GCC-3.4.6源代码学习笔记（112）

5.12.3.2.1.2.1.4.


为产生代码准备


cp_parser_function_definition_after_declarator

最后调用的函数是
expand_or_defer_fn

。在适当的情况下，它将为该函数产生
RTL
代码；否则为
RTL
代码的产生做一些准备。

2973

void

2974

expand_or_defer_fn

(tree fn)

in semantics.c

2975

{

2976

/*
When the parser calls us after finishing the body of a template

2977

function, we don't really want to expand the body.
*/

2978

if (processing_template_decl
)

2979

{

2980

/*
Normally, collection only occurs in rest_of_compilation. So,

2981

if
we don't collect here, we never collect junk generated

2982

during the processing of templates until we hit a

2983

non-template function.
*/

2984

ggc_collect ();

2985

return
;

2986

}

2987

2988

/*
Replace AGGR_INIT_EXPRs with appropriate CALL_EXPRs.
*/

2989

walk_tree_without_duplicates
(&DECL_SAVED_TREE (fn),

2990

simplify_aggr_init_exprs_r,

2991

NULL);

2992

2993

/*
If this is a constructor or destructor body, we have to clone

2994

it.
*/

2995

if (maybe_clone_body (fn))

2996

{

2997

/*
We don't want to process FN again, so pretend we've written

2998

it
out, even though we haven't.
*/

2999

TREE_ASM_WRITTEN (fn) = 1;

3000

return
;

3001

}

3002

3003

/*
There's no reason to do any of the work here if we're only doing

3004

semantic analysis; this code just generates RTL.
*/

3005

if (flag_syntax_only

)

3006

return
;

3007

3008

/*
Compute the appropriate object-file linkage for inline functions.
*/

3009

if (DECL_DECLARED_INLINE_P
(fn))

3010

import_export_decl (fn);

3011

3012

/*
If this function is marked with the constructor attribute, add it

3013

to the
list of functions to be called along with constructors

3014

from
static duration objects.
*/

3015

if (DECL_STATIC_CONSTRUCTOR
(fn))

3016

static_ctors

= tree_cons
(NULL_TREE, fn, static_ctors

);

3017

3018

/*
If this function is marked with the destructor attribute, add it

3019

to the
list of functions to be called along with destructors from

3020

static
duration objects.
*/

3021

if (DECL_STATIC_DESTRUCTOR
(fn))

3022

static_dtors

= tree_cons
(NULL_TREE, fn, static_dtors

);

3023

3024

function_depth

++;

3025

3026

/*
Expand or defer, at the whim of the compilation unit manager.
*/

3027

cgraph_finalize_function
(fn, function_depth

> 1);

3028

3029

function_depth

--;

3030

}

2989
行对函数定义树进行遍历，处理对经过具名返回值优化处理的函数的调用。我们将结合具名返回值优化来看。

上面判断
DECL_STATIC_CONSTRUCTOR
或
DECL_STATIC_DESTRUCTOR
成立，如果函数的声明中带有
__attribute__ ((constructor))

或
__attribute__ ((destructor))

。属性
constructor
使得该函数，在进入
main ()
执行前，被自动调用。类似的，属性
destructor
使得该函数，
main ()
已经完成或
exit ()
已经被调用后，被自动调用。具有这些属性的函数对于初始化那些在程序执行期间被隐含使用的数据很有用。

5.12.3.2.1.2.1.4.1.


更新函数调用图


前面的章节
函数调用关系图

中描述了数据结构
cgraph_node

及
cgraph_edge

。另外还学习了函数
cgraph_node

，这个函数为每个函数在第一次调用时构建唯一的
cgraph_node

节点，而在后面的调用中返回这个唯一的节点。这是在编译过程中，为由
decl

指定的函数第一次调用
cgraph_finalize_function

及
cgraph_node

。

157

void

158

cgraph_finalize_function

(tree decl,
bool nested)

in cgraphunit.c

159

{

160

struct
cgraph_node *node = cgraph_node
(decl);

161

162

if (node->local.finalized)

163

{

164

/* As an GCC
extension we allow redefinition of the function. The

165

semantics when
both copies of bodies differ is not well defined.

166

We replace the
old body with new body so in unit at a time mode

167

we always use
new body, while in normal mode we may end up with

168

old body
inlined into some functions and new body expanded and

169

inlined in
others.

