open coded defer 是怎么实现的
1.14 defer
正常处理流程
在 Go 1.14 中，增加了一种新的 defer 实现：open coded defer。当函数内 defer 不超过 8 个时，则会使用这种实现。
形如下面这样的代码:
defer f1(a)
if cond {
defer f2(b)
}
body...
会被翻译为:
deferBits |= 1<<0 // 因为函数主 block 有 defer 语句，所以这里要设置第 0 位为 1
tmpF1 = f1 // 将函数地址搬运到预留的栈上空间
tmpA = a // 将函数参数复制到预留的栈上空间
if cond {
deferBits |= 1<<1 // 因为进入了 if block，所以这里要设置第 1 位为 1
tmpF2 = f2 // 将函数地址搬运到预留的栈上空间
tmpB = b // 将函数参数复制到预留的栈上空间
}
body...
exit: // 退出函数 body
if deferBits & 1<<1 != 0 { // 检查第 1 个 bit，如果是 1，执行保存好的函数
deferBits &^= 1<<1
tmpF2(tmpB)
}
if deferBits & 1<<0 != 0 { // 检查第 0 个 bit，如果是 1，执行保存好的函数
deferBits &^= 1<<0
tmpF1(tmpA)
}
即每次进入 function 的一个 block，都需要将 deferbits 的对应位置置一，并将 defer 的函数地址和参数赋值给临时对象(就是栈的某个槽位置)。
这样看我们不一定知道他具体是怎么实现的，可以写个 block 更多的例子来看看实际生成的代码是什么样的:
1 package main
2
3 import "fmt"
4
5 var i = 100
6
7 func main() {
8 if i == 0 {
9 defer fmt.Println("1")
10 }
11
12 if i == 1 {
13 defer fmt.Println("2")
14 }
15
16 if i == 2 {
17 defer fmt.Println("3")
18 }
19
20 if i == 3 {
21 defer fmt.Println("4")
22 }
23
24 if i == 4 {
25 defer fmt.Println("5")
26 }
27
28 if i == 5 {
29 defer fmt.Println("6")
30 }
31 }
下面是 go tool compile -S 生成的汇编代码，简单划分一下 block 来理解一下:
"".main STEXT size=1125 args=0x0 locals=0x160
if i == 0:
0x0055 00085 (open-coded-defer.go:8) CMPQ "".i(SB), $0
0x005d 00093 (open-coded-defer.go:8) JNE 1075 // 1075 是条废指令，其实本质是跳过这条 if 的 DEFER PREPARE BLOCK，去地址 188
DEFER PREPARE BLOCK 1，主要工作是将 fmt.Println 的参数和函数地址搬运到栈上该 defer 对应的区域:
0x0063 00099 (open-coded-defer.go:9) XORPS X0, X0
0x0066 00102 (open-coded-defer.go:9) MOVUPS X0, ""..autotmp_3+72(SP)
0x006b 00107 (open-coded-defer.go:9) LEAQ type.string(SB), AX
0x0072 00114 (open-coded-defer.go:9) MOVQ AX, ""..autotmp_3+72(SP)
0x0077 00119 (open-coded-defer.go:9) LEAQ ""..stmp_0(SB), CX
0x007e 00126 (open-coded-defer.go:9) MOVQ CX, ""..autotmp_3+80(SP)
0x0083 00131 (open-coded-defer.go:9) LEAQ fmt.Println·f(SB), CX
0x008a 00138 (open-coded-defer.go:9) MOVQ CX, ""..autotmp_25+192(SP)
0x0092 00146 (open-coded-defer.go:9) LEAQ ""..autotmp_3+72(SP), DX
0x0097 00151 (open-coded-defer.go:9) MOVQ DX, ""..autotmp_26+320(SP)
0x009f 00159 (open-coded-defer.go:9) MOVQ $1, ""..autotmp_26+328(SP)
0x00ab 00171 (open-coded-defer.go:9) MOVQ $1, ""..autotmp_26+336(SP)
0x00b7 00183 (open-coded-defer.go:9) MOVB $1, ""..autotmp_24+55(SP)
// 根据 zero flag 设置值，如果 zero flag = 1，那么 DI = 1，如果 zero flag = 0，那么 DI = 0
