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[88.138.106.230]) by smtp.gmail.com with ESMTPSA id 143sm14842876wmb.33.2019.10.28.07.59.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Oct 2019 07:59:43 -0700 (PDT) From: Richard Henderson To: qemu-devel@nongnu.org Subject: [PULL v2 06/12] exec: Use const alias for TARGET_PAGE_BITS_VARY Date: Mon, 28 Oct 2019 15:59:31 +0100 Message-Id: <20191028145937.10914-7-richard.henderson@linaro.org> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20191028145937.10914-1-richard.henderson@linaro.org> References: <20191028145937.10914-1-richard.henderson@linaro.org> MIME-Version: 1.0 X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 2a00:1450:4864:20::42d X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: peter.maydell@linaro.org Errors-To: qemu-devel-bounces+patch=linaro.org@nongnu.org Sender: "Qemu-devel" Using a variable that is declared "const" for this tells the compiler that it may read the value once and assume that it does not change across function calls. For target_page_size, this means we have only one assert per function, and one read of the variable. This reduces the size of qemu-system-aarch64 by 8k. Reviewed-by: Alex Bennée Reviewed-by: Paolo Bonzini Signed-off-by: Richard Henderson --- include/exec/cpu-all.h | 14 ++++++--- exec-vary.c | 66 +++++++++++++++++++++++++++++++++++++----- 2 files changed, 68 insertions(+), 12 deletions(-) -- 2.17.1 diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h index 255bb186ac..0b449b98ba 100644 --- a/include/exec/cpu-all.h +++ b/include/exec/cpu-all.h @@ -210,10 +210,16 @@ static inline void stl_phys_notdirty(AddressSpace *as, hwaddr addr, uint32_t val /* page related stuff */ #ifdef TARGET_PAGE_BITS_VARY -extern bool target_page_bits_decided; -extern int target_page_bits; -#define TARGET_PAGE_BITS ({ assert(target_page_bits_decided); \ - target_page_bits; }) +typedef struct { + bool decided; + int bits; +} TargetPageBits; +#if defined(CONFIG_ATTRIBUTE_ALIAS) || !defined(IN_EXEC_VARY) +extern const TargetPageBits target_page; +#else +extern TargetPageBits target_page; +#endif +#define TARGET_PAGE_BITS ({ assert(target_page.decided); target_page.bits; }) #else #define TARGET_PAGE_BITS_MIN TARGET_PAGE_BITS #endif diff --git a/exec-vary.c b/exec-vary.c index 48c0ab306c..8725fd0285 100644 --- a/exec-vary.c +++ b/exec-vary.c @@ -19,11 +19,55 @@ #include "qemu/osdep.h" #include "qemu-common.h" + +#define IN_EXEC_VARY 1 + #include "exec/exec-all.h" #ifdef TARGET_PAGE_BITS_VARY -int target_page_bits; -bool target_page_bits_decided; +# ifdef CONFIG_ATTRIBUTE_ALIAS +/* + * We want to declare the "target_page" variable as const, which tells + * the compiler that it can cache any value that it reads across calls. + * This avoids multiple assertions and multiple reads within any one user. + * + * This works because we finish initializing the data before we ever read + * from the "target_page" symbol. + * + * This also requires that we have a non-constant symbol by which we can + * perform the actual initialization, and which forces the data to be + * allocated within writable memory. Thus "init_target_page", and we use + * that symbol exclusively in the two functions that initialize this value. + * + * The "target_page" symbol is created as an alias of "init_target_page". + */ +static TargetPageBits init_target_page; + +/* + * Note that this is *not* a redundant decl, this is the definition of + * the "target_page" symbol. The syntax for this definition requires + * the use of the extern keyword. This seems to be a GCC bug in + * either the syntax for the alias attribute or in -Wredundant-decls. + * + * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91765 + */ +# pragma GCC diagnostic push +# pragma GCC diagnostic ignored "-Wredundant-decls" + +extern const TargetPageBits target_page + __attribute__((alias("init_target_page"))); + +# pragma GCC diagnostic pop +# else +/* + * When aliases are not supported then we force two different declarations, + * by way of suppressing the header declaration with IN_EXEC_VARY. + * We assume that on such an old compiler, LTO cannot be used, and so the + * compiler cannot not detect the mismatched declarations, and all is well. + */ +TargetPageBits target_page; +# define init_target_page target_page +# endif #endif bool set_preferred_target_page_bits(int bits) @@ -36,11 +80,11 @@ bool set_preferred_target_page_bits(int bits) */ #ifdef TARGET_PAGE_BITS_VARY assert(bits >= TARGET_PAGE_BITS_MIN); - if (target_page_bits == 0 || target_page_bits > bits) { - if (target_page_bits_decided) { + if (init_target_page.bits == 0 || init_target_page.bits > bits) { + if (init_target_page.decided) { return false; } - target_page_bits = bits; + init_target_page.bits = bits; } #endif return true; @@ -49,9 +93,15 @@ bool set_preferred_target_page_bits(int bits) void finalize_target_page_bits(void) { #ifdef TARGET_PAGE_BITS_VARY - if (target_page_bits == 0) { - target_page_bits = TARGET_PAGE_BITS_MIN; + if (init_target_page.bits == 0) { + init_target_page.bits = TARGET_PAGE_BITS_MIN; } - target_page_bits_decided = true; + init_target_page.decided = true; + + /* + * For the benefit of an -flto build, prevent the compiler from + * hoisting a read from target_page before we finish initializing. + */ + barrier(); #endif }