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X-Received-From: 2a00:1450:4864:20::443 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: pbonzini@redhat.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Richard Henderson writes: > 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: Paolo Bonzini > Signed-off-by: Richard Henderson Reviewed-by: Alex Benn=C3=A9e > --- > v2: Notice CONFIG_ATTRIBUTE_ALIAS, and work around Xcode 9 lossage. > --- > include/exec/cpu-all.h | 14 +++++++--- > exec-vary.c | 60 ++++++++++++++++++++++++++++++++++++------ > 2 files changed, 62 insertions(+), 12 deletions(-) > > diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h > index 255bb186ac..76515dc8d9 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..e0befd502a 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 use= r. > + * > + * This works because we initialize the target_page data very early, in a > + * location far removed from the functions that require the final result= s. > + * > + * 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 val= ue. > + * > + * 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=3D91765 > + */ > +# 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 declaratio= ns, > + * 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 we= ll. > + */ > +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 >=3D TARGET_PAGE_BITS_MIN); > - if (target_page_bits =3D=3D 0 || target_page_bits > bits) { > - if (target_page_bits_decided) { > + if (init_target_page.bits =3D=3D 0 || init_target_page.bits > bits) { > + if (init_target_page.decided) { > return false; > } > - target_page_bits =3D bits; > + init_target_page.bits =3D bits; > } > #endif > return true; > @@ -49,9 +93,9 @@ bool set_preferred_target_page_bits(int bits) > void finalize_target_page_bits(void) > { > #ifdef TARGET_PAGE_BITS_VARY > - if (target_page_bits =3D=3D 0) { > - target_page_bits =3D TARGET_PAGE_BITS_MIN; > + if (init_target_page.bits =3D=3D 0) { > + init_target_page.bits =3D TARGET_PAGE_BITS_MIN; > } > - target_page_bits_decided =3D true; > + init_target_page.decided =3D true; > #endif > } -- Alex Benn=C3=A9e