// 详情参见 intel 手册的 sete/setz 指令
// 也就是说，只要走了上面的 DEFER PREPARE BLOCK，那么这里就 DI = 1，否则 DI = 0
0x00bc 00188 (open-coded-defer.go:8) SETEQ DL
if i == 1:
0x00bf 00191 (open-coded-defer.go:12) CMPQ "".i(SB), $1
0x00c7 00199 (open-coded-defer.go:12) JNE 278
DEFER PREPARE BLOCK 2, 和 BLOCK 1 工作差不多，是把第二个 defer 的参数和函数地址保存在栈上:
0x00c9 00201 (open-coded-defer.go:13) XORPS X0, X0
0x00cc 00204 (open-coded-defer.go:13) MOVUPS X0, ""..autotmp_7+56(SP)
0x00d1 00209 (open-coded-defer.go:13) MOVQ AX, ""..autotmp_7+56(SP)
0x00d6 00214 (open-coded-defer.go:13) LEAQ ""..stmp_1(SB), BX
0x00dd 00221 (open-coded-defer.go:13) MOVQ BX, ""..autotmp_7+64(SP)
0x00e2 00226 (open-coded-defer.go:13) MOVQ CX, ""..autotmp_27+184(SP)
0x00ea 00234 (open-coded-defer.go:13) LEAQ ""..autotmp_7+56(SP), BX
0x00ef 00239 (open-coded-defer.go:13) MOVQ BX, ""..autotmp_28+296(SP)
0x00f7 00247 (open-coded-defer.go:13) MOVQ $1, ""..autotmp_28+304(SP)
0x0103 00259 (open-coded-defer.go:13) MOVQ $1, ""..autotmp_28+312(SP)
0x010f 00271 (open-coded-defer.go:13) ORL $2, DX // 设置 DX 的第二个标记位为 1
0x0112 00274 (open-coded-defer.go:13) MOVB DL, ""..autotmp_24+55(SP) // TODO
if i == 2:
0x0116 00278 (open-coded-defer.go:16) CMPQ "".i(SB), $2
0x011e 00286 (open-coded-defer.go:16) JNE 377
DEFER PREPARE BLOCK 3，和上面类似，就不解释了:
0x0120 00288 (open-coded-defer.go:17) XORPS X0, X0
0x0123 00291 (open-coded-defer.go:17) MOVUPS X0, ""..autotmp_11+136(SP)
0x012b 00299 (open-coded-defer.go:17) MOVQ AX, ""..autotmp_11+136(SP)
0x0133 00307 (open-coded-defer.go:17) LEAQ ""..stmp_2(SB), BX
0x013a 00314 (open-coded-defer.go:17) MOVQ BX, ""..autotmp_11+144(SP)
0x0142 00322 (open-coded-defer.go:17) MOVQ CX, ""..autotmp_29+176(SP)
0x014a 00330 (open-coded-defer.go:17) LEAQ ""..autotmp_11+136(SP), BX
0x0152 00338 (open-coded-defer.go:17) MOVQ BX, ""..autotmp_30+272(SP)
0x015a 00346 (open-coded-defer.go:17) MOVQ $1, ""..autotmp_30+280(SP)
0x0166 00358 (open-coded-defer.go:17) MOVQ $1, ""..autotmp_30+288(SP)
0x0172 00370 (open-coded-defer.go:17) ORL $4, DX // 设置 DX 的第三个标记位为 1
0x0175 00373 (open-coded-defer.go:17) MOVB DL, ""..autotmp_24+55(SP)
if i == 3
0x0179 00377 (open-coded-defer.go:20) CMPQ "".i(SB), $3
0x0181 00385 (open-coded-defer.go:20) JNE 467
DEFER PREPARE BLOCK 4:
0x0183 00387 (open-coded-defer.go:21) XORPS X0, X0
0x0186 00390 (open-coded-defer.go:21) MOVUPS X0, ""..autotmp_15+120(SP)
0x018b 00395 (open-coded-defer.go:21) MOVQ AX, ""..autotmp_15+120(SP)
