Next up we add an optimization to only save and restore the flags if we can't find a spot within the block where the flags are dead.

#include "dr_api.h"

#ifdef WINDOWS
# define DISPLAY_STRING(msg) dr_messagebox(msg)
#else
# define DISPLAY_STRING(msg) dr_printf("%s\n", msg)
#endif

+ #define TESTALL(mask, var) (((mask) & (var)) == (mask))
+ #define TESTANY(mask, var) (((mask) & (var)) != 0)

typedef struct bb_counts { 
   uint64 blocks;
   uint64 total_size;
} bb_counts;


static bb_counts counts_as_built;
void *as_built_lock;

static bb_counts counts_dynamic;

+ /* Protected by the as_built_lock */
+ static uint64 bbs_eflags_saved;


static void
event_exit(void);
static dr_emit_flags_t
event_basic_block(void *drcontext, void *tag, instrlist_t *bb,
                  bool for_trace, bool translating);

DR_EXPORT void 
dr_init(client_id_t id)
{
    /* register events */
    dr_register_exit_event(event_exit);
    dr_register_bb_event(event_basic_block);

    /* initialize lock */
    as_built_lock = dr_mutex_create();
}

static void
event_exit(void)
{
    /* Display results - we must first snpritnf the string as on windows
 dr_printf(), dr_messagebox() and dr_fprintf() can't print floats. */
    char msg[512];
    int len;
    len = snprintf(msg, sizeof(msg)/sizeof(msg[0]),
                   "Number of blocks built  : %"UINT64_FORMAT_CODE"\n"
                   "     Average size       : %5.2lf instructions\n"
+                    "     Num saved eflags : %"UINT64_FORMAT_CODE"\n"
                   "Number of blocks executed  : %"UINT64_FORMAT_CODE"\n"
                   "     Average weighted size : %5.2lf instructions\n",
                   counts_as_built.blocks,
                   counts_as_built.total_size / (double)counts_as_built.blocks,
+                    bbs_eflags_saved,
                   counts_dynamic.blocks,
                   counts_dynamic.total_size / (double)counts_dynamic.blocks);
    DR_ASSERT(len > 0);
    msg[sizeof(msg)/sizeof(msg[0])-1] = '\0';
    DISPLAY_STRING(msg);

    /* free mutex */
    dr_mutex_destroy(as_built_lock);
}

static dr_emit_flags_t
event_basic_block(void *drcontext, void *tag, instrlist_t *bb,
                  bool for_trace, bool translating)
{
    uint num_instructions = 0;
    instr_t *instr, *where = NULL;
+     bool eflags_saved = true;

    /* count the number of instructions in this block */
    for (instr = instrlist_first(bb); instr != NULL; instr = instr_get_next(instr)) {
+         /* Since it doesn't matter where we insert, look for a place
+          * where the eflags are dead. */
+         uint flags = instr_get_arith_flags(instr);
+         if (TESTALL(EFLAGS_WRITE_6, flags) && !TESTANY(EFLAGS_READ_6, flags)) {
+             where = instr;
+             eflags_saved = false;
+         }
        num_instructions++;
    }

    /* update the as-built counts */
    dr_mutex_lock(as_built_lock);
    counts_as_built.blocks++;
    counts_as_built.total_size += num_instructions;
+     if (eflags_saved)
+         bbs_eflags_saved++;
    dr_mutex_unlock(as_built_lock);

    /* increment counters */
+     if (eflags_saved) {
+         where = instrlist_first(bb);
        dr_save_arith_flags(drcontext, bb, where, SPILL_SLOT_1);
+     }
#ifdef X86_32
    /* Since the counters are 64-bit we must use an add an addc to increment.
 The operations is still effectively atomic since we're only increasing
 the count. */
    instrlist_meta_preinsert(bb, where,
        LOCK(INSTR_CREATE_add(drcontext,
                              OPND_CREATE_ABSMEM((byte *)&counts_dynamic.blocks, OPSZ_4),
                              OPND_CREATE_INT8(1))));
    instrlist_meta_preinsert(bb, where,
        LOCK(INSTR_CREATE_adc(drcontext,
                              OPND_CREATE_ABSMEM((byte *)&counts_dynamic.blocks + 4, OPSZ_4),
                              OPND_CREATE_INT8(0))));

    instrlist_meta_preinsert(bb, where,
        LOCK(INSTR_CREATE_add(drcontext,
                              OPND_CREATE_ABSMEM((byte *)&counts_dynamic.total_size, OPSZ_4),
                              OPND_CREATE_INT_32OR8(num_instructions))));
    instrlist_meta_preinsert(bb, where,
        LOCK(INSTR_CREATE_adc(drcontext,
                              OPND_CREATE_ABSMEM((byte *)&counts_dynamic.total_size + 4, OPSZ_4),
                              OPND_CREATE_INT8(0))));
#else /* X86_64 */
    instrlist_meta_preinsert(bb, where,
        LOCK(INSTR_CREATE_inc(drcontext,
                              OPND_CREATE_ABSMEM((byte *)&counts_dynamic.blocks, OPSZ_8))));
    instrlist_meta_preinsert(bb, where,
        LOCK(INSTR_CREATE_add(drcontext,
                              OPND_CREATE_ABSMEM((byte *)&counts_dynamic.total_size, OPSZ_8),
                              OPND_CREATE_INT_32OR8(num_instructions))));
#endif
+     if (eflags_saved) {
        dr_restore_arith_flags(drcontext, bb, where, SPILL_SLOT_1);
+     }

    return DR_EMIT_DEFAULT;
}

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