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>

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
 }

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