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exec: Use const alias for TARGET_PAGE_BITS_VARY 

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 <alex.bennee@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2019-09-13 11:21:53 -04:00
parent db8aaae822
commit bbc17caf81
2 changed files with 68 additions and 12 deletions
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66
exec-vary.c
View File

@ -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
}

14
include/exec/cpu-all.h
View File

@ -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