170

171

??? It may make
more sense to use one body for inlining and other

172

body for
expanding the function but this is difficult to do.
*/

173

174

/* If
node->output is set, then this is a unit-at-a-time compilation

175

and we have
already begun whole-unit analysis. This is *not*

176

testing for
whether we've already emitted the function. That

177

case can be
sort-of legitimately seen with real function

178

redefinition
errors. I would argue that the front end should

179

never present
us with such a case, but don't enforce that for now.
*/

180

if (node->output)

181

abort ();

182

183

/* Reset our
datastructures so we can analyze the function again.
*/

184

memset (&node->local, 0, sizeof
(node->local));

185

memset (&node->global, 0, sizeof
(node->global));

186

memset (&node->rtl, 0, sizeof
(node->rtl));

187

node->analyzed = false;

188

node->local.redefined_extern_inline =
true;

189

while
(node->callees)

190

cgraph_remove_edge (node,
node->callees->callee);

191

192

/* We may need to
re-queue the node for assembling in case

193

we already
proceeded it and ignored as not needed.

*/

194

if (node->reachable && !flag_unit_at_a_time

)

195

{

196

struct
cgraph_node *n;

197

198

for
(n = cgraph_nodes_queue

;
n; n = n->next_needed)

199

if (n == node)

200

break
;

201

if
(!n)

202

node->reachable = 0;

203

}

204

}

205

206

notice_global_symbol
(decl);

207

node->decl = decl;

208

node->local.finalized = true;

209

210

/* If not unit at a
time, then we need to create the call graph

211

now, so that
called functions can be queued and emitted now.

*/

212

if (!flag_unit_at_a_time

)

213

{

214

cgraph_analyze_function (node);

215

cgraph_decide_inlining_incrementally
(node);

216

}

在这个第一次调用中，
node->local.finalized

一定是
false
，而它很快在
208
行被更新为
true
。在
206
行，
notice_global_symbol

收集第一个可见的全局符号。因为全局符号指针这个编译单元是唯一的，这个信息对于编译器为匿名对象产生唯一名字是有用的，比如，在这里收集的符号将被用于产生匿名名字空间的内部名字。

1034

void

1035

notice_global_symbol

(tree decl)

in varasm.c

1036

{

1037

const
char
**type = &first_global_object_name

;

1038

1039

if (first_global_object_name

1040

|| !TREE_PUBLIC (decl) || DECL_EXTERNAL
(decl)

1041

|| !DECL_NAME (decl)

1042

|| (TREE_CODE (decl) != FUNCTION_DECL

1043

&& (TREE_CODE (decl) !=
VAR_DECL

1044

|| (DECL_COMMON (decl)

1045

&& (DECL_INITIAL (decl)
== 0

1046

|| DECL_INITIAL (decl) ==
error_mark_node))))

1047

|| GET_CODE (DECL_RTL (decl)) != MEM)

1048

return
;

1049

1050

/* We win when
global object is found, but it is usefull to know about weak

1051

symbol as well so we can
produce nicer unique names.
*/

1052

if (DECL_WEAK (decl) || DECL_ONE_ONLY (decl))

1053

type = &weak_global_object_name

;

1054

1055

if (!*type)

1056

{

1057

const
char
*p;

1058

char *name;

1059

rtx decl_rtl = DECL_RTL (decl);

1060

1061

p = (* targetm

.strip_name_encoding) (XSTR (XEXP
(decl_rtl, 0), 0));

1062

name = xstrdup (p);

1063

1064

*type = name;

1065

}

1066

}

在
cgraph_finalize_function

的
212
行，如果我们在编译指令中指定了高于
-O2
的优化选项，
flag_unit_at_a_time

被设置为
1
。通常，我们倾向使用
–O3
选项来榨取编译器的所有优化能力。在这里假定
flag_unit_at_a_time

为
1
。

cgraph_finalize_function (continue)

218

if (decide_is_function_needed
(node, decl))

219

cgraph_mark_needed_node
(node);

220

221

/* If not unit at a
time, go ahead and emit everything we've found

222

to be reachable
at this time.
*/

223

if (!nested)

224

{

225

if (!cgraph_assemble_pending_functions ())

226

ggc_collect ();

227

}

228

229

/* If we've not yet
emitted decl, tell the debug info about it.