0x0190 00400 (open-coded-defer.go:21) LEAQ ""..stmp_3(SB), BX
0x0197 00407 (open-coded-defer.go:21) MOVQ BX, ""..autotmp_15+128(SP)
0x019f 00415 (open-coded-defer.go:21) MOVQ CX, ""..autotmp_31+168(SP)
0x01a7 00423 (open-coded-defer.go:21) LEAQ ""..autotmp_15+120(SP), BX
0x01ac 00428 (open-coded-defer.go:21) MOVQ BX, ""..autotmp_32+248(SP)
0x01b4 00436 (open-coded-defer.go:21) MOVQ $1, ""..autotmp_32+256(SP)
0x01c0 00448 (open-coded-defer.go:21) MOVQ $1, ""..autotmp_32+264(SP)
0x01cc 00460 (open-coded-defer.go:21) ORL $8, DX // 设置 DX 的第四个标记位为 1
0x01cf 00463 (open-coded-defer.go:21) MOVB DL, ""..autotmp_24+55(SP)
if i == 4:
0x01d3 00467 (open-coded-defer.go:24) CMPQ "".i(SB), $4
0x01db 00475 (open-coded-defer.go:24) JNE 554
DEFER PREPARE BLOCK 5:
0x01dd 00477 (open-coded-defer.go:25) XORPS X0, X0
0x01e0 00480 (open-coded-defer.go:25) MOVUPS X0, ""..autotmp_19+104(SP)
0x01e5 00485 (open-coded-defer.go:25) MOVQ AX, ""..autotmp_19+104(SP)
0x01ea 00490 (open-coded-defer.go:25) LEAQ ""..stmp_4(SB), BX
0x01f1 00497 (open-coded-defer.go:25) MOVQ BX, ""..autotmp_19+112(SP)
0x01f6 00502 (open-coded-defer.go:25) MOVQ CX, ""..autotmp_33+160(SP)
0x01fe 00510 (open-coded-defer.go:25) LEAQ ""..autotmp_19+104(SP), BX
0x0203 00515 (open-coded-defer.go:25) MOVQ BX, ""..autotmp_34+224(SP)
0x020b 00523 (open-coded-defer.go:25) MOVQ $1, ""..autotmp_34+232(SP)
0x0217 00535 (open-coded-defer.go:25) MOVQ $1, ""..autotmp_34+240(SP)
0x0223 00547 (open-coded-defer.go:25) ORL $16, DX // 设置 DX 的第五个标记位为 1
0x0226 00550 (open-coded-defer.go:25) MOVB DL, ""..autotmp_24+55(SP)
if i == 5:
0x022a 00554 (open-coded-defer.go:28) CMPQ "".i(SB), $5
0x0232 00562 (open-coded-defer.go:28) JNE 641
DEFER PREPARE BLOCK 6:
0x0234 00564 (open-coded-defer.go:29) XORPS X0, X0
0x0237 00567 (open-coded-defer.go:29) MOVUPS X0, ""..autotmp_23+88(SP)
0x023c 00572 (open-coded-defer.go:29) MOVQ AX, ""..autotmp_23+88(SP)
0x0241 00577 (open-coded-defer.go:29) LEAQ ""..stmp_5(SB), AX
0x0248 00584 (open-coded-defer.go:29) MOVQ AX, ""..autotmp_23+96(SP)
0x024d 00589 (open-coded-defer.go:29) MOVQ CX, ""..autotmp_35+152(SP)
0x0255 00597 (open-coded-defer.go:29) LEAQ ""..autotmp_23+88(SP), AX
0x025a 00602 (open-coded-defer.go:29) MOVQ AX, ""..autotmp_36+200(SP)
0x0262 00610 (open-coded-defer.go:29) MOVQ $1, ""..autotmp_36+208(SP)
0x026e 00622 (open-coded-defer.go:29) MOVQ $1, ""..autotmp_36+216(SP)
