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Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash
Date: Tue, 5 Apr 2016 08:41:43 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

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.


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