
add a big comment saying it's only valid in that case.

Anyway, I think the following logic is all we need for opcode 31:

bool is_load_store(int ext_opcode)
{
        upper = ext_opcode >> 5;
        lower = ext_opcode & 0x1f;

        /* Short circuit as many misses as we can */
        if (lower < 3 || lower > 23)
            return false;

        if (lower == 3)
            if (upper >= 16)
                return true;

            return false;

        if (lower == 6)
            if (upper <= 1)
                return true;
            return false;

        if (lower == 7 || lower == 12)
            return true;

        if (lower >= 20) /* && lower <= 23 (implicit) */
            return true;

        return false;
}


Which is not pretty, but I think it's preferable to the full search over the
array.

cheers
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