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Allocating an arbitrarily-sized array of tbs results in either (a) a lot of memory wasted or (b) unnecessary flushes of the code cache when we run out of TB structs in the array. An obvious solution would be to just malloc a TB struct when needed, and keep the TB array as an array of pointers (recall that tb_find_pc() needs the TB array to run in O(log n)). Perhaps a better solution, which is implemented in this patch, is to allocate TB's right before the translated code they describe. This results in some memory waste due to padding to have code and TBs in separate cache lines--for instance, I measured 4.7% of padding in the used portion of code_gen_buffer when booting aarch64 Linux on a host with 64-byte cache lines. However, it can allow for optimizations in some host architectures, since TCG backends could safely assume that the TB and the corresponding translated code are very close to each other in memory. See this message by rth for a detailed explanation: https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg05172.html Subject: Re: GSoC 2017 Proposal: TCG performance enhancements Backports commit 6e3b2bfd6af488a896f7936e99ef160f8f37e6f2 from qemu
43 lines
1.2 KiB
C
43 lines
1.2 KiB
C
/*
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* Internal structs that QEMU exports to TCG
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*
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* Copyright (c) 2003 Fabrice Bellard
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef QEMU_TB_CONTEXT_H
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#define QEMU_TB_CONTEXT_H
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#include "qemu/thread.h"
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#define CODE_GEN_PHYS_HASH_BITS 15
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#define CODE_GEN_PHYS_HASH_SIZE (1 << CODE_GEN_PHYS_HASH_BITS)
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typedef struct TranslationBlock TranslationBlock;
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typedef struct TBContext TBContext;
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struct TBContext {
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TranslationBlock **tbs;
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TranslationBlock *tb_phys_hash[CODE_GEN_PHYS_HASH_SIZE];
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size_t tbs_size;
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int nb_tbs;
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/* statistics */
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int tb_flush_count;
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int tb_phys_invalidate_count;
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};
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#endif
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