core/x86/instr.c
/* If val is true, indicates that instr's target fragment should be
 *   entered through its prefix, which restores ecx.
 * If val is false, indicates that instr should target the normal entry
 *   point and not the prefix.
 *
 * Some code manipulations need to store a target address in a
 * register and then jump there, but need the register to be restored
 * as well.  DR provides a single-instruction prefix that is
 * placed on all fragments (basic blocks as well as traces) that
 * restores ecx.  It is on traces for internal DR use.  To have
 * it added to basic blocks as well, call
 * dr_add_prefixes_to_basic_blocks() during initialization.
 */
If val is true, indicates that instr's target fragment should be
entered through its prefix, which restores ecx.
If val is false, indicates that instr should target the normal entry
point and not the prefix.

Some code manipulations need to store a target address in a
register and then jump there, but need the register to be restored
as well.  DR provides a single-instruction prefix that is
placed on all fragments (basic blocks as well as traces) that
restores ecx.  It is on traces for internal DR use.  To have
it added to basic blocks as well, call
dr_add_prefixes_to_basic_blocks() during initialization.
If val is true indicates that instr ' s target fragment should be entered through its prefix which restores ecx . If val is false indicates that instr should target the normal entry point and not the prefix . Some code manipulations need to store a target address in a register and then jump there but need the register to be restored as well . DR provides a single - instruction prefix that is placed on all fragments ( basic blocks as well as traces ) that restores ecx . It is on traces for internal DR use . To have it added to basic blocks as well call dr_add_prefixes_to_basic_blocks () during initialization .
void
instr_branch_set_prefix_target(instr_t *instr, bool val)
{
    if (val)
        instr->flags |= INSTR_BRANCH_TARGETS_PREFIX;
    else
        instr->flags &= ~INSTR_BRANCH_TARGETS_PREFIX;
}
void instr_branch_set_prefix_target ( instr_t * instr , bool val ) { if ( val ) instr -> flags |= INSTR_BRANCH_TARGETS_PREFIX ; else instr -> flags &= ~ INSTR_BRANCH_TARGETS_PREFIX ; }
If val is true, indicates that instr's target fragment should be
   entered through its prefix, which restores ecx.
If val is true indicates that instr ' s target fragment should be entered through its prefix which restores ecx .