0x027a 00634 (open-coded-defer.go:29) ORL $32, DX // 设置 DX 的第六个标志位为 1
0x027d 00637 (open-coded-defer.go:29) MOVB DL, ""..autotmp_24+55(SP)
FUNCTION END, FLAG CHECKS, 这里要检查上面设置的所有的 open coded defer bits 了，可以看到与 flag 的设置顺序是相反的:
0x0281 00641 (open-coded-defer.go:31) TESTB $32, DL
0x0284 00644 (open-coded-defer.go:31) JNE 1011
0x028a 00650 (open-coded-defer.go:31) TESTB $16, DL
0x028d 00653 (open-coded-defer.go:31) JNE 947
0x0293 00659 (open-coded-defer.go:31) TESTB $8, DL
0x0296 00662 (open-coded-defer.go:31) JNE 883
0x029c 00668 (open-coded-defer.go:31) TESTB $4, DL
0x029f 00671 (open-coded-defer.go:31) JNE 819
0x02a5 00677 (open-coded-defer.go:31) TESTB $2, DL
0x02a8 00680 (open-coded-defer.go:31) JNE 755
0x02aa 00682 (open-coded-defer.go:31) TESTB $1, DL
0x02ad 00685 (open-coded-defer.go:31) JNE 703
0x02af 00687 (open-coded-defer.go:31) MOVQ 344(SP), BP
0x02b7 00695 (open-coded-defer.go:31) ADDQ $352, SP
0x02be 00702 (open-coded-defer.go:31) RET
DEFER 逻辑执行部分
// defer fmt.Println("6"):
0x02bf 00703 (open-coded-defer.go:31) ANDL $-2, DX
0x02c2 00706 (open-coded-defer.go:31) MOVB DL, ""..autotmp_24+55(SP)
0x02c6 00710 (open-coded-defer.go:31) MOVQ ""..autotmp_26+320(SP), AX
0x02ce 00718 (open-coded-defer.go:31) MOVQ ""..autotmp_26+328(SP), CX
0x02d6 00726 (open-coded-defer.go:31) MOVQ ""..autotmp_26+336(SP), DX
0x02de 00734 (open-coded-defer.go:31) MOVQ AX, (SP)
0x02e2 00738 (open-coded-defer.go:31) MOVQ CX, 8(SP)
0x02e7 00743 (open-coded-defer.go:31) MOVQ DX, 16(SP)
0x02ec 00748 (open-coded-defer.go:31) CALL fmt.Println(SB)
0x02f1 00753 (open-coded-defer.go:31) JMP 687
// defer fmt.Println("5"):
0x02f3 00755 (open-coded-defer.go:31) ANDL $-3, DX
0x02f6 00758 (open-coded-defer.go:31) MOVB DL, ""..autotmp_37+54(SP)
0x02fa 00762 (open-coded-defer.go:31) MOVB DL, ""..autotmp_24+55(SP)
0x02fe 00766 (open-coded-defer.go:31) MOVQ ""..autotmp_28+312(SP), AX
0x0306 00774 (open-coded-defer.go:31) MOVQ ""..autotmp_28+304(SP), CX
0x030e 00782 (open-coded-defer.go:31) MOVQ ""..autotmp_28+296(SP), BX
0x0316 00790 (open-coded-defer.go:31) MOVQ BX, (SP)
0x031a 00794 (open-coded-defer.go:31) MOVQ CX, 8(SP)
0x031f 00799 (open-coded-defer.go:31) MOVQ AX, 16(SP)
0x0324 00804 (open-coded-defer.go:31) CALL fmt.Println(SB)
0x0329 00809 (open-coded-defer.go:31) MOVBLZX ""..autotmp_37+54(SP), DX
0x032e 00814 (open-coded-defer.go:31) JMP 682
// defer fmt.Println("4"):