*/

230

if (!TREE_ASM_WRITTEN (decl))

231

(*debug_hooks

->deferred_inline_function) (decl);

232

233

/* We will never
really output the function body, clear the SAVED_INSNS array

234

early then.
*/

235

if (DECL_EXTERNAL (decl))

236

DECL_SAVED_INSNS (decl) = NULL;

237

}

函数
decide_is_function_needed

确定
decl
所引用的函数是否在编译单元以外，或者在系统配置的戏法中可见；或者（如果不是
unit-at-a-time
—一个单元一起处理）为我们还没遇到的部分所可见。在
93
行，属性“
used
”表示该变量的代码必须被生成，就算看起来这个变量没有被引用（编译器会清除死代码）。

73

static
bool

74

decide_is_function_needed

(struct
cgraph_node *node, tree decl)

in cgraphunit.c

75

{

76

/* If we decided it
was needed before, but at the time we didn't have

77

the body of the
function available, then it's still needed. We have

78

to go back and
re-check its dependencies now.
*/

79

if (node->needed)

80

return
true;

81

82

/* Externally visible
functions must be output. The exception is

83

COMDAT functions
that must be output only when they are needed.

*/

84

if (TREE_PUBLIC (decl)
&& !DECL_COMDAT (decl) && !DECL_EXTERNAL (decl))

85

return
true;

86

87

/* Constructors and
destructors are reachable from the runtime by

88

some
mechanism.
*/

89

if (DECL_STATIC_CONSTRUCTOR (decl) ||
DECL_STATIC_DESTRUCTOR (decl))

90

return
true;

91

92

/* If the user told
us it is used, then it must be so.
*/

93

if (lookup_attribute
("used", DECL_ATTRIBUTES (decl)))

94

return
true;

95

96

/* ??? If the
assembler name is set by hand, it is possible to assemble

97

the name later
after finalizing the function and the fact is noticed

98

i
n assemble_name then. This is arguably a
bug.
*/

99

if (DECL_ASSEMBLER_NAME_SET_P (decl)

100

&& TREE_SYMBOL_REFERENCED
(DECL_ASSEMBLER_NAME (decl)))

101

return
true;

102

103

if (flag_unit_at_a_time

)

104

return
false;

105

106

/* If not doing unit
at a time, then we'll only defer this function

107

if its marked for
inlining. Otherwise we want to emit it now.

*/

108

109

/* "extern
inline" functions are never output locally.
*/

110

if (DECL_EXTERNAL (decl))

111

return
false;

112

/* We want to emit
COMDAT functions only when absolutely necessary.
*/

113

if (DECL_COMDAT (decl))

114

return
false;

115

if (!DECL_INLINE (decl)

116

||
(!node->local.disregard_inline_limits

117

/* When
declared inline, defer even the uninlinable functions.

118

This allows them to be eliminated when
unused.
*/

119

&& !DECL_DECLARED_INLINE_P
(decl)

120

&& (!node->local.inlinable ||
!cgraph_default_inline_p (node))))

121

return
true;

122

123

return
false;

124

}

如果该声明是可见的，
cgraph_mark_needed_node

需要把相关的
cgraph_node
节点设置为
needed
并且标记其中的子节点为可到达。

286

void

287

cgraph_mark_needed_node

(struct
cgraph_node *node)

in cgraph.c

288

{

289

node->needed = 1;

290

cgraph_mark_reachable_node
(node);

291

}

在
cgraph_finalize_function

中，域
local.finalized

已经被设置为
true
。在函数
cgraph_node

中，从
123
到
128
行，域
origin

指向包含这个函数的节点；域
nested

指向这个被包含函数的节点（即在其中所调用的函数），而域
next_nested

指向被包含的兄弟函数（
sibling functions
）。

260

void

261

cgraph_mark_reachable_node

(struct
cgraph_node *node)

in cgraph.c

262

{

263

if (!node->reachable &&
node->local.finalized)

264

{

265

notice_global_symbol
(node->decl);

266

node->reachable = 1;

267

268

node->next_needed = cgraph_nodes_queue

;

269

cgraph_nodes_queue

= node;

270

271

/* At the moment
frontend automatically emits all nested functions.
*/

272

if (node->nested)

273

{

274

struct
cgraph_node *node2;

275

276

for
(node2 = node->nested; node2; node2 = node2->next_nested)

277

if (!node2->reachable)

278

cgraph_mark_reachable_node (node2);

279

}

280

}

281

}

所有外部可见的函数都应该被串接起来。这个链表为
cgraph_nodes_queue

所指向。

回到
cgraph_finalize_function

，如果这不是一个嵌套函数（在函数作用域中或类定义中定义的内联函数），而且
flag_unit_at_a_time

是
true
，
cgraph_assemble_pending_functions

直接返回
false
，触发垃圾回收。