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From: Richard Henderson <rth@twiddle.net>
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Date: Tue, 5 Apr 2016 08:41:43 -0700
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Subject: Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc,
	and flags with xxhash
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To: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Sergey Fedorov <serge.fdrv@gmail.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On 04/04/2016 10:30 PM, Emilio G. Cota wrote:
> Tests show that the other element checked for in tb_find_physical,
> cs_base, is always a match when tb_phys+pc+flags are a match,
> so hashing cs_base is wasteful. It could be that this is an ARM-only
> thing, though.

The cs_base field is only used by i386 (in 16-bit modes), and sparc (for a TB 
consisting of only a delay slot).

It may well still turn out to be reasonable to ignore cs_base for hashing.

> +static inline
> +uint32_t tb_hash_func(tb_page_addr_t phys_pc, target_ulong pc, uint64_t flags)
>   {
> -    return (pc >> 2) & (CODE_GEN_PHYS_HASH_SIZE - 1);
> +    struct key {
> +        tb_page_addr_t phys_pc;
> +        target_ulong pc;
> +        uint64_t flags;

The flags field should be uint32_t, not uint64_t.
See its definition in TranslationBlock.

> +    } QEMU_PACKED k;
> +    unsigned int hash;
> +
> +    k.phys_pc = phys_pc;
> +    k.pc = pc;
> +    k.flags = flags;
> +    hash = qemu_xxh32((uint32_t *)&k, sizeof(k) / sizeof(uint32_t), 1);

I'm less than happy about putting data into a struct and hashing that memory. 
Why don't you just take the ideas from xxh32/xxh64 and perform the hash 
directly right here?

In particular, you've 3 data elements here, a maximum of 5 32-bit words, thus 
the loop within xxh32 will never roll, so it's mostly dead code.


r~