0x0333 00819 (open-coded-defer.go:31) ANDL $-5, DX
0x0336 00822 (open-coded-defer.go:31) MOVB DL, ""..autotmp_37+54(SP)
0x033a 00826 (open-coded-defer.go:31) MOVB DL, ""..autotmp_24+55(SP)
0x033e 00830 (open-coded-defer.go:31) MOVQ ""..autotmp_30+288(SP), AX
0x0346 00838 (open-coded-defer.go:31) MOVQ ""..autotmp_30+280(SP), CX
0x034e 00846 (open-coded-defer.go:31) MOVQ ""..autotmp_30+272(SP), BX
0x0356 00854 (open-coded-defer.go:31) MOVQ BX, (SP)
0x035a 00858 (open-coded-defer.go:31) MOVQ CX, 8(SP)
0x035f 00863 (open-coded-defer.go:31) MOVQ AX, 16(SP)
0x0364 00868 (open-coded-defer.go:31) CALL fmt.Println(SB)
0x0369 00873 (open-coded-defer.go:31) MOVBLZX ""..autotmp_37+54(SP), DX
0x036e 00878 (open-coded-defer.go:31) JMP 677
// defer fmt.Println("3"):
0x0373 00883 (open-coded-defer.go:31) ANDL $-9, DX
0x0376 00886 (open-coded-defer.go:31) MOVB DL, ""..autotmp_37+54(SP)
0x037a 00890 (open-coded-defer.go:31) MOVB DL, ""..autotmp_24+55(SP)
0x037e 00894 (open-coded-defer.go:31) MOVQ ""..autotmp_32+264(SP), AX
0x0386 00902 (open-coded-defer.go:31) MOVQ ""..autotmp_32+256(SP), CX
0x038e 00910 (open-coded-defer.go:31) MOVQ ""..autotmp_32+248(SP), BX
0x0396 00918 (open-coded-defer.go:31) MOVQ BX, (SP)
0x039a 00922 (open-coded-defer.go:31) MOVQ CX, 8(SP)
0x039f 00927 (open-coded-defer.go:31) MOVQ AX, 16(SP)
0x03a4 00932 (open-coded-defer.go:31) CALL fmt.Println(SB)
0x03a9 00937 (open-coded-defer.go:31) MOVBLZX ""..autotmp_37+54(SP), DX
0x03ae 00942 (open-coded-defer.go:31) JMP 668
// defer fmt.Println("2"):
0x03b3 00947 (open-coded-defer.go:31) ANDL $-17, DX
0x03b6 00950 (open-coded-defer.go:31) MOVB DL, ""..autotmp_37+54(SP)
0x03ba 00954 (open-coded-defer.go:31) MOVB DL, ""..autotmp_24+55(SP)
0x03be 00958 (open-coded-defer.go:31) MOVQ ""..autotmp_34+224(SP), AX
0x03c6 00966 (open-coded-defer.go:31) MOVQ ""..autotmp_34+240(SP), CX
0x03ce 00974 (open-coded-defer.go:31) MOVQ ""..autotmp_34+232(SP), BX
0x03d6 00982 (open-coded-defer.go:31) MOVQ AX, (SP)
0x03da 00986 (open-coded-defer.go:31) MOVQ BX, 8(SP)
0x03df 00991 (open-coded-defer.go:31) MOVQ CX, 16(SP)
0x03e4 00996 (open-coded-defer.go:31) CALL fmt.Println(SB)
0x03e9 01001 (open-coded-defer.go:31) MOVBLZX ""..autotmp_37+54(SP), DX
0x03ee 01006 (open-coded-defer.go:31) JMP 659
// defer fmt.Println("1"):
0x03f3 01011 (open-coded-defer.go:31) ANDL $-33, DX
0x03f6 01014 (open-coded-defer.go:31) MOVB DL, ""..autotmp_37+54(SP)
