1. 首先介绍测试用例，这是一个简单的if-then-else结构，输入为 int 类型的单变量，输出为 int 类型的结果。如果条件 a < 1 成立，则将输入直接返回；如果条件不成立，则返回 1。
int foo(int a) {
if (a < 1)
return a;
else
return 1;
}2. 我们用GCC编译上述用例，打印出构造CFG之前的GIMPLE序列。在划分BB块之前，所有GIMPLE表达式在同一个序列中。
;; Function foo (foo, funcdef_no=0, decl_uid=1793, cgraph_uid=0, symbol_order=0)
foo (int a)
{
int D.1800;
if (a <= 0) goto <D.1798>; else goto <D.1799>;
<D.1798>:
D.1800 = a;
goto <D.1801>;
<D.1799>:
D.1800 = 1;
goto <D.1801>;
<D.1801>:
return D.1800;
}
3. GIMPLE 序列是一段线性结构，以双向链表的形式保存。next指向下一条gimple表达式，prev指向上一条gimple表达式，label也是单独的一条gimple表达式。
(gdb) pt gimple_seq
type = struct gimple {
gimple_code code : 8;
unsigned int no_warning : 1;
unsigned int visited : 1;
unsigned int nontemporal_move : 1;
unsigned int plf : 2;
unsigned int modified : 1;
unsigned int has_volatile_ops : 1;
unsigned int pad : 1;
unsigned int subcode : 16;
unsigned int uid;
location_t location;
unsigned int num_ops;
basic_block bb;
gimple *next;
gimple *prev;
} *
(gdb) p debug(seq)
if (a <= 0) goto <D.1798>; else goto <D.1799>;
(gdb) p debug(seq->next)
<D.1798>:
(gdb) p debug(seq->next->next)
D.1800 = a;
(gdb) p debug(seq->next->next->next)
goto <D.1801>;
4. BB块构造算法：
4.1 顺序遍历GIMPLE序列，把正在访问的stmt 的bb属性设置为当前BB。
4.2 如果遇到符合stmt_starts_bb_p 和 stmt_ends_bb_p 条件的stmt，则创建一个新的BB。
/* Insert SEQ after BB and build a flowgraph. */
static basic_block
make_blocks_1 (gimple_seq seq, basic_block bb)
{
gimple_stmt_iterator i = gsi_start (seq);
gimple *stmt = NULL;
bool start_new_block = true;
bool first_stmt_of_seq = true;
// 顺序遍历GIMPLE序列
while (!gsi_end_p (i))
{
gimple *prev_stmt;
prev_stmt = stmt;
stmt = gsi_stmt (i);
if (stmt && is_gimple_call (stmt))
gimple_call_initialize_ctrl_altering (stmt);
/* If the statement starts a new basic block or if we have determined
in a previous pass that we need to create a new block for STMT, do
so now. */
if (start_new_block || stmt_starts_bb_p (stmt, prev_stmt))
{
if (!first_stmt_of_seq)
gsi_split_seq_before (&i, &seq);
bb = create_basic_block (seq, bb);
start_new_block = false;
}
/* Now add STMT to BB and create the subgraphs for special statement
codes. */
gimple_set_bb (stmt, bb);
/* If STMT is a basic block terminator, set START_NEW_BLOCK for the
next iteration. */
if (stmt_ends_bb_p (stmt))
{
……
start_new_block = true;
}
gsi_next (&i);
first_stmt_of_seq = false;
}
return bb;
}5. 完成BB块的构造后，GIMPLE序列整体的组织方式发生了改变，由线性序列变成了以BB块为节点的图结构。在每个BB块的内部，GIMPLE表达式仍然以链表的形式线性保存。TODO ：BB块之间会通过edge联系起来，形成流图
;; Function foo (foo, funcdef_no=0, decl_uid=1793, cgraph_uid=0, symbol_order=0)
;; 1 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2 3 4 5
;; 2 succs { 3 4 }
;; 3 succs { 5 }
;; 4 succs { 5 }
;; 5 succs { 1 }
foo (int a)
{
int D.1800;
<bb 2> [0.00%]:
if (a <= 0)
goto <bb 3>; [0.00%]
else
goto <bb 4>; [0.00%]
<bb 3> [0.00%]:
D.1800 = a;
goto <bb 5> (<L2>); [0.00%]
<bb 4> [0.00%]:
D.1800 = 1;
<L2> [0.00%]:
return D.1800;
}