0x03fa 01018 (open-coded-defer.go:31) MOVB DL, ""..autotmp_24+55(SP)
0x03fe 01022 (open-coded-defer.go:31) MOVQ ""..autotmp_36+216(SP), AX
0x0406 01030 (open-coded-defer.go:31) MOVQ ""..autotmp_36+200(SP), CX
0x040e 01038 (open-coded-defer.go:31) MOVQ ""..autotmp_36+208(SP), BX
0x0416 01046 (open-coded-defer.go:31) MOVQ CX, (SP)
0x041a 01050 (open-coded-defer.go:31) MOVQ BX, 8(SP)
0x041f 01055 (open-coded-defer.go:31) MOVQ AX, 16(SP)
0x0424 01060 (open-coded-defer.go:31) CALL fmt.Println(SB)
0x0429 01065 (open-coded-defer.go:31) MOVBLZX ""..autotmp_37+54(SP), DX
0x042e 01070 (open-coded-defer.go:31) JMP 650
如果第一个 if 不满足，会跳到这里:
0x0433 01075 (open-coded-defer.go:31) LEAQ type.string(SB), AX
0x043a 01082 (open-coded-defer.go:31) LEAQ fmt.Println·f(SB), CX
0x0441 01089 (open-coded-defer.go:8) JMP 188
这段代码没什么用，只有在 panic 时，才可能跳到这里，但本例中无 panic
0x0446 01094 (open-coded-defer.go:8) CALL runtime.deferreturn(SB)
0x044b 01099 (open-coded-defer.go:8) MOVQ 344(SP), BP
0x0453 01107 (open-coded-defer.go:8) ADDQ $352, SP
0x045a 01114 (open-coded-defer.go:8) RET
函数 prologue 栈检查的时候可能会跳到这里
0x045b 01115 (open-coded-defer.go:8) NOP
0x045b 01115 (open-coded-defer.go:7) CALL runtime.morestack_noctxt(SB)
0x0460 01120 (open-coded-defer.go:7) JMP 0
这里的 deferbits 用了 DX 寄存器，这个寄存器并不是固定的，换一段代码可能就是别的了。读者可以自己写一段简单的代码来实践一下。
整体流程还好，不过比起以前堆上分配一个 _defer 链表，函数执行完之后直接遍历链表还是复杂太多了。
超过 8 个 defer 时会退化回以前的 defer 链，也可以观察一下:
package main
func main() {
defer println(1)
defer println(1)
defer println(1)
defer println(1)
defer println(1)
defer println(1)
defer println(1)
defer println(1)
defer println(1)
defer println(1)
}
编译出的汇编就不贴了，和以前的没什么区别。
结合代码分析过程和 proposal 中的描述，总结:
- open coded defer 在函数内部总 defer 数量少于 8 时才会使用，大于 8 时会退化回老的 defer 链，这个权衡是考虑到程序文件的体积(个人觉得应该也有栈膨胀的考虑在)
- 使用 open coded defer 时，在进入函数内的每个 block 时，都会设置这个 block 相应的 bit(ORL 指令)，在执行到相应的 defer 语句时，把 defer 语句的参数和调用函数地址推到栈的相应位置
- 在栈上需要为这些 defer 调用的参数、函数地址，以及这个用来记录 defer bits 的 byte 预留空间，所以毫无疑问，函数的 framesize 会变大
panic 处理部分
proposal 有两部分，一部分是讲 open coded defer 的实现，另一部分是说 panic 的处理，现在在函数中发生 panic 时，我们没有办法获得原来的 _defer 结构体了，所以必须在编译期，给每个函数准备一项 FUNCDATA，存储每一个 defer block 的地址。这样 runtime 就可以通过 FUNCDATA 和偏移找到需要执行的 defer block 在什么地方。
0x004a 00074 (defer.go:8) FUNCDATA $2, gclocals·951c056c1b0a4d6097d387fbe928bbbf(SB)
0x004a 00074 (defer.go:8) FUNCDATA $3, "".main.stkobj(SB)
0x004a 00074 (defer.go:8) FUNCDATA $5, "".main.opendefer(SB)
....
"".main.opendefer SRODATA dupok size=29
0x0000 30 c1 01 04 30 80 01 01 60 18 00 30 78 01 48 18 0...0...`..0x.H.
0x0010 00 30 70 01 30 18 00 30 68 01 18 18 00 .0p.0..0h....
这里的 main.opendefer 在编译为二进制之后，可能会被优化掉。不过我们理解基本的执行流程就可以了。
跟踪 runtime.gopanic 的流程，可以看到现在的 defer 上有 openDefer 的 bool 值:
(dlv) p d
*runtime._defer {
siz: 48,
started: true,
heap: false,
openDefer: false,
sp: 824634391712,
pc: 17590156,
fn: *runtime.funcval {fn: 17565744},
_panic: *runtime._panic nil,
link: *runtime._defer {
siz: 8,
started: false,
heap: true,
openDefer: true,
sp: 824634392456,
pc: 17001293,
fn: *runtime.funcval nil,
_panic: *runtime._panic nil,
link: *runtime._defer nil,
fd: unsafe.Pointer(0x10fea48),
varp: 824634392528,
framepc: 17000984,},
fd: unsafe.Pointer(0x0),
varp: 0,
framepc: 0,}
(dlv) n
执行具体的 open coded defer 逻辑在 runtime.runOpenDeferFrame，也没啥神奇的:
func runOpenDeferFrame(gp *g, d *_defer) bool {
done := true
fd := d.fd // 这里就是 proposal 里说的 funcdata
// Skip the maxargsize
_, fd = readvarintUnsafe(fd)
deferBitsOffset, fd := readvarintUnsafe(fd)
nDefers, fd := readvarintUnsafe(fd)
deferBits := *(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset)))
for i := int(nDefers) - 1; i >= 0; i-- {
// read the funcdata info for this defer
var argWidth, closureOffset, nArgs uint32
argWidth, fd = readvarintUnsafe(fd)
closureOffset, fd = readvarintUnsafe(fd)
nArgs, fd = readvarintUnsafe(fd)
if deferBits&(1<<i) == 0 {
for j := uint32(0); j < nArgs; j++ {
_, fd = readvarintUnsafe(fd)
_, fd = readvarintUnsafe(fd)
_, fd = readvarintUnsafe(fd)
}
continue
}
closure := *(**funcval)(unsafe.Pointer(d.varp - uintptr(closureOffset)))
d.fn = closure
deferArgs := deferArgs(d)
// If there is an interface receiver or method receiver, it is
// described/included as the first arg.
for j := uint32(0); j < nArgs; j++ {
var argOffset, argLen, argCallOffset uint32
argOffset, fd = readvarintUnsafe(fd)
argLen, fd = readvarintUnsafe(fd)
argCallOffset, fd = readvarintUnsafe(fd)
memmove(unsafe.Pointer(uintptr(deferArgs)+uintptr(argCallOffset)),
unsafe.Pointer(d.varp-uintptr(argOffset)),
uintptr(argLen))
}
deferBits = deferBits &^ (1 << i)
*(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset))) = deferBits
p := d._panic
reflectcallSave(p, unsafe.Pointer(closure), deferArgs, argWidth)
if p != nil && p.aborted {
break
}
d.fn = nil
// These args are just a copy, so can be cleared immediately
memclrNoHeapPointers(deferArgs, uintptr(argWidth))
if d._panic != nil && d._panic.recovered {
done = deferBits == 0
break
}
}
return done
